# Proposed host galaxies of repeating fast radio burst sources detected by
CHIME/FRB

Adaeze L. Ibik David A. Dunlap Department of Astronomy & Astrophysics,
University of Toronto, 50 St. George Street, Toronto, ON M5S 3H4, Canada
Dunlap Institute for Astronomy & Astrophysics, University of Toronto, 50 St.
George Street, Toronto, ON M5S 3H4, Canada  Maria R. Drout David A. Dunlap
Department of Astronomy & Astrophysics, University of Toronto, 50 St. George
Street, Toronto, ON M5S 3H4, Canada  Dunlap Institute for Astronomy &
Astrophysics, University of Toronto, 50 St. George Street, Toronto, ON M5S
3H4, Canada  B. M. Gaensler Dunlap Institute for Astronomy & Astrophysics,
University of Toronto, 50 St. George Street, Toronto, ON M5S 3H4, Canada
David A. Dunlap Department of Astronomy & Astrophysics, University of Toronto,
50 St. George Street, Toronto, ON M5S 3H4, Canada  Present address: Division
of Physical and Biological Sciences, University of California Santa Cruz, 1156
High Street, Santa Cruz, CA 95064, USA  Paul Scholz Dunlap Institute for
Astronomy & Astrophysics, University of Toronto, 50 St. George Street,
Toronto, ON M5S 3H4, Canada  Daniele Michilli MIT Kavli Institute for
Astrophysics and Space Research, Massachusetts Institute of Technology, 77
Massachusetts Ave, Cambridge, MA 02139, USA  Department of Physics,
Massachusetts Institute of Technology, 77 Massachusetts Ave, Cambridge, MA
02139, USA  Mohit Bhardwaj Department of Physics, Carnegie Mellon University,
5000 Forbes Avenue, Pittsburgh, 15213, PA, USA  Victoria M. Kaspi Department
of Physics, McGill University, 3600 rue University, Montréal, QC H3A 2T8,
Canada  Trottier Space Institute, McGill University, 3550 rue University,
Montréal, QC H3A 2A7, Canada  Ziggy Pleunis Dunlap Institute for Astronomy &
Astrophysics, University of Toronto, 50 St. George Street, Toronto, ON M5S
3H4, Canada  Tomas Cassanelli Department of Electrical Engineering,
Universidad de Chile, Av. Tupper 2007, Santiago 8370451, Chile  Amanda M. Cook
David A. Dunlap Department of Astronomy & Astrophysics, University of Toronto,
50 St. George Street, Toronto, ON M5S 3H4, Canada  Dunlap Institute for
Astronomy & Astrophysics, University of Toronto, 50 St. George Street,
Toronto, ON M5S 3H4, Canada  Fengqiu A. Dong Department of Physics and
Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver,
BC V6T 1Z1 Canada  Jane F. Kaczmarek Dominion Radio Astrophysical Observatory,
Herzberg Research Centre for Astronomy and Astrophysics, National Research
Council Canada, PO Box 248, Penticton, BC V2A 6J9, Canada  CSIRO Astronomy and
Space Science, Parkes Observatory, P.O. Box 276, Parkes NSW 2870, Australia
Calvin Leung MIT Kavli Institute for Astrophysics and Space Research,
Massachusetts Institute of Technology, 77 Massachusetts Ave, Cambridge, MA
02139, USA  Department of Physics, Massachusetts Institute of Technology, 77
Massachusetts Ave, Cambridge, MA 02139, USA  Katherine J. Lu  Department of
Physics, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, 15213,
PA, USA  Kiyoshi W. Masui MIT Kavli Institute for Astrophysics and Space
Research, Massachusetts Institute of Technology, 77 Massachusetts Ave,
Cambridge, MA 02139, USA  Department of Physics, Massachusetts Institute of
Technology, 77 Massachusetts Ave, Cambridge, MA 02139, USA  Aaron B. Pearlman
Department of Physics, McGill University, 3600 rue University, Montréal, QC
H3A 2T8, Canada  Trottier Space Institute, McGill University, 3550 rue
University, Montréal, QC H3A 2A7, Canada  Masoud Rafiei-Ravandi Department of
Physics, McGill University, 3600 rue University, Montréal, QC H3A 2T8, Canada
Trottier Space Institute, McGill University, 3550 rue University, Montréal, QC
H3A 2A7, Canada  Ketan R Sand Department of Physics, McGill University, 3600
rue University, Montréal, QC H3A 2T8, Canada  Trottier Space Institute, McGill
University, 3550 rue University, Montréal, QC H3A 2A7, Canada  Kaitlyn Shin
MIT Kavli Institute for Astrophysics and Space Research, Massachusetts
Institute of Technology, 77 Massachusetts Ave, Cambridge, MA 02139, USA
Department of Physics, Massachusetts Institute of Technology, 77 Massachusetts
Ave, Cambridge, MA 02139, USA  Kendrick M. Smith Perimeter Institute for
Theoretical Physics, 31 Caroline Street N, Waterloo, ON N25 2YL, Canada
Ingrid H. Stairs Department of Physics and Astronomy, University of British
Columbia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1 Canada

###### Abstract

We present a search for host galaxy associations for the third set of
repeating fast radio burst (FRB) sources discovered by the CHIME/FRB
Collaboration. Using the $\sim$ 1 arcmin CHIME/FRB baseband localizations and
probabilistic methods, we identify potential host galaxies of two FRBs,
20200223B and 20190110C at redshifts of 0.06024(2) and 0.12244(6),
respectively. We also discuss the properties of a third marginal candidate
host galaxy association for FRB 20191106C with a host redshift of 0.10775(1).
The three putative host galaxies are all relatively massive, fall on the
standard mass-metallicity relationship for nearby galaxies, and show evidence
of ongoing star formation. They also all show signatures of being in a
transitional regime, falling in the “green valley” which is between the bulk
of star-forming and quiescent galaxies. The plausible host galaxies identified
by our analysis are consistent with the overall population of repeating and
non-repeating FRB hosts while increasing the fraction of massive and bright
galaxies. Coupled with these previous host associations, we identify a
possible excess of FRB repeaters whose host galaxies have
$M_{\mathrm{u}}-M_{\mathrm{r}}$ colors redder than the bulk of star-forming
galaxies. Additional precise localizations are required to confirm this trend.

Fast radio bursts, galaxies, star formation, radio transient sources

††journal: ApJ

##  1 Introduction

Over 600 short-duration energetic radio pulses known as fast radio bursts
(FRBs; Lorimer et al., 2007) have been reported111https://www.chime-
frb.ca/catalog (CHIME/FRB Collaboration et al., 2021; Nimmo et al., 2023).
Multiple FRBs that are coincident in sky location and dispersion measure
(DM)—“FRB repeaters”, are particularly interesting since they provide more
opportunities for detailed follow-up studies. While we do not yet know whether
repeaters and one-off FRBs are two separate classes, the existence of
repeaters demonstrates that the progenitors of at least some FRBs are not
cataclysmic (for a review of FRB models, see Platts et al. 2019). Recently,
the magnetar theory for FRBs (Beloborodov, 2017; Metzger et al., 2019) was
confirmed as one possible origin of FRB with the discovery of a very luminous
FRB-like burst from the Galactic magnetar SGR $1935+2154$ (CHIME/FRB
Collaboration et al., 2020; Bochenek et al., 2020). However, it remains the
only FRB source confirmed to be a magnetar, emphasizing the need for
additional probe of their nature.

The host galaxies of various types of transients have often provided useful
insight into their origins—allowing constraints on the ages and physical
properties required to produce them (e.g., Eftekhari et al., 2019, 2021). To
date, approximately 34 FRBs have been localized to their host galaxies (e.g.,
Chatterjee et al., 2017; Bhandari et al., 2020; Heintz et al., 2020; Bhandari
et al., 2021; Bhardwaj et al., 2021b; Niu et al., 2021; Bhandari et al.,
2022a; Michilli et al., 2022; Rajwade et al., 2022; Gordon et al., 2023;
Sharma et al., 2023; Jankowski et al., 2023). This is due to the difficulty of
pinpointing FRBs within the arcsecond precision required for robust host
associations (Eftekhari & Berger, 2017) and multi-wavelength follow-up (e.g.,
Scholz et al., 2017). The host populations of both repeating and non-repeating
FRBs have diverse properties (star-forming/quiescent, low/high metallicity,
massive/dwarf, spiral/elliptical) (e.g., Heintz et al., 2020; Bhandari et al.,
2021, 2022b) and it is not clear what type of host is preferred by repeaters.
While the nature and the origin of repeating FRBs are still debated, finding
more host galaxies will provide insights into the origin of the FRB
population. In addition to understanding the nature of FRBs, FRBs with known
host galaxies are useful cosmological probes because the relationship between
their DMs and redshifts could give insight into the missing baryons in the
intergalactic medium (IGM) (Macquart et al., 2020).

Recently, the Canadian Hydrogen Intensity Mapping Experiment Fast Radio Burst
(CHIME/FRB) project (CHIME/FRB Collaboration et al., 2018) published 25 new
repeaters and a further 14 candidates as described by CHIME/FRB Collaboration
et al. (2023). This supplements 21 previously published CHIME/FRB repeaters
(CHIME/FRB Collaboration et al., 2019a, b; Fonseca et al., 2020; Bhardwaj et
al., 2021a; Lanman et al., 2022; Mckinven & CHIME/FRB Collaboration, 2022),
all of which are observed in the frequency range 400–800 MHz. This sample
represents by far the largest population of repeaters detected by a single
survey to date and thus offers the opportunity to learn about various
properties of the FRB population. CHIME/FRB repeaters show a lower average
dispersion measure than those of apparently non-repeating sources (CHIME/FRB
Collaboration et al., 2023) even though this could be a selection effect (see
Connor et al., 2020).

Other previously established characteristic properties of repeaters such as
wider bursts and narrower emitting bandwidths than non-repeating FRBs were
confirmed in the additional set of repeaters (CHIME/FRB Collaboration et al.,
2021; Pleunis et al., 2021). These observational differences could signal a
distinct astrophysical origin for repeaters. Additional multi-wavelength
studies with a larger sample, including host galaxy identification, could help
to disentangle potential multiple populations.

Typically, robust host associations require $\lesssim$ 1 arcsecond
localizations for the FRBs, and hence detection of a burst with
interferometers (Eftekhari & Berger, 2017). Indeed, some FRB repeaters (e.g.,
FRB 20180916B and 20201124A; Marcote et al. 2020, Nimmo et al. 2022)
discovered by CHIME/FRB have subsequently been associated with their host
galaxies by means of very long-baseline interferometry (VLBI). However,
arcsecond localizations are not available for most CHIME/FRB bursts. At best,
CHIME/FRB can achieve $\sim$1 arcminute localization for bursts when baseband
data are recorded. CHIME/FRB baseband data are channelized raw voltages of the
FRB signals from the telescope feeds. Localizations with precision $\sim$1
arcminute using baseband data from CHIME/FRB can be obtained following the
techniques described by Michilli et al. (2021). While these baseband positions
are larger than the size of a typical galaxy in the sky, for objects in the
relatively nearby universe, it is possible to make probable host galaxy
associations due to the low density of galaxies at lower redshifts. This
technique has previously been used to associate five FRBs detected by
CHIME/FRB with likely hosts: FRBs 20200120E (Bhardwaj et al., 2021a),
20181030A (Bhardwaj et al., 2021b), 20220912A (Mckinven & CHIME/FRB
Collaboration, 2022), 20180814A, and 20190303A (Michilli et al., 2022) and the
reliability of the method was supported by the eventual VLBI confirmation of
the host galaxies for FRB 20200120E (Kirsten et al., 2021).

Future telescopes such as the CHIME/FRB Outrigger project (currently under
construction) will be able to improve the number of precisely localized FRBs
(Leung et al., 2021; Mena-Parra et al., 2022; Cassanelli et al., 2022). Other
such facilities include the Real-time fast transients at the Karl Janskly Very
Large Array (realfast VLA; Law et al., 2018), the fast radio transient-
detection program at MeerKAT (MeerTRAP; Rajwade et al., 2022), and the Deep
Synoptic Array (DSA-110; Ravi et al., 2022). Until sub-arcseconds localization
is possible for CHIME/FRB events, the baseband position remains a useful
technique for associating local universe FRBs to their host galaxies (Michilli
et al., 2022).

In this paper, we perform a search for likely host galaxies for 20 repeaters
and 3 repeater candidates from the recently published sample by CHIME/FRB
Collaboration et al. (2023) for which $\sim$arcmin baseband localization were
available. In total, we identify highly probable hosts for two FRBs and one
marginally significant candidate host for a third. A thorough description of
our search for the probable hosts of the events is described in §2 and details
of the observations of likely hosts are explained in §3. The observed and
inferred properties of the hosts are described in §4 including a comparison
between the three hosts and previously known hosts of FRBs in §4.5. Finally,
we summarize the result of the host associations in §5.

##  2 Search for Probable Host Galaxies

Here we describe the process used to search for probable host galaxies to the
CHIME/FRB repeaters. Specifically, after describing our initial sample (§2.1),
we determine the maximum redshift for each FRB based on its DM (§2.2), search
archival optical catalogs for galaxies within the localization regions for
each burst (§2.3), and perform a probability of chance coincidence analysis
(§2.4). Finally, we summarize these results for three galaxies identified as
potential hosts to CHIME repeating FRBs in §2.5.

###  2.1 Initial FRB Sample

Of the 25 ‘gold’ and 14 ‘silver’ repeating FRBs presented by CHIME/FRB
Collaboration et al. (2023), we first restrict ourselves to objects that have
baseband localizations for multiple bursts because of their small localization
uncertainties. This criterion is met for 20 gold and 3 silver candidates. From
there, we continue the analysis with events that are in the survey coverage
area of either the Sloan Digital Sky Survey (SDSS) DR12 (Alam et al., 2015) or
Dark Energy Spectroscopy Instrument (DESI), Data Release 9 (DR9) (Dey et al.,
2019) Legacy Imaging Survey photometric catalog, as these are required to
assess the population of possible host galaxies within the CHIME/FRB
localizations regions. This reduced the sample list to the 12 gold and 1
silver (FRB 20190303D) candidates listed in Table 1.

Table 1: Summary of FRB repeater candidates searched for likely host galaxies

FRB Name | DM | DMMWc | zmax |  $N_{\mathrm{s}}$d | Max $P_{\mathrm{\texttt{PATH}}}$e | Max $P_{\mathrm{\texttt{PATH}}}$e | mr,galf |  $P_{\mathrm{cc,gal}}$g
---|---|---|---|---|---|---|---|---
| (pc cm-3) | (pc cm-3) |  |  | $\mathrm{P(U)}=0.0$ | $\mathrm{P(U)}=0.1$ | (AB mag) |
FRB 20200223B | 201.8(4) | 45.6 | 0.19 | 7 | 0.994 | 0.899 | 16.08 | 0.010
FRB 20191106C | 332.2(7) | 25.0 | 0.36 | 11 | 0.951 | 0.815 | 17.31 | 0.063
FRB 20190110C | 221.6(1.6) | 37.1 | 0.22 | 9 | 0.918 | 0.779 | 18.00 | 0.102
FRB 20190804E | 363.2(3) | 43.4 | 0.37 | 9 | 0.638 | 0.549 | 19.13 | 0.305
FRB 20200118D | 625.7(1.4) | 76.6 | 0.62 | 14 | 0.619 | 0.520 | 19.71 | 0.491
FRB 20201114A | 321.1(8) | 37.8 | 0.33 | 60 | 0.537 | 0.454 | 17.68 | 0.469
FRB 20200913C | 574.2(1.6) | 47.8 | 0.60 | 74 | 0.490 | 0.407 | 17.87 | 0.600
FRB 20200929C | 413.3(3) | 38.4 | 0.44 | 41 | 0.474 | 0.388 | 17.04 | 0.144
FRB 20191013D | 524.1(6.5) | 43.1 | 0.55 | 111 | 0.465 | 0.396 | 17.49 | 0.401
FRB 20190609Ca | 479.2(3) | 113.4 | 0.43 | 3 | 0.409 | 0.354 | 22.58 | 0.745
FRB 20201221B | 509.5(1.1) | 50.9 | 0.53 | 45 | 0.210 | 0.175 | 19.48 | 0.686
FRB 20190303Db | 716.6(1.6) | 36.5 | 0.77 | 28 | 0.118 | 0.098 | 20.55 | 0.750
FRB 20200619A | 439.6(0.4) | 50.8 | 0.45 | 350 | 0.080 | 0.067 | 18.18 | 0.753

Notes:
a PATH probability using the SDSS list of host candidates while others used
the DESI list of host candidates.
b This is the only silver candidate in our sample. The rest of the candidates
are from the gold sample.
c DM contribution expected for the disk of the Milky Way from the NE2001 model
(Cordes & Lazio, 2002).
d $N_{\mathrm{s}}$ is the number of extended optical objects detected in the
FRB 90% error region.
e Max $P_{\mathrm{\texttt{PATH}}}$ is the maximum probability estimated by
PATH for each FRB region given two different priors, $\mathrm{P(U)}=0.0$ and
$\mathrm{P(U)}=0.1$. FRBs are ordered in the decreasing order of Max
$P_{\mathrm{\texttt{PATH}}}$.
f mr,gal(AB) is the r-band AB magnitude of the plausible host of the FRB as
identified by PATH.
g $P_{\mathrm{cc,gal}}$ is the probability of finding a galaxy with the r-band
magnitude of the most probable host or brighter ($<$mr,gal) within each FRB
90% error region.

###  2.2 Determination of zmax

To begin, we calculate the maximum redshift for each FRB in our sample. This
is possible because the DM measured for each burst gives us an integrated
column density of electrons along the line of sight. This DM will contain
contributions from materials in the Milky Way (MW) disk and halo, the inter-
galactic medium (IGM), the host galaxy interstellar medium (ISM), and
potentially the local material around the FRB. However, if electrons in the
IGM are the dominant contribution, the DM of FRBs is approximately linearly
related to redshift.

For each of the FRBs in Table 1, we determine the maximum redshift
$z_{\mathrm{max}}$ using DMs taken from CHIME/FRB Collaboration et al. (2023).
To do this, we subtract the DM contribution expected for the disk of the Milky
Way from the NE2001 model (Cordes & Lazio, 2002), DMMW(NE2001), but do not
attempt to correct for contributions from either the MW halo (DMhalo) or the
FRB host galaxy (DMhost).

This method provides us with a conservative estimate for the “excess” DM
associated with each burst (DMexcess). We then calculate the maximum redshift,
zmax for each repeater event by attributing this entire DMexcess to the IGM
using the FRUITBAT software (Batten, 2019). FRUITBAT uses the given DM to
estimate the maximum redshift given the cosmology of Komatsu et al. (2009) and
the DM-redshift relationship of Zhang (2018). In Table 1, we list the DM,
DMMW(NE2001), and zmax for all bursts in our sample. The derived zmax values
span 0.19 $<$ zmax $<$ 0.77.

We acknowledge that this method is different from that described by Bhardwaj
et al. (2021a) and used in other CHIME/FRB papers. However, we emphasize that
zmax is not utilized in the probabilistic analysis below. We consider all
galaxies in the field of the FRB. Rather, zmax is only used to ensure that any
individual host with a high probability of association has z < zmax (see
Section 2.5). However, using the method described by Bhardwaj et al. (2021a)
does not change the set of possible host galaxies. We, therefore, opt for the
most conservative approach.

###  2.3 Optical Galaxy Catalog Search

We searched the SDSS and DESI catalogs for host galaxy candidates within the
90%-confidence baseband positions of the FRBs. For 12 of the 13 events that
are in the survey coverage area of both DESI and SDSS, we adopted the DESI
catalog which is more sensitive to fainter sources (r-band 5$\sigma$ depths of
$\sim$23.4 mag), while the SDSS catalog (r-band 5$\sigma$ depths of $\sim$22.7
mag) was used to analyze FRB 20190609C. The impact of completeness in these
catalogs, especially for faint galaxies, will be discussed below. To identify
likely host galaxies, we first exclude any object classified as a point source
within the SDSS or DESI catalogs. Multiple extended sources were found within
the 90%-confidence positional error region for each FRB. We record the number
of extended objects within the localization regions for each FRB as
$N_{\mathrm{s}}$ in Table 1.

###  2.4 Host Probability Analysis

We use two complementary methods to assess the likelihood that a galaxy within
the CHIME/FRB error region can be robustly associated with a host galaxy. We
use the result of these methods together to decide on when/whether likely host
associations can be made.

_First method:_ First, for cases where more than one galaxy was identified in
DESI/SDSS imaging, we analyze the probability of each being associated with
the FRB using the formalism described in the Probabilistic Association of
Transients to their Hosts (PATH) software (Aggarwal et al., 2021). PATH takes
as input the FRB position and its uncertainty, and the r-band magnitudes and
half-light radii of all candidate galaxies. It then calculates a probability
for each galaxy using Bayes’ theorem and accounting for the FRB localization
error region. We initially adopt priors that assume (i) the probability that
each candidate galaxy is the host and is inversely proportional to the angular
surface density of galaxies with magnitudes equal to or brighter than the
candidate’s magnitude (based on galaxy counts from Driver et al. 2016), (ii)
the probability of finding the host galaxy of the FRB at a given physical
offset is an exponentially declining function, and (iii) that the probability
that the true host was undetected, $\mathrm{P(U)}$, was zero
($\mathrm{P(U)}=0$). The impact of adjusting this final assumption on the
resultant probabilities is examined below, as well as how reasonable the
assumption is given the depth of the DESI/SDSS surveys at the maximum
redshifts of specific bursts (see § 2.5). For the CHIME/FRB host candidates
described above, we obtained the half-light radii in arcseconds from DESI and
used the Petrosian radius instead in the absence of a half-light radius for
the FRB with only SDSS data.

Applying the PATH framework, we obtain a probability of associating the FRB
with each of the candidates identified in its localization region and report
the highest probability value, Max $P_{\mathrm{\texttt{PATH}}}$ in Table 1. In
total, we find three FRBs where the maximum PATH probability
($P_{\mathrm{\texttt{PATH}}}$) for the $\mathrm{P(U)}=0$ case is $>$ 90%.
Given that we initially ran PATH adopting a probability of zero that the true
host was undetected (and hence not in our candidate list), this is effectively
a statement that PATH significantly prefers one of the candidate hosts over
all _detected_ galaxies222The resulting PATH probabilities for all galaxies
within these fields are given in Appendix A. For the remaining 10 FRBs, PATH
gives a maximum probability to any individual galaxy of $\lesssim$ 75% _even_
in the case that the probability of an undetected host is zero. For these
cases, we argue that it is not possible to make a likely host association with
current data, while the three FRBs with higher Max
$P_{\mathrm{\texttt{PATH}}}$ warrant further examination.

In particular, it is possible that some potential host galaxies were missing
from our initial candidate lists. The DESI detection limit corresponds to an
absolute r-band magnitude of $-$15.9 to $-$17.48 at redshifts between 0.19 and
0.36 (the range of $z_{\mathrm{max}}$ values for the three bursts with the
highest PATH probability hosts). At these depths, we are sensitive to galaxies
fainter than 0.1L∗ including all of the superluminous supernova and long-
duration gamma-ray burst host galaxies at $z<$0.3 of Lunnan et al. (2014).
However, some of the faintest dwarf galaxies, such as the Mr $>$ $-$16.5 host
of FRB121102 (Chatterjee et al., 2017), could have eluded detection at the
high-end of this distance range.

Given the possibility of undetected dwarf galaxies, we re-run PATH, now
setting an arbitrary prior on the probability that the true host was
undetected as $\mathrm{P(U)}=0.1$ (recommended by the authors of PATH). The
results are also listed in Table 1. For one FRB, 20200223B, the galaxy with
the maximum PATH probability is approximately 90%, while for the other two
(20190110C, 20191106C), the maximum PATH probability falls to 82% $-$ 78%. We
will further discuss the appropriateness of these values of P(U) for each of
these specific FRBs—given our survey depth at their maximum distance—in
Section 2.5.

_Second method:_ Given that the appropriate value for $\mathrm{P(U)}$ for each
burst is uncertain, we also calculate the probability that the galaxy _most
preferred by PATH_ would be found within the CHIME localization region for
that FRB by chance. To calculate this probability of chance coincidence
($P_{\mathrm{cc,gal}}$), we use a Monte Carlo approach. Specifically, for each
FRB, we query random patches of the sky within the SDSS footprint having the
size of the FRB localization region for 1000 times. We used SDSS instead of
DESI because it was easier to query the entire SDSS catalog multiple times for
many error regions. We determine the number of galaxies within that patch that
have r-band magnitudes brighter than or equal to that of the most probable
host galaxy according to PATH (mr $\leq$ mr,gal). We then use the fractions of
these runs that have at least one galaxy that meets this requirement to
determine the probability that the most plausible identified host was within
the FRB 90%-confidence localization region by chance. In Table 1, we list
$P_{\mathrm{cc,gal}}$ as well as the AB r-band magnitude (mr,gal) values for
the preferred host of these FRBs.

We note that these values are most useful in cases where the maximum PATH
probability is high. For example, while the galaxy with the highest PATH
probability within the localization region of FRB 20200929C has a small
$P_{\mathrm{cc,gal}}$ of 14.4%, PATH only assigns a probability of $\sim$38
$-$ 47% that this is the true host because there are two galaxies of similar
brightness within the localization region. On the other hand, for three FRBs
(20200223B, 20190110C, 20191106C) with maximum PATH probability at
$\mathrm{P(U)}=0$ $>$90%, we calculate that there was a $\leq$10% chance that
the preferred galaxy is located within the CHIME/FRB localization region by
chance.

We emphasize that these $P_{\mathrm{cc,gal}}$ values describe the probability
that an _individual_ galaxy would be located within an area the size of an
_individual_ FRB localization region. They do not account for “look-elsewhere”
effects associated with the fact that we have searched multiple CHIME/FRB
localization regions as part of this overall study. If we instead perform our
Monte Carlo analysis considering a region with the _combined_ area of all 13
FRBs listed in Table 1, we find that: (i) in 71.7% of trials, there is at
least one galaxy with $m_{r}<16.28$ (the SDSS magnitude of the preferred host
for FRB 20200223B), (ii) in 72.9% of trials, there are at least 3 galaxies
with $m_{r}<17.43$ (the SDSS magnitude of the preferred host for FRB
20191106C), and (iii) in 73.6% of trials, there are at least 8 galaxies with
$m_{r}<18.18$ (the SDSS magnitude of the preferred host for FRB 20190110C).
For cases (ii) and (iii), we quote values for at least 3 and 8 galaxies,
respectively, because that was the total number of galaxies with those
brightnesses found in the combined localization regions of all 13 FRBs. These
numbers indicate that although there is a relatively low $P_{\mathrm{cc,gal}}$
of finding a bright galaxy in the localization region of an individual FRB
(e.g. $1.0\%$ for FRB 20200223B). When considering the search region as a
whole, we do not see evidence for a strong overabundance of bright galaxies
within the _combined_ 13 CHIME/FRB repeaters regions. This implies that we
cannot make associations for our galaxies using $P_{\mathrm{cc}}$ based on
galaxy number counts _alone_.

###  2.5 Summary of Possible Host Associations

In the analysis above, we identified three galaxies that we felt were worthy
of further investigation due to PATH strongly preferring an individual galaxy
over any other source _detected_ in the field. We found that when “look-
elsewhere” effects were considered, there is not a _strong_ overabundance of
bright galaxies ($m_{r}<18$ mag) within the 13 combined CHIME/FRBs
localization regions. However, if we assume that FRBs must come from galaxies,
then the Bayesian approach of PATH—which takes into account not only galaxy
number counts but also information on transients offsets—offers a means to
robustly consider the relative probabilities of possible hosts for an
individual FRB. We, therefore, rely primarily on the PATH analysis. Here, we
synthesize information available for the three galaxies necessary to infer
whether a likely host association can be made.

_FRB 20200223B:_ There are seven extended sources in the DESI catalog in the
field of FRB 20200223B. These are marked as green boxes in the left panel of
Figure 1, and have apparent $r$-band magnitudes in the range 16.08 $<$ mr $<$
23.97. The morphology of Source 1 suggests a spiral galaxy while sources 2-7
are faint objects in the field. Of these, Source 1 (SDSS
J003304.68$+$284952.6; hereafter A) with mr(AB) $=$ 16.080$\pm$0.001 mag and
an effective radius of 5.81$\pm$0.02 arcseconds is the galaxy preferred by
PATH (see Table 6 in Appendix A for PATH probabilities assigned to all
galaxies in the field of this FRB).

When adopting $\mathrm{P(U)}=0$, PATH gives a probability of 99.4% that the
FRB is associated with galaxy A (Table 1). Given that the DESI depth
corresponds to an absolute magnitude of roughly $-$15.90 mag at
$z_{\mathrm{max}}<$0.19, our initial search should have been sensitive to all
FRB hosts discovered to date. The probabilities obtained using the prior,
$\mathrm{P(U)}=0$ are likely reasonable for this FRB. In addition, we found
that the probability of finding an m${}_{\mathrm{r}}=$16.1 mag galaxy in the
CHIME localization region for FRB 20200223B by chance was $\sim$1.0%.

Finally, we note that the photometric redshift for this galaxy
($z_{\mathrm{phot}}=$0.06$\pm$0.01) is less than $z_{\mathrm{max}}$ for this
FRB ($z_{\mathrm{max}}$ $<$ 0.19). We, therefore, consider this host
association robust. A is located at $\alpha$(J2000) $=$ 00h33m04$\fs$68
(0.015$\arcsec$) and $\delta$(J2000) = $+$28°49′52$\farcs$60 (0.021$\arcsec$)
(Alam et al., 2015).

_FRB 20190110C:_ We found nine objects that are not point sources in the field
of FRB 20190110C by searching the DESI DR9 catalog (shown in green boxes on
the middle panel of Fig 1). The nine sources have apparent DESI $r$-band
magnitudes in the range 18.0 $<$ mr $<$ 23.9. By visual inspection, source 1
looks like an irregularly shaped object with an extended clump classified as
source 5 by DESI. We, therefore, consider them the same as opposed to distinct
sources. Source 1, SDSS J163716.43$+$412636.2 (hereafter, B) has a probability
of being associated with the FRB of $P_{\mathrm{\texttt{PATH}}}$ $=$ 91.8%
(when adopting $\mathrm{P(U)}=0$) and has a $z_{\mathrm{phot}}=$0.10 $\pm$0.02
compared to $z_{\mathrm{max}}$ $<$ 0.22 for this target. It has apparent
r-band magnitude, mr(AB) 18.009 $\pm$ 0.006 and an effective radius of 3.38
$\pm$ 0.03 arcseconds.

The DESI depth of mr(AB) $\sim$24 mag corresponds to an absolute magnitude of
roughly $-$16.26 mag at $z_{\mathrm{max}}<$0.22, and thus we were sensitive to
all FRB hosts discovered to date. Given this, the probabilities obtained using
the prior, $\mathrm{P(U)}=0$ is likely reasonable for this FRB. In addition,
we found that the probability of finding an m${}_{\mathrm{r}}=18.0$ mag galaxy
in the CHIME localization region by chance was 10.2%. We, therefore, conclude
that B is the likely host galaxy of FRB 20190110C. Galaxy B is located at
$\alpha$(J2000) $=$ 16h37m16$\fs$43 (0.022$\arcsec$) and $\delta$(J2000) =
$+$41°26′36$\farcs$30 (0.024$\arcsec$) (Alam et al., 2015). We report the PATH
probability of all the sources in the field of this FRB in Table 7 of Appendix
A.

Figure 1: DESI $r$-band images showing the 90% baseband localization region of
FRBs 20200223B, 20190110C and 20191106C (orange ellipses). Green boxes show
the positions of host galaxy candidates. The red background contours are from
high-resolution LOFAR Two-metre Sky Survey (LoTSS; Shimwell et al., 2022)
radio data. _Left:_ FRB 20200223B: There are 7 sources within the localization
region from the DESI catalog. Galaxy A, marked as 1 in the image, is a spiral
galaxy at $z_{\mathrm{spec}}=0.06$ and is the most plausible host galaxy of
FRB 20200223B. The red background contours from LoTSS fit the morphology of A
and are thus consistent with star formation activities of the galaxy.
_Middle:_ FRB 20190110C: We present 9 sources from the DESI catalog in this
FRB region. Galaxy B, marked as 1 is the plausible host galaxy of FRB
20190110C at $z_{\mathrm{spec}}=0.122$. The white markers represent VLASS and
FIRST radio point sources in the field of the FRB spatially coincident with
another nearby galaxy named source 2. There is no radio source that is
coincident with galaxy B. _Right:_ FRB 20191106C: There are 11 sources within
the localization region from the DESI catalog. Source 1 (C) is a galaxy at
$z_{\mathrm{spec}}=0.108$ and is the most likely host of FRB 20191106C. There
is a LoTSS radio source just outside the region shown as a red star but the
one that is spatially coincident with galaxy C is shown as a red contour.

_FRB 20191106C:_ 11 objects that are not point sources were found in the DESI
catalog within the baseband region of FRB 20191106C (See the green boxes in
Figure 1. The eleven sources have apparent DESI $r$-band magnitudes in the
range 17.3 $<$ mr $<$ 23.9.

We note that the DM for FRB 20191106C is larger than those of the other two
FRBs discussed in this section, leading to a higher $z_{\mathrm{max}}$. As a
result, the DESI limit corresponds to a slightly higher absolute magnitude of
roughly $-$17.48 mag at $z_{\mathrm{max}}<0.36$ and it is possible that some
dwarf galaxies were missing from our initial candidate list (for example, the
known host of FRB 121102 at $-16.5$ mag; Chatterjee et al. 2017). Given these,
it is likely more appropriate to use $\mathrm{P(U)}=0.1$ for this FRB.

Even when adopting $\mathrm{P(U)}=0.1$, PATH significantly prefers source 1
(SDSS J131819.23+425958.9, hereafter, C) over any other objects in the field.
It assigns a probability of 81.5% to galaxy C compared to $\lesssim$1% for any
other galaxy in the field (see Table 8 in Appendix A where we report the PATH
probability of all the sources in the field of this FRB). However, we found
that the probability of finding an m${}_{\mathrm{r}}=$17.3 mag galaxy in the
CHIME localization region by chance is 6.3%. Therefore, we argue that C is
likely a good candidate host of the FRB. It is located at $\alpha$(J2000) $=$
13h18m19$\fs$23 (0.01$\arcsec$) and $\delta$(J2000) = $+$42°59′58$\farcs$97
(0.01$\arcsec$) (Alam et al., 2015) and looks like an elliptical/irregular
galaxy with mr(AB) of 17.306 $\pm$ 0.003.

_Summary:_ Based on the results above, we conclude that Galaxies A and B are
highly probable hosts for FRB 20200223B and FRB 20190110C, respectively. Both
galaxies have $>$90% probability of being the true host based on PATH and the
depth of the archival surveys is sufficient to detect all previously
discovered FRB host galaxies. As described above, although it is not possible
to make host associations based on galaxy number counts in the 13 CHIME/FRB
localization regions alone, if we assume that FRBs come from galaxies, the
Bayesian approach of PATH provides a means to assess the most likely host. In
contrast, the association of galaxy C to FRB 20191106C is more marginal due to
the shallower depth of archival surveys which means that the probability of
unseen galaxies as the host is non-negligible. This leads to a PATH
probability below 90% when adopting a value of P(U) that is greater than 0.
However, given that PATH strongly preferred this host over any other source
found in the field, we chose to also describe the properties of this galaxy in
the section below.

Finally, we note that the FRB host galaxy association method described in this
manuscript is different from that used in previously published CHIME/FRB
papers (Bhardwaj et al., 2021a, b; Michilli et al., 2022). The latter involves
obtaining spectroscopic redshifts of all the galaxy candidates within the FRB
field and uses the $z_{\mathrm{spec}}$ $<z_{\mathrm{max}}$ condition to decide
on host association. Given that we are looking at many FRB fields, some of
which have high DMs, it is beyond the scope of the present work to conduct
spectroscopic analysis for all the candidates in each field. Hence, we adopt
the probabilistic method as described here. However, future more detailed
studies involving spectroscopic redshifts could be of use.

##  3 Observations of Likely Host Associations

Here we describe the full set of archival and follow-up observations obtained
for the plausible host galaxies of FRBs 20200223B, 20191106C, and 20190110C.
These will be used to measure detailed host properties in §4.

###  3.1 Optical and Infrared Photometry

We retrieved photometric data for each of the 3 host galaxies from SDSS, DESI,
the Wide-field Infrared Survey Explorer (WISE) (Wright et al., 2010), 2MASS
(Cutri et al., 2003), the Galaxy Evolution Explorer (GALEX) (Bianchi et al.,
2017) and the Spitzer Enhanced Imaging Product (SEIP) source list (SSTSL2)
(Spitzer Science Center & Infrared Science Archive (IRSA)(2021), SSC). We will
use these optical-infrared spectral energy distributions to estimate various
properties of the galaxies in §4.2. We corrected all data for Galactic
foreground extinction using the dust reddening map of Schlegel et al. (1998)
and assuming an R${}_{\mathrm{v}}=$3.1 Milky Way extinction curve. $E(\bv)$
values are 0.0489(8), 0.0081(5), and 0.0200(8) for galaxies A, B, and C
respectively.

The infrared colors of galaxies A, B, and C in the WISE bands can be seen in
Figure 2. Using the WISE color-map diagnostic plot (Wright et al., 2010;
Nikutta et al., 2014), galaxy A falls in a region also occupied by spiral,
star-forming, starburst, low-ionization nuclear emission-line regions (LINER)
galaxies, and luminous infrared galaxies (LIRG) (Jarrett et al., 2017). While
the morphology of galaxy B is ambiguous, the infrared colors also overlap with
spiral galaxies and LIRGs, implying a star-forming galaxy. Finally, we note
that Galaxy C was previously classified as an elliptical galaxy by Kuminski &
Shamir (2016) with $\sim$88% certainty and an ellipticity of 0.4 (Simard et
al., 2011). However, the infrared colors overlap with various classes of star-
forming galaxies (spiral, starburst, and LIRGs), which is inconsistent with a
typical quiescent elliptical galaxy. Moreover, this infrared color result is
in line with the result of Ellison et al. (2016) who use artificial neural
networks to model the total infrared luminosity, where they predict that C is
a star-forming galaxy. We conclude that C could be among the rare population
of elliptical galaxies with ongoing star formation.

Figure 2: WISE diagnostic plot adapted from Wright et al. (2010) showing the
location of various classes of objects with the 3 plausible host galaxies
over-plotted as orange, purple, and red squares for galaxies A, B, and C. The
infrared colors from the 3 plausible host galaxies all seem to be associated
with various types of star-forming galaxies.

###  3.2 Optical Spectroscopy

For all three likely host galaxies, we use optical spectroscopy to determine
their spectroscopic redshifts and emission line fluxes— particularly, Hα, Hβ,
[OIII]($\lambda 5006\AA$), and [NII]($\lambda 6582\AA$)— that will then be
used to estimate other galaxy properties in §4.1. In all cases, we convert
redshifts to luminosity distances assuming a flat
$\Lambda_{\mathrm{\textit{CDM}}}$ cosmology with $H_{\mathrm{0}}$ $=$ 67.7 km
s-1 Mpc-1, $\Omega_{\mathrm{m}}$ = 0.31, and $\Omega_{\mathrm{\wedge}}$ = 0.70
(Planck Collaboration et al., 2020).

####  3.2.1 Archival Spectroscopy

For galaxies A and C, archival spectra are available. For FRB 20200223B, the
likely host galaxy, A, was observed and released as part of the Large Sky Area
Multi-Object Fiber Spectroscopic Telescope (LAMOST; Cui et al., 2012) Data
Release 5 (DR5). We retrieve the redshift and relevant emission line fluxes.
The spectroscopic redshift is $z_{\mathrm{spec}}=$ 0.06024(2) which
corresponds to a luminosity distance of 278.97(9)Mpc. For FRB 20191106C, the
likely host galaxy, C, was observed as part of SDSS DR12 and the redshift was
reported by Alam et al. (2015). The spectroscopic redshift is
$z_{\mathrm{spec}}=$ 0.10775(1), which corresponds to a luminosity distance of
515.32(4) Mpc. Emission line fluxes for galaxy C were extracted from the SDSS-
III spectroscopic survey (Eisenstein et al., 2011; Ahumada et al., 2020).

For both A and C, we correct the previously published LAMOST/SDSS line fluxes
for foreground Milky Way extinction. We determine correction factors based on
the Cardelli et al. (1989) extinction law (RV $=$ 3.1) at the rest wavelengths
of Hα, Hβ, [OIII]($\lambda 5006\AA$) and [NII]($\lambda 6582\AA$). In
addition, we also check for intrinsic host galaxy extinction by calculating
the Balmer decrement from the Hα, Hβ line fluxes. Compared to the theoretical
Case-B recombination line ratio of Hα/Hβ $=$ 2.86, we do not find evidence for
significant additional extinction.

####  3.2.2 Gemini North Spectroscopy

For FRB 20190110C, there is no archival spectroscopy available for its
plausible host galaxy, B. Hence we obtained an observation with the Gemini
Multi-Object Spectrograph (GMOS) of the Gemini-North telescope in Hawaii. The
data were obtained on 2022 August 20 during semester 2022B. We used the 1.0"
long-slit, R400 grating and OG515 ($>$ 520 nm) blocking filter, at central
wavelengths of 720/730 nm. In total, we obtained five 1200 second exposures
with a mean airmass of 1.2.

The data were reduced using the standard packages of gmos and gemini routine
in PYRAF (Science Software Branch at STScI, 2012). This included overscan
correction, flat fielding, sky subtraction, wavelength calibration,
extraction, and flux calibration. Chip gaps and cosmic rays were removed from
individual exposures and the different exposure spectra were combined to
produce a single spectrum. In order to estimate the redshift of galaxy A from
the spectrum, we measured the observed central wavelengths of the Hα, Hβ,
[NII], and SII emission lines. We recorded a redshift of $z_{\mathrm{spec}}=$
0.12244(6), which corresponds to a luminosity distance of 591.2(3) Mpc.

The spectrum was then corrected for cosmological expansion using the redshift
and corrected for Milky Way extinction using the Cardelli et al. (1989)
extinction law. As above, we find no evidence for significant additional
internal extinction when calculating the Balmer decrement. The line fluxes of
Hα, Hβ, [OIII]($\lambda 5006\AA$), and [NII]($\lambda 6582\AA$) were then
measured by fitting a gaussian profile to the rest-frame spectrum. The process
was carried out in an automated manner and repeated 100 times to estimate the
error in the line fluxes due to uncertainty at the continuum level.

Figure 3: Gemini North spectra for the plausible host galaxy of FRB 20190110C,
B. Gray lines highlight the location of prominent emission lines that were
used for analysis.

###  3.3 Radio Emission

We searched for the presence of a persistent radio source (PRS) within the
error regions of the three FRBs using archival radio catalogs from the VLA Sky
Survey (VLASS; Lacy et al., 2020), the NRAO VLA Sky Survey (NVSS; Condon et
al., 1998), the Faint Images of the Radio Sky at Twenty-Centimeters (FIRST;
Becker et al., 1995) survey, the TIFR-GMRT Sky Survey (TGSS; Intema et al.,
2017) and the high-resolution component of the LOFAR Two-metre Sky Survey
(LoTSS; Shimwell et al., 2022).

There are LoTSS radio sources at the locations of the host galaxies of FRB
20200223B and FRB 20191106C, with integrated flux densities of 2.96 $\pm$ 0.59
mJy and 3.29 $\pm$ 0.14 mJy, respectively, at a frequency of 144 MHz. We
measured upper limits on the presence of a radio source at the location of B,
which is the plausible host of FRB 20190110C. In Table 2, we report detections
and upper limits on radio emission in the vicinity of the plausible host
galaxies from these datasets. In §4.3, we will investigate the possible origin
of any emission detected.

Table 2: Summary of PRS searches for the host galaxies of FRB 20200223B (A),
FRB 20190110C (B) and FRB 20191106C (C)

|  |  | A | B | C |
---|---|---|---|---|---|---
Survey | Frequency | Angular | Flux density | Flux density | Flux density | References
| ( MHz) | Resolution ( ′′) | ( mJy) |  mJy |  mJy |
FIRST | 1400 | 5 |  $<$0.75a |  $<$0.75a |  $<$0.75a | Becker et al. (1995)
NVSS | 1400 | 45 |  $<$12.5a |  $<$12.5a |  $<$12.5a | Condon et al. (1998)
VLASS | 3000 | 2.5 |  $<$0.69a |  $<$0.60a |  $<$0.63a | Lacy et al. (2020)
TGSS ADR1 | 150 | 25 |  $<$17.5a |  $<$17.5a |  $<$17.5a | Intema et al. (2017)
LoTSS | 144 | 6 | 2.96 $\pm$ 0.59 |  $<$0.42a | 3.29 $\pm$ 0.14 | Shimwell et al. (2022)

Notes.
a 5$\sigma$ RMS sensitivity or upper limit.

##  4 Properties of Likely Host Associations

Here we described the properties of galaxies A, B, and C. We investigate the
implication of the results from optical spectroscopy used to estimate galaxy
properties of A, B, and C in §4.1. We then report on the SED fitting using
Prospector for galaxies A and B in §4.2, and place constraints on the presence
of radio emission from these galaxies in §4.3. We check for any intervening
structures that could contribute to the DM and discuss their effect on DMhost
for the FRB in our sample in §4.4. Finally, in § 4.5 we compare the properties
of these galaxies to the hosts of other FRBs.

Table 3: Properties of the plausible host galaxies of FRB 20200223B (A), FRB
20190110C (B), and FRB 20191106C (C)

Observed property | A Values | B Values | C Values
---|---|---|---
Right Ascension (J2000) | 00h33m04$\fs$68 (0.015$\arcsec$) | 16h37m16$\fs$43 (0.022$\arcsec$) | 13h18m19$\fs$23 (0.01$\arcsec$)
Declination (J2000) |  $+$28°49′52$\farcs$60 (0.021$\arcsec$) |  $+$41°26′36$\farcs$30 (0.024$\arcsec$) |  $+$42°59′58$\farcs$97 (0.01$\arcsec$)
Galaxy name | SDSS J003304.68$+$284952.6 | SDSS 163716.43$+$412636.2 | SDSS J131819.23+425958.9
Galactic Longitude ($l$) and Latitude ($b$) | 118.08, $-$33.86 | 65.57, $+$42.09 | 105.68, $+$73.22
Apparent r-band mag (AB) | $16.080\pm 0.001$ | 18.009$\pm$0.006 | 17.306$\pm$0.003
$E(\bv)$ (mag) | 0.0483(8) | 0.0078(5) | 0.0197(8)
spectroscopic redshift, $z_{\mathrm{spec}}$ | 0.06024(2) | 0.12244(6) | 0.10775(1)
Luminosity distance (Mpc) | 278.97(9) | 591.2(3) | 515.32(4)
Effective radius, Reff (kpc)e | 7.83$\pm$0.03 | 9.65$\pm$0.09 | 4.01$\pm$0.01
Mr |  $-$21.15$\pm$0.01 |  $-$20.71$\pm$0.02 |  $-$21.20$\pm$0.01
Mu $-$ Mr | 1.84$\pm$0.05 | 1.64$\pm$0.11 | 2.08$\pm$0.07
Hα | 8.63$\times$10-15f | 4.8042(8)$\times$10-16 | 6.09$\times$10-15f
Hβ | 1.17$\times$10-15f | 6.43(8)$\times$10-17 | 8.88$\times$10-16f
[OIII]($\lambda 5006\AA$) | 2.94$\times$10-15f | 2.93(7)$\times$10-17 | 1.27$\times$10-16f
[NII]($\lambda 6582\AA$) | 4.17$\times$10-15f | 1.703(9)$\times$10-16 | 2.09$\times$10-15f
Inferred Property |  |  |
SFR${}_{\mathrm{H_{\mathrm{\alpha}}}}$ ($M_{\mathrm{\odot}}$ yr-1)a | N/A | 0.1575(6) | 1.53f
Oxygen Abundance [$Log_{\mathrm{10}}$ (O/H)]a \+ 12 | N/A | 9.102(2) | 9.10f
SFR ($M_{\mathrm{\odot}}$ yr-1)b | 0.59${}_{-0.04}^{+0.04}$ | 0.54${}_{-0.04}^{+0.04}$ | 4.75${}_{-1.27}^{+1.29}$c
Stellar mass (M⊙)b |  $5.6_{-0.93}^{+1.14}$ $\times$ $10^{10}$ |  $2.5_{-0.17}^{+0.10}$ $\times$ $10^{10}$ |  $4.5_{-1.2}^{+1.2}$ $\times$ $10^{10c}$
Mass weighted Age (Gyr)b | 3.37${}_{-0.73}^{+0.47}$ | 2.59${}_{-1.24}^{+0.80}$ | -
Specific star formation rate [log(sSFR)] (yr-1) |  $-$10.98${}_{-0.03}^{+0.00}$d |  $-$10.66${}_{-0.14}^{+0.07}$d |  $-$10.61${}_{-0.21}^{+0.14}$c

Notes.
aValues obtained using the optical spectrum of the host galaxies.
bValue estimated from Prospector.
c Published values obtained from literature (Chang et al., 2015).
d Values obtained using SFR and the stellar mass values from the Prospector
analysis.
e Values are half-light radius taken from DESI legacy catalog (Dey et al.,
2019)
f Values published without errors. Our analysis of these data shows that the
errors will not change the result of our analysis.
N/A represents parameter estimates that are not reliable, and hence not
published.

###  4.1 Optical Emission Line Diagnostics

For the three host galaxies, we use the corrected emission line fluxes (see
§3.2) to infer their dominant source of photo-ionization, star formation
rates, and metallicities.

_BPT diagram:_ In order to probe the major source of photo-ionization in each
of the three galaxies, we first placed all 3 of them on the updated “Baldwin,
Phillips & Terlevich” Baldwin et al. (1981) (BPT) diagram of Trouille et al.
(2011) shown in Figure 4. Galaxies A, B, and C are designated by orange,
magenta, and red squares, while in the background we plot the density
distribution of nearby (0.02 $<z<$ 0.35) emission line galaxy ratios with S/N
$>$5 from the SDSS spectroscopic data from Data Release 8 (DR8: Aihara et al.,
2011). Comparison with other overplotted FRB hosts line ratios is made in
§4.5.

From this, we see that B and C galaxies fall within the region where the
optical emission line ratios are dominated by star formation, while A appears
to have a non-negligible contribution from an AGN to its optical line ratios.
We note that there are no error bars associated with emission line fluxes for
galaxies A and C (which were taken from LAMOST and SDSS, respectively, see
above). From examining these archival spectra, we find that both
$H_{\mathrm{\alpha}}$ and [NII] are strongly detected while [OIII] (and
$H_{\mathrm{\beta}}$ in the case of galaxy A) fall in noisy regions of the
spectra. However, if we assume a conservative error of 30% (corresponding to a
3$\sigma$ detection of these lines), the resulting errors would not be
significant enough to shift either galaxy from the star-forming or composite
region of the BPT diagram, respectively. As a result, we consider these broad
classifications robust and calculate other properties of star formation in the
host galaxies (star formation rate, metallicity) for B and C only. We do not
calculate the star formation rate and metallicity for A due to the AGN
contamination.

_Star formation rates:_ In the absence of internal extinction as was evident
from the Balmer decrement of each of the three galaxies, we calculate star
formation rates from the Hα line fluxes for B and C without Balmer decrement
correction using the equations from Kennicutt (1998):
SFR($H_{\mathrm{\alpha}}$) $=$ 7.9$\times$10-42
(L${}_{\mathrm{H_{\mathrm{\alpha}}}}$) From this we find star formation rates
of 0.1575(6) $M_{\mathrm{\odot}}$ yr-1 at L${}_{\mathrm{H_{\mathrm{\alpha}}}}$
= 1.994(7) $\times$ 1040 ergs s-1 for B and 1.53 $M_{\mathrm{\odot}}$ yr-1 at
L${}_{\mathrm{H_{\mathrm{\alpha}}}}$ = 1.94 $\times$ 1041 ergs s-1 for C (See
Table 3). These values are consistent with galaxies with active star
formation.

_Metallicity:_ We calculate metallicities using Equation 1 from Pettini &
Pagel (2004) (12 $+$ log (O/H) $=$ 8.90 $+$ 0.57 $\times$ N2), where N2 $=$
[NII]$\lambda$6583/Hα. Using these ratios to estimate the metallicity of the
galaxy resulted in an oxygen abundance of 9.102(2) which is equivalent to
2.76(1) Z☉ metallicity for B, and 9.10 which is equivalent to 2.7 Z☉
metallicity for C. These are high metallicities expected from an old
population.

Figure 4: BPT diagram showing the dominant source of ionization using the
nebular emission line ratios from the 3 plausible host galaxies and other
previously published FRB hosts. The greyscale background represents the
density of galaxies from SDSS DR8 (Aihara et al., 2011). The dashed line
(Kauffmann et al., 2003) indicates the empirical division between star-forming
galaxies and AGNs while the dotted line (Kewley et al., 2001) shows the
theoretical demarcation. FRB repeaters and one-offs are represented with
squares and circles respectively while the ones with robust host associations
($\sim<$1 arcsecond) have red edge color. The hollow green squares are other
CHIME/FRB repeaters while the filled red, purple, and orange squares indicate
the 3 host galaxies proposed here for CHIME/FRB repeaters. The location of the
two galaxies (B and C) clearly suggests star formation while the location of A
indicates an AGN contribution to the line fluxes. The representative repeating
and non-repeating FRBs’ line ratios taken from Heintz et al. (2020); Bhandari
et al. (2021); Michilli et al. (2022); Bhardwaj et al. (2021b); Fong et al.
(2021); Mannings et al. (2021), indicates that most FRB host galaxies,
especially repeaters, are mostly star-forming galaxies. See §4.5 for details.

###  4.2 Prospector SED Fitting

We fitted the observed photometric data and spectroscopic redshifts using the
stellar population synthesis modeling code Prospector (Johnson et al., 2021),
in order to determine the stellar mass of galaxies A and B (the mass of Galaxy
C properties was previously published by Chang et al. 2015, hence there was no
need to carry out SED fitting for it). Prospector uses the stellar population
synthesis library provided in the Flexible Stellar Populations Synthesis
(FSPS) stellar population code python-fsps (Conroy et al., 2009) to fit the
observed data. We assumed a Chabrier initial mass function (2003) and allowed
a delayed-$\tau$ star formation history (SFH) model (Carnall et al., 2019)
that has 5 free parameters (stellar mass, stellar metallicity, galaxy age,
dust attenuation optical depth for a foreground screen, and star formation
timescale for an exponentially declining SFH). Reasonable priors as listed in
Table 5 were set for the free parameters. The likelihood given the free
parameters was then sampled using a Markov Chain Monte Carlo (MCMC) approach
through the emcee code (Foreman-Mackey et al., 2013) and the posterior
distributions were calculated. The extinction-corrected photometry was
converted to flux densities as listed in Table 4.

The best-fit SED profile of A (left panel) and B (right panel) are shown in
Figure 5. Since we did not fit the spectra of galaxy B, we plot only the
photometry and the fitted model. In Table 3 we present values for stellar
mass, age, and SFR from our Prospector analysis of galaxies A and B along with
values for stellar mass and SFR for galaxy C from (Chang et al., 2015).
Prospector estimates SFR using the posteriors for the stellar mass, galaxy
age, and decay time scale. Based on these results, we find that galaxies A, B
and C are all relatively massive with M${}_{\rm{gal}}\approx 2-6\times
10^{10}$ M⊙. For galaxies B and C we combine these masses with SFRs estimated
from the spectroscopic analysis described above to compute specific star
formation rates (see Table 3).

Table 4: Photometric data used for Prospector SED fitting for Galaxies, A and
B

Dataset | Filter | Central | A Flux density | B Flux density
---|---|---|---|---
|  | Wavelength (Å) |  |
GALEX | FUV | 1528 | 1.68$\times$10-8 | 4.85$\times$10-9
| NUV | 2271 | 2.23$\times$10-8 | 6.98$\times$10-9
SDSS | u | 3543 | 6.83$\times$10-8 | 1.23$\times$10-8
| g | 4770 | 2.18$\times$10-7 | 3.49$\times$10-8
| r | 6231 | 3.72$\times$10-7 | 5.54$\times$10-8
| i | 7625 | 4.95$\times$10-7 | 7.43$\times$10-8
| z | 9134 | 6.38$\times$10-7 | 7.91$\times$10-8
2MASS | J | 12350 | 9.28$\times$10-7 | -
| K | 16620 | 5.95$\times$10-7 | -
| Ks | 21590 | 7.58$\times$10-7 | -
WISE | W1 | 33526 | 2.19$\times$10-7 | 4.32$\times$10-8
| W2 | 46028 | 1.49$\times$10-7 | 2.79$\times$10-8
Spitzera | 3.6$\mu$m | 35510 | - | 2.71$\times$10-8
| 4.5$\mu$m | 44930 | - | 1.98$\times$10-8
| 5.8$\mu$m | 57300 | - | 1.86$\times$10-8
| 8.0$\mu$m | 78730 | - | 1.24$\times$10-8

Note: All flux densities are in maggies.
a Flux densities are for 3.8 arcsecond diameter aperture.

Table 5: Priors used for Prospector SED fitting for Galaxies, A and B  Prior description |  |  | A | B
---|---|---|---|---
Total stellar mass formed | log(M∗/M⊙) | log-uniform | min$=$9, max$=$11 | min$=$9, max$=$12
Stellar metallicity | log(Z/Z⊙) | top-hat | min$=$ $-$2, max$=$1 | min$=$ $-$5, max$=$ 1
Diffuse V-band dust optical depth | dust2 | top-hat | min$=$0, max$=$0.3 | min$=$0, max$=$5
Stellar population age of the galaxy | tage (Gyrs) | log-uniform | min$=$5, max$=$12 | min$=$1, max$=$10
E-folding time of the SFH |  $\tau$ (Gyrs) | log-uniform | min$=$0.05, max$=$10 | min$=$0.05, max$=$10

Figure 5: _Left:_ Best-fit Prospector model spectrum for A, the plausible host
of FRB 20200223B, plotted along with the flux density in different wavelength
bands. _Right:_ Best-fit Prospector model spectrum for B, the plausible host
of FRB 20190110C. Different wavelength broadband filters are represented in
different colors from ultraviolet to infrared from GALEX, SDSS, 2MASS,
SPITZER, and WISE catalogs. The best-fit model spectra are used to estimate
different physical properties for A and B, as summarized in Table 3.

###  4.3 Constraints on the presence of persistent radio sources

In the field of FRB 20200223B, we found a source in the high-resolution LOFAR
Two-metre Sky Survey (LoTSS; Shimwell et al., 2022) LoTSS catalog, ILT
J003304.67+284952.4 with an integrated flux density of 2.96$\pm$0.59 mJy and
an isotropic radio luminosity of 2.7(5) $\times$ 1029 erg s-1 Hz-1 at 144 MHz
given the luminosity distance of 277.81 Mpc. The radio source is resolved and
extended in the LoTSS, with a full-width half maximum (FWHM) of the major axis
width of 19.53" (LoTSS beam size, $\theta=6"$) and also spatially coincident
with galaxy A. The radio contours match the optical morphology implying that
the radio emission originates from star-formation activity from the galaxy
(see the left panel of Figure 1). We used the VLASS upper limit of $<$0.69 mJy
(5$\sigma$) at 3 GHz and the LoTSS detection at 144 MHz to estimate an upper
limit on the spectral index (S${}_{\mathrm{\nu}}\approx\nu^{\alpha}$) of the
radio source as $\alpha_{\mathrm{radio}}<-0.48\pm 0.09$ which is consistent
with star formation in the galaxy. Also, the radio-to-optical flux ratio of
$<$ 0.4 is consistent with star formation as opposed to AGN activity (radio-
to-optical flux ratio, $F_{\mathrm{1.4\,GHz}}/F_{\mathrm{r-mag}}$ $>$ 1.4)
(e.g., Machalski & Condon, 1999; Vega et al., 2008; Padovani et al., 2009).
Therefore, we conclude that the radio source is likely a low-frequency
emission coming from star formation in the host galaxy. Due to the large beam
size, $\theta=6"$ of LoTSS, we cannot rule out a faint PRS embedded in this
emission.

In the field of FRB 20190110C, we found unresolved radio sources in FIRST
(FIRST J163717.8$+$412634), VLASS (VLASS 1QLCIR J163717.83$+$412633.9: Epoch 1
and 2), and LoTSS (ILT J163717.72$+$412633.8) catalogs in the field spatially
coincident with one another but not with the plausible host galaxy, B. There
is no source found in NVSS down to a 5$\sigma$ threshold of 12.5 mJy/beam. The
integrated radio flux density for the FIRST source is 0.91$\pm$0.15 mJy; for
the first epoch of VLASS is 0.85$\pm$0.11 mJy; for the second epoch of VLASS
is 0.72$\pm$0.11 mJy and for the LoTSS source is 8.67$\pm$0.14 mJy. The
detailed analysis of this radio source will be published by Ibik et al. (in
prep.). We report the measured upper limits from the B location in Table 2.

In the field of FRB 20191106C, we found five LoTSS sources with one (ILT
J131819.22+425958.9, shown in the right panel of Figure 1) spatially
coincident with the plausible host galaxy, C. For simplicity, we show only the
radio source contours that are spatially coincident with the plausible host
galaxy. This source, ILT J131819.22+425958.9, has an integrated flux density
of 3.29$\pm$0.14 mJy and a radio luminosity of 1.05$\pm$0.04 $\times$ 1030 erg
s-1 Hz-1 at 144 MHz given the luminosity distance of 516.60 Mpc. The radio
source is slightly resolved in the LoTSS, with an FWHM major axis width of
7.76", and is offset by 4.0$\pm$1.2 kpc from the centre of the host galaxy. We
used the VLASS upper limit of $<$0.63 mJy (5$\sigma$) at 3 GHz and the LoTSS
detection at 144 MHz to estimate a spectral index
$\alpha_{\mathrm{radio}}<-0.54\pm 0.02$. The spectral index and the radio-to-
optical ratio of $<$ 0.9 are both consistent with star formation as opposed to
AGN activity. Therefore, we conclude that the radio source is likely a low-
frequency emission coming from star formation in the host galaxy. Due to the
large beam size, $\theta=6"$ of LoTSS, we cannot rule out a faint PRS if
embedded in the emission.

###  4.4 Estimates of Host Dispersion Measures

To characterize DMhost of a galaxy, we first need to properly assess any
additional foreground contributions to the DM from intervening structures.
Typically, using the equation, DMhost $=$ DMobserved $-$ DMMW(NE2001) $-$
DMhalo $-$ DMIGM, any missed or unaccounted intervening media will reduce the
estimated value of DMhost. We searched within our FRB baseband positions for
foreground galaxy clusters using data from Wen et al. (2009, 2012); Wen & Han
(2015); Wen et al. (2018); Banerjee et al. (2018), Galactic H II-regions using
the Hα maps from Green (2019); Anderson et al. (2014), Galactic star-forming
regions using CO maps from Avedisova (2002); Rice et al. (2016), nearby
galaxies from GLADE v2.3 (Dálya et al., 2018) and dwarf galaxies from
McConnachie (2012).

The results show that there are no star-forming or H II-regions in the
foreground of the searched FRB repeaters in our sample listed in Table 1. We
will now estimate DMhost for the 3 FRBs with plausible hosts discussed in this
work and discuss the result of the few cases where there is at least one
galaxy cluster or a galaxy in the region.

For the case of FRB 20200223B, in the absence of any object along the line-of-
sight of the plausible host, we use $z_{\mathrm{spec}}=0.06024(2)$ to obtain
DMIGM = 57.12(2) pc cm-3 following Equation 2 of Macquart et al. (2020). We
chose a range of DMhalo with the lower bound and a higher bound that will
produce a physical DMhost for each FRB but still within the given range
reported by Dolag et al. (2015); Yamasaki & Totani (2020); Cook et al. (2023)
which is 30 $-$ 98 pc cm-3 for this FRB. Taking note of the DMMW(NE2001) $=$
45(9) pc cm-3, we estimate DM${}_{\mathrm{host}}=1$ \- $73$ pc cm-3.

We do not find any object along the line-of-sight of the host galaxy of FRB
20190110C. We therefore use z${}_{\mathrm{spec}}=0.12244(6)$ for the host
galaxy to directly estimate DMIGM and hence DMhost. For DMMW(NE2001) $=$ 37(7)
pc cm-3 and assuming DMhalo = 30 $-$ 65 pc cm-3, we obtain DMIGM = 117.75(6)
pc cm-3 and DM${}_{\mathrm{host}}=2$ \- $33$ pc cm-3.

In the field of FRB 20191106C, there is a galaxy cluster, WHL J131827.1+425803
(Wen & Han, 2015) with z${}_{\mathrm{spec}}=0.47$. Given
$z_{\mathrm{max}}=0.36$, we do not expect that the galaxy cluster will affect
the DM of the FRB. There are many other galaxies in the field but none of them
has a redshift that could contribute to the DM of the FRB. In the absence of
any intervening medium, we estimate DM${}_{\mathrm{host}}=$ 103 $-$ 187 pc
cm-3 from DMIGM = 103.29(1) pc cm-3 at z${}_{\mathrm{spec}}=0.10775(1)$ of the
host galaxy, assuming DMhalo = 30 $-$ 111 pc cm-3 and DMMW(NE2001) $=$ 25(5)
pc cm-3.

For the rest of this section, we take note of any possible intervening
structures for the other 10 repeaters in our sample, for which we do not yet
have host associations, but for which one should beware if host galaxy
associations are made in the future. We found a nearby galaxy cluster (WHL
J172540.4+550250) (Wen et al., 2009) with redshift $z=0.43$ in the field of
FRB 20190804E. However, we do not think that this galaxy cluster could affect
the DM of the FRB as it has $z_{\mathrm{max}}=0.37$. In the field of FRB
20201114A with $z_{\mathrm{max}}=0.33$, we found a GLADE galaxy (HyperLEDA
catalog number: 2745781) with z${}_{\mathrm{spec}}=0.08$ (Dálya et al., 2018)
and 4 other galaxies from the DESI catalog, all having $z<z_{\mathrm{max}}$.

We also found 4 galaxy clusters, namely WHL J010806.9+182752
(z${}_{\mathrm{spec}}=0.17$), WHL J010830.7+183100
(z${}_{\mathrm{spec}}=0.56$), WHL J010833.0+182514
(z${}_{\mathrm{spec}}=0.46$) (Wen & Han, 2015), and AMF9 J010830.7+182754
(z${}_{\mathrm{spec}}=0.19$) (Banerjee et al., 2018) in the field of FRB
20200929C. Since their redshifts are lower compared to the FRB’s
$z_{\mathrm{max}}=0.44$, it is plausible that WHL J010806.9+182752 and AMF9
J010830.7+182754 could affect the DM of the FRB depending on the location and
redshift of the host galaxy.

###  4.5 Comparison to Previous FRB Hosts

Based on the results found above, the three probable FRB host galaxies
presented here are all relatively massive galaxies with signatures of ongoing
star formation and roughly solar metallicity. Here, we compare these
properties to those of other known FRB hosts. To aid this comparison, we plot
galaxies A, B, and C along with other FRB hosts on a BPT diagram (Figure 4), a
mass-star formation rate diagram (Figure 6), a color-magnitude diagram (Figure
7) and a mass-metallicity diagram (Figure 8). In all plots, FRB hosts from the
literature are shown in green, with repeaters designated as squares and one-
offs as circles. Previous repeating FRBs that were identified by CHIME/FRB are
shown as open squares. FRBs with robust host association are shown as red-
edge-colored markers.

_BPT diagram:_ First, to compare the dominant sources of photoionization in
FRBs host galaxies, we add FRBs from the literature to the BPT diagram
described in §4.1 to get Figure 4. A total of 15 FRB hosts’ emission line
fluxes were taken from Heintz et al. (2020), Mannings et al. (2021), Fong et
al. (2021), Bhardwaj et al. (2021b), Bhandari et al. (2021), and Michilli et
al. (2022) and shown in green squares and circles for 5 repeating (the host of
FRB 20190303A are two merging galaxies which leads to 6 open squares in the
Figure) and 10 non-repeating FRBs. Out of the 15 FRBs, 3 repeaters and 9 one-
offs have robust (FRBs with localization region of $\sim<$1 arcsecond) host
associations (see markers with red edge colors in Fig 4). The plot shows that
the 3 host galaxies’ (A, B, C) line ratios are broadly consistent with those
of most FRB hosts, although galaxy A would be the first host for a _repeating_
FRB with signatures of AGN activity. Galaxies B and C, support the trend
identified by Bhandari et al. (2021) that most repeating FRBs happen in
galaxies where star formation is the dominant source of photo-ionization, thus
disfavoring progenitor channels where AGN activity is required (e.g. Katz,
2017; Vieyro et al., 2017; Gupta & Saini, 2018).

Figure 6: Star formation rate versus stellar mass distribution of FRB hosts in
three redshift ranges. The squares and circles in each plot represent
repeating and one-off FRBs respectively (Mahony et al., 2018; Heintz et al.,
2020; Mannings et al., 2021; Fong et al., 2021; Bhardwaj et al., 2021a;
Bhandari et al., 2021; Bhardwaj et al., 2021b; Niu et al., 2021; Bhandari et
al., 2022a; Michilli et al., 2022; Sharma et al., 2023)) while the ones with
robust host associations ($\sim<$1 arcsecond) have red edge color. The hollow
green squares are other CHIME/FRB repeaters while the filled red, purple, and
orange squares indicate the 3 host galaxies proposed here for CHIME/FRB
repeaters. The grey background is a kernel density distribution of the PRIMUS
survey galaxy sample for the top panel and COSMOS-15b for the bottom panel.
While the hosts of repeating FRBs show diverse SFR behavior, only a few FRBs
are associated with transitioning and quiescent galaxies. Most FRB hosts are
slightly offset below the star-forming main sequence when compared to the
PRIMUS dataset in the redshift range 0 $<$ z $<$ 0.2. While this effect is
less strong compared to COSMIS-15b data, in each case, a subset of FRB-
repeater hosts (including those presented here) still exhibit low star
formation for their galaxy mass. We discuss this issue further in §4.5.

_SFR-Mass:_ In Figure 6, we compare the rate of star formation versus the
stellar mass of FRB host galaxies in three redshift bins, 0 $<$ z $<$ 0.2, 0.2
$<$ z $<$ 0.4, and 0.4 $<$ z $<$ 0.6. SFRs and masses for 24 FRB hosts were
extracted from Mahony et al. (2018), Heintz et al. (2020), Mannings et al.
(2021), Fong et al. (2021), Bhardwaj et al. (2021a), Bhandari et al. (2021),
(Bhardwaj et al., 2021b), Niu et al. (2021), Bhandari et al. (2022a), Michilli
et al. (2022), and Sharma et al. (2023) and shown in green squares and circles
for 10 repeating (the host of FRB 20190303A are two merging galaxies which
leads to 11 open squares in the Figure) and 14 non-repeating FRBs. Out of the
24 FRBs, 7 repeaters and 11 one-offs have robust (FRBs with localization
region of $\sim<$1 arcsecond) host associations (see markers with red edge
colors in Fig 6). We plot the location of galaxies A, B, and C using SFRs, and
stellar masses measured from Prospector SED fitting described above.

In the top left panel, kernel density contours represent galaxies from the
PRism MUlti-object Survey (PRIMUS) survey (Moustakas et al., 2013) with
redshifts z $<$ 0.5. In the bottom left panel, kernel density contours
represent galaxies from the COSMOS-15 sample that were modeled by Leja et al.
(2020), referred to as COSMOS-15b. Bhandari et al. (2021) previously
identified that most FRB host galaxies are offset slightly below the star-
forming main sequence, showing low star formation rates for their masses when
compared to the PRIMUS dataset. This offset is visible in our figure (see
Figure 6), where the star-forming main sequence is shifted about half an order
of magnitude lower in the upper panels (PRIMUS) of Figure 6. In agreement with
the result of Gordon et al. (2023), when compared to the lower panel
(COSMOS15b), the galaxies mostly fall within the star-forming main sequence in
closer alignment to many of the known FRB hosts. While reconciling this
discrepancy is beyond the scope of this work, we note the following:
i) Our datasets are a mix of both spectroscopic measurements and SED-derived
information and so some are model-dependent. This makes it difficult to select
direct analogous survey data for comparison; ii) We note that below redshift
$<$ 0.2, the PRIMUS sample has fewer galaxies compared to higher redshift
field galaxies and lacks quality SFR and stellar mass measurements within this
redshift range; iii) The PRIMUS dataset redshifts are however measured from
spectroscopy and the derived galaxy property values are from parametrized star
formation history; iv) The COSMOS-15b dataset on the other hand has redshift
measurements from a mixture of spectroscopic and photometric data while its
galaxy properties are derived values from a non-parametric continuity SFH.

Compared to _both_ comparison datasets, galaxies A, B, and C (and many of the
other CHIME repeaters) lie along and below the star-forming main sequence. In
addition, the WISE plot location of galaxy A, showing ongoing star formation
coupled with emission line signatures showing AGN activity, is also consistent
with a galaxy in a transition phase. Our findings show that most FRBs appear
to come from star-forming galaxies across the FRB redshift range observed so
far. While the hosts of repeating FRBs show diverse SFR behavior, only a few
FRBs are associated with transitioning and quiescent galaxies. Most FRB hosts
are slightly offset below the star-forming main sequence when compared to the
PRIMUS dataset in the redshift range 0 $<$ z $<$ 0.2. While this effect is
less strong compared to the COSMOS-15b data, in each case, a subset of FRB-
repeater hosts (including those presented here) still exhibit low star
formation for their galaxy mass.

Overall, even though most FRB hosts prefer star-forming hosts, there may be an
observational bias against finding FRBs in star-forming regions due to large
possible levels of pulse dispersion and scattering in such regions. This could
lead to decreased sensitivity to repeating bursts (Gordon et al., 2023). The
difference in the SED-modelling methods used for the two field galaxy datasets
as well as insufficient data for a given redshift range are possible factors
that could affect the interpretation of this result.

Figure 7:  Rest-frame color–magnitude diagram in three redshift ranges showing
a grey background of the galaxy population from the PRIMUS survey (Moustakas
et al., 2013) for the top panel and the field galaxies from the COSMOS-15
survey (Leja et al., 2020) for the bottom panel. The squares and circles in
each plot represent repeating and one-off FRBs respectively (Mahony et al.,
2018; Heintz et al., 2020; Mannings et al., 2021; Fong et al., 2021; Bhandari
et al., 2021; Bhardwaj et al., 2021a, b; Bhandari et al., 2022a; Michilli et
al., 2022; Sharma et al., 2023) while the ones with robust host associations
($\sim<$1 arcsecond) have red edge color. The hollow green squares are other
CHIME/FRB repeaters while the filled red, purple, and orange squares indicate
the 3 host galaxies proposed here for CHIME/FRB repeaters. Many FRBs come from
bright galaxies and there is a diversity of early-type, late-type, and “green-
valley” (parameter space between the two peaks— early-type and late-type)
galaxies among repeaters and one-offs. Approximately half of the FRB-repeater
hosts show evidence of being in the transitional green-valley and early-type
irrespective of datasets. See §4.5 for details.

Gordon et al. (2023) also used the mass-doubling number criterion (the number
of times the stellar mass doubles within the age of the universe at redshift
at the redshift of the galaxy assuming a constant specific star formation
rate, sSFR) described by Tacchella et al. (2022) to classify galaxies as star-
forming, transitioning and quiescent. We adopted this method for all the
galaxies in our comparison sample as well as galaxies A, B, and C (even though
we caution that the criterion was developed for galaxy properties derived
using non-parametric star formation histories). Our results of the literature
comparison sample show that most FRB hosts are star-forming, few are
transitioning and one is quiescent. In addition, Galaxies A and B are both
transitioning while C is star-forming according to the criteria with mass-
doubling numbers of 0.1363, 0.2686, and 0.9547 respectively. Together, we note
that all four galaxies classified as transitioning by this metric were
repeaters. The overall outcome (that many FRB hosts are star-forming) is
however consistent with the conclusions of Gordon et al. (2023) and Sharma et
al. (2023).

_Color-Magnitude:_ To further examine the possibility that a subset of
repeating FRB hosts are in a transitional phase, in Figure 7, we compare the
color–magnitude properties of the FRB hosts in three redshift bins, 0 $<$ z
$<$ 0.2, 0.2 $<$ z $<$ 0.4 and 0.4 $<$ z $<$ 0.6. This can be a useful
indicator of the class of stellar population in the galaxies, which is less
model-dependent. Specifically, the plot of the rest-frame
M${}_{\mathrm{u}}-M_{\mathrm{r}}$ colors versus their absolute r-band
magnitudes, Mr is overplotted with the kernel density contours of field
galaxies. The top panel again compares to galaxies from the PRIMUS survey with
redshifts z $<$ 0.5 and the lower panel with galaxies of the COSMOS-15b (Leja
et al., 2020) dataset retrieved from the parent catalog (Laigle et al., 2016).
A total of 21 FRB hosts’ color-magnitude values were extracted from Mahony et
al. (2018), Heintz et al. (2020), Mannings et al. (2021), Fong et al. (2021),
Bhandari et al. (2021), Bhardwaj et al. (2021a), Bhardwaj et al. (2021b),
Bhandari et al. (2022a), Michilli et al. (2022), and Sharma et al. (2023) and
shown in green squares and circles for 8 repeating (the host of FRB 20190303A
are two merging galaxies which leads to 9 open squares in the Figure) and 13
non-repeating FRBs. Out of the 21 FRBs, 6 repeaters and 11 one-offs have
robust (FRBs with localization region of $\sim<$1 arcsecond) host associations
(see markers with red edge colors in Fig 7). Galaxies A, B, and C are bright
but fall within the overall range of properties seen in the literature for the
FRB hosts. Many FRBs come from bright galaxies and there is a diversity of
early-type, late-type, and “green-valley” (parameter space between the two
peaks— early-type and late-type) galaxies among repeaters and one-offs.

Our result shows that the observed colors for all three probable hosts
presented in this manuscript fall within the “green valley” when compared to
the PRIMUS dataset, which would be consistent with having low star formation
rates for their stellar masses (We note that the PRIMUS dataset has very few
quiescent galaxies in the redshift range 0 $<$ z $<$ 0.2 which is why there is
no corresponding “early-type” island in the left top panel). In particular,
with the addition of these hosts, _half_ of the FRB repeaters shown in Figure
7 have colors between 1.6 mag $<$ M${}_{\mathrm{u}}-M_{\mathrm{r}}$ $<$ 2.1
mag.

When comparing to the COSMOS-15b dataset, we again see that the three probable
hosts described here show redder colors than a majority of late-time galaxies.
(We note that the Leja et al. 2020 sample has very few quiescent galaxies,
which is why there is a small corresponding “early-type” islands in all the
lower panels). We note that this is a slightly different behaviour that was
observed with the SFR-Stellar mass plot for the COSMOS-15b dataset (lower
panels Figure 6) where many FRBs fell directly on the SFR main sequence. Given
that the exact same set of galaxies are plotted in both figures, this
distinction must stem from methods used to measure star-formation rate and
stellar mass for the COSMOS-15b and FRB datasets (see above).

Approximately half of the FRB-repeater hosts show evidence of being in the
transitional green-valley and early-type, irrespective of datasets. The
moderate and low SFR seen for most FRB hosts are consistent with the location
on the color-magnitude diagram. Most of the hosts are in the green valley and
have a redder color possibly because of their low current star formation. This
result is however non-intuitive as previous analysis shows that FRB hosts are
mostly star-forming (which is mostly associated with late-type galaxies). It
is possible that many FRBs are coming from star-forming regions of either
late-type or green-valley galaxies. Indeed, we see evidence above for ongoing
star formation within the potential FRB hosts presented here. Alternatively,
this may be evidence for an additional, older, FRB progenitor channel, which
is slightly overrepresented in the repeater population. Indeed, there has been
recent evidence for some FRBs coming from older environments, such as FRB
20200120E found in a globular cluster of a spiral galaxy (Bhardwaj et al.,
2021a; Kirsten et al., 2021). Finally, it is possible that this outcome is a
selection bias since it may be easier to see FRBs from early-type galaxies
given that they are not obscured by a high degree of dispersion and scattering
from ionised gas in the hosts (Mannings et al., 2021).

_Mass-Metallicity:_ Finally, Figure 8 shows the distribution of host galaxies’
metallicities and stellar masses. These are shown in comparison to the
Tremonti et al. (2004) stellar mass versus gas-phase metallicity relationship
found from SDSS data of redshifts z $\sim$ 0.1. The solid black line is the
median, and the grey dashed and dotted lines cover the 68% and 95% confidence
regions. Host information for 19 other FRBs was extracted from Heintz et al.
(2020); Mannings et al. (2021); Bhardwaj et al. (2021b); Bhandari et al.
(2021); Michilli et al. (2022); Sharma et al. (2023) consisting of 6 repeaters
(the host of FRB 20190303A are two merging galaxies which leads to 7 open
squares in the Figure) and 13 one-offs. Out of the 19 FRBs, 3 repeaters and 11
one-offs have robust (FRBs with localization region of $\sim<$1 arcsecond)
host associations (see markers with red edge colors in Fig 8). In most cases,
we extract raw emission line fluxes and calculate metallicity ourselves using
the O3N2 calibration (Hirschauer et al., 2018). In 3 cases where the line
fluxes were not available and we could not identify the metallicity
calibration used, we took published metallicities directly, acknowledging that
a small error may be present in their location within Figure 8 due to
systematic offsets between the various strong line metallicity diagnostics
(Kewley & Ellison, 2008). Galaxies B and C are both relatively high-mass
galaxies that fall directly on the mass-metallicity relationship of Tremonti
et al. (2004). Overall, there is no metallicity preference between repeaters
and one-offs but most of them are slightly below the solid line. This supports
the conclusion that FRBs, as a whole, do not require the low metallicity, high
specific star-formation rate environments preferred by superluminous
supernovae and long-duration gamma-ray bursts (Lunnan et al., 2014).

_Trends with FRB repeat rate:_ We used the repeat rates reported by CHIME/FRB
Collaboration et al. (2023) for 8 CHIME/FRB events in our sample to compare
with host properties (metallicity, SFR, color). Out of these, 7 of them are
clustered at similar low repeat rates ($<$1 hr-1) with only one having a high
repeat rate ($>$1 hr-1). The results show no relationship between repeat rates
and any of the properties given the current data. However, this is a very
small dataset; a larger sample collected in the future could provide more
insight.

_Trends with FRB DM host: _ We retrieved the FRB DMhost values of 24 FRBs
containing 8 repeaters and 16 one-offs from Law et al. (2022). We included the
DMhost estimates from FRB 20200223B, FRB 20190110C, and FRB 20191106C thereby
increasing the sample of repeaters to 11 events. We applied the two-sample
Kolmogorov-Smirnov test to the DMhost histogram distribution for repeaters and
non-repeaters. The result is a p-value of 0.4112 ($\sim$41%). Assuming a null
hypothesis at a 95% confidence interval that the repeaters and non-repeaters
have the same local environment, then a p-value that is less than 0.05 (5%)
will reject the null hypothesis. In this case, the p-value is greater than 5%,
thus in agreement with the null hypothesis stating that the two distributions
are the same. We note that there is an outlier among the repeaters which is
FRB 190520. However, repeating the test with the outlier removed does not
change the conclusion.

Figure 8: A plot of metallicity versus the stellar mass of galaxies. FRB
repeaters and one-offs are represented with squares and circles respectively
(Heintz et al., 2020; Mannings et al., 2021; Bhardwaj et al., 2021b; Bhandari
et al., 2021; Michilli et al., 2022; Sharma et al., 2023) while the ones with
robust host associations ($\sim<$1 arcsecond) have red edge color. The hollow
green squares are other CHIME/FRB repeaters while the filled red and purple
squares indicate 2 out of the 3 host galaxies proposed here for CHIME/FRB
repeaters. The third galaxy, A was not included in this plot since the nebular
emission is contaminated by AGN activity. The solid black line is the median
of the SDSS metallicity-stellar mass distribution, and the grey dashed and
dotted lines cover the 68% and 95% confidence regions (Tremonti et al., 2004).
Overall, there is no metallicity preference between repeaters and one-offs but
most of them are slightly below the solid line. See §4.5 for details.

In summary, the sample of about 24 known FRB of host galaxies (repeating and
non-repeating; Heintz et al., 2020; Bhandari et al., 2021; Niu et al., 2021;
Bhandari et al., 2022a; Michilli et al., 2022) covers a broad range of
$r$-band absolute magnitude, Mu-Mr colors, stellar masses, star formation
rates, and metallicities. The three new probable hosts identified here are
consistent with this broader population. However, it is notable that through a
combination of intermediate M${}_{\mathrm{u}}-M_{\mathrm{r}}$ colors (galaxies
A, B, C), low star formation rate for a given stellar mass (galaxies A, B, C),
mass-doubling timescale (galaxies A, B, C) and/or an elliptical morphology
with signatures of ongoing star formation (galaxy C), all three show some
evidence of being in a transitional or star-forming regime. Coupled with the
previously published hosts, there is tentative evidence that FRB repeaters, in
particular, are over-represented in the star-forming and “green valley” with
many showing M${}_{\mathrm{u}}-M_{\mathrm{r}}$ colors redder than the majority
of star-forming galaxies.

##  5 Summary

We searched for host galaxy associations for 13 FRB repeaters out of the 25
“gold” and 14 “silver” samples recently published by CHIME/FRB Collaboration
et al. (2023). The CHIME/FRB baseband localization regions were searched for
host galaxies in SDSS and DESI. We considered two methods to assess the
probability of host association for candidate galaxies within the field.
First, we run candidate host galaxies through PATH to estimate their
probabilities of association with an FRB. Second, we consider the probability
that a galaxy as bright as the host preferred by PATH would be found within
the CHIME localization region by chance. When we consider the “look-elsewhere”
effects associated with the fact that we searched 13 CHIME/FRB localization
regions, we do not find evidence for a significant overabundance of bright
galaxies. However, under the assumption that FRBs come from galaxies, the
Bayesian approach of PATH still allows us to consider the most probable host
for an individual FRB.

In the end, we consider two FRBs (FRB 20200223B, FRB 20190110C) for which we
have galaxy candidates with a maximum association probability that is greater
than 90%. We also cautiously discuss a third FRB (FRB 20191106C) with a
plausible host galaxy for which the archival surveys have a shallower depth at
the maximum redshift of the FRB. We obtained a Max
P${}_{\mathrm{\texttt{PATH}}}$ $=$ 81.5% at $\mathrm{P(U)}=0.1$, but this
galaxy was strongly preferred over any other galaxy in the field. The
resulting discussed host galaxies for the 3 FRBs (FRB 20200223B, FRB
20190110C, and FRB 20191106C) are SDSS J003304.68+284952.6, SDSS
163716.43+412636.2, and SDSS J131819.23+425958.9, respectively.

FRB 20200223B is likely located in a spiral, star-forming galaxy with evidence
for AGN activity. FRB 20190110C likely comes from a high metallicity,
irregular galaxy showing evidence of quenching star formation, while the
potential host galaxy of FRB 20191106C is a high metallicity, possibly
elliptical with evidence for ongoing star formation. The latter two galaxies
follow the standard mass-metallicity relationship observed for nearby
galaxies. Moreover, all 3 galaxies are in the transitional phase with regard
to star formation and show redder colors than expected for late-type galaxies.
This is consistent with the broader repeater host population, which appears to
prefer the region between early and late-type galaxies as discussed in §4.5.
In addition, the high metallicity and low specific star-formation rate
obtained for these galaxies are different from what is seen for the hosts of
superluminous supernovae and long-duration gamma-ray bursts. However, it is
critical to both confirm these host galaxies via arcsecond localization and to
continue to increase the number of FRBs with robust host associations, since
probabilistic host identification methods are biased towards bright galaxies.

_Facilities:_ CHIME, Gemini North

_Softwares:_ FRUITBAT (Batten, 2019), PATH (Aggarwal et al., 2021), PYRAF
(Science Software Branch at STScI, 2012), Prospector (Johnson et al., 2021),
emcee (Foreman-Mackey et al., 2013), python-fsps (Conroy et al., 2009),
Astropy (Astropy Collaboration et al., 2018), Matplotlib (Hunter, 2007), NumPy
(Harris et al., 2020), SAOImage DS9(Joye & Mandel, 2003)

##  6 Acknowledgement

We thank Kasper Heintz and Joel Leja for the supplementary information and
code that facilitated this work. We sincerely appreciate Prof. Edward Wright
and Chao-Wei Tsai for granting permission to use the WISE diagnostic plot from
their paper.

We acknowledge that CHIME is located on the traditional, ancestral, and
unceded territory of the Syilx/Okanagan people. We are grateful to the staff
of the Dominion Radio Astrophysical Observatory, which is operated by the
National Research Council of Canada. CHIME is funded by a grant from the
Canada Foundation for Innovation (CFI) 2012 Leading Edge Fund (Project 31170)
and by contributions from the provinces of British Columbia, Quebec, and
Ontario. The CHIME/FRB Project is funded by a grant from the CFI 2015
Innovation Fund (Project 33213) and by contributions from the provinces of
British Columbia and Quebec, and by the Dunlap Institute for Astronomy and
Astrophysics at the University of Toronto. Additional support was provided by
the Canadian Institute for Advanced Research (CIFAR), McGill University, and
the McGill Space Institute thanks to the Trottier Family Foundation and the
University of British Columbia. The CHIME/FRB baseband system is funded in
part by a Canada Foundation for Innovation John R. Evans Leaders Fund award to
IHS.

The Dunlap Institute is funded through an endowment established by the David
Dunlap family and the University of Toronto.

Based on observations obtained at the Gemini Observatory, which is operated by
the Association of Universities for Research in Astronomy, Inc., under a
cooperative agreement with the NSF on behalf of the Gemini partnership: the
National Science Foundation (United States), the Science and Technology
Facilities Council (United Kingdom), the National Research Council (Canada),
CONICYT (Chile), the Australian Research Council (Australia), Ministério da
Ciência e Tecnologia (Brazil), and SECYT (Argentina). We appreciate the Gemini
team for granting us observing time for the program ID: GN-2022B-Q-308.

B.M.G. acknowledges the support of the Natural Sciences and Engineering
Research Council of Canada (NSERC) through grant RGPIN-2022-03163, and of the
Canada Research Chairs program. MRD acknowledges support from the NSERC
through grant RGPIN-2019-06186, the Canada Research Chairs Program, the
Canadian Institute for Advanced Research (CIFAR), and the Dunlap Institute at
the University of Toronto. V.M.K. holds the Lorne Trottier Chair in
Astrophysics & Cosmology, a Distinguished James McGill Professorship, and
receives support from an NSERC Discovery grant (RGPIN-228738-13), and from the
FRQNT CRAQ. K.W.M. holds the Adam J. Burgasser Chair in Astrophysics and is
supported by an NSF Grant (2008031). A.B.P. is a Banting Fellow, McGill Space
Institute (MSI) Fellow, and a Fonds de Recherche du Quebec – Nature et
Technologies (FRQNT) postdoctoral fellow. Z.P. and P. S. are Dunlap Fellows.
FRB Research at UBC is funded by an NSERC Discovery Grant and by the Canadian
Institute for Advanced Research. The CHIME/FRB baseband system is funded in
part by a CFI John R. Evans Leaders Fund award to IHS. M.B. is a Mcwilliams
fellow. F.A.D is supported by the UBC Four Year Fellowship. A.M.C is funded by
an NSERC Doctoral Postgraduate Scholarship. K.S. is supported by the NSF
Graduate Research Fellowship Program.

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##  Appendix A PATH Probabilities of all candidates

Here we present the Tables of probabilities of all the sources in the field of
FRB 20200223B, FRB 20190110C, and FRB 20191106C. R50 is the half-light radius
of the galaxy from the DESI survey, P0.0 is the probability that the object is
the preferred host galaxy of the FRB given the prior $\mathrm{P(U)}=0.0$ and
P0.1 is the probability that the object is the preferred host galaxy of the
FRB given the prior $\mathrm{P(U)}=0.1$.

Table 6: PATH probability for the sources in the field of FRB 20200223B  Source | RA | DEC | R50 | r-band | P0.0 | P0.1
---|---|---|---|---|---|---
| ° | ° | " | mag |  |
1 | 8.269527 | 28.831267 | 5.814174 | 16.080395 | 0.994104 | 0.899497
2 | 8.266982 | 28.829577 | 0.405767 | 22.06333 | 0.001767 | 0.001599
3 | 8.267366 | 28.82594 | 0.646236 | 21.993284 | 0.001427 | 0.001291
4 | 8.263246 | 28.828137 | 0.434656 | 23.971949 | 0.000322 | 0.000291
5 | 8.263912 | 28.828986 | 0.877458 | 23.089138 | 0.000708 | 0.00064
6 | 8.268353 | 28.826364 | 0.439845 | 22.411278 | 0.000992 | 0.000897
7 | 8.271681 | 28.835329 | 0.450015 | 22.615425 | 0.000682 | 0.000617
Table 7: PATH probability for the sources in the field of FRB 20190110C  Source | RA | DEC | R50 | r-band | P0.0 | P0.1
---|---|---|---|---|---|---
| ° | ° | " | mag |  |
1 | 249.318485 | 41.443412 | 3.379357 | 18.00912 | 0.917943 | 0.779452
2 | 249.324262 | 41.442808 | 0.662556 | 22.32065 | 0.013446 | 0.011418
3 | 249.336069 | 41.448172 | 0.39331 | 21.21785 | 0.025025 | 0.02125
4 | 249.315487 | 41.448435 | 0.337111 | 23.374989 | 0.003773 | 0.003204
5 | 249.317587 | 41.444105 | 2.032882 | 21.900526 | 0.016519 | 0.014027
6 | 249.319692 | 41.437475 | 1.434205 | 23.942047 | 0.002159 | 0.001833
7 | 249.319796 | 41.440578 | 0.571469 | 22.459723 | 0.009711 | 0.008246
8 | 249.32092 | 41.448989 | 0.321152 | 23.473305 | 0.004379 | 0.003719
9 | 249.325318 | 41.441917 | 0.459996 | 23.032875 | 0.007045 | 0.005982
Table 8: PATH probability for the sources in the field of FRB 20191106C  Source | RA | DEC | R50 | r-band | P0.0 | P0.1
---|---|---|---|---|---|---
| ° | ° | " | mag |  |
1 | 199.580129 | 42.999713 | 1.601871 | 17.306004 | 0.951259 | 0.815378
2 | 199.567778 | 43.005932 | 0.361793 | 23.50386 | 0.001473 | 0.001262
3 | 199.568633 | 43.007172 | 1.032621 | 21.765076 | 0.006658 | 0.005707
4 | 199.571617 | 42.998974 | 0.700418 | 22.764788 | 0.00331 | 0.002837
5 | 199.57512 | 42.99899 | 0.467756 | 22.17816 | 0.006046 | 0.005182
6 | 199.575607 | 43.007772 | 0.982723 | 21.274609 | 0.012805 | 0.010976
7 | 199.577994 | 43.000793 | 1.937793 | 23.018389 | 0.003076 | 0.002636
8 | 199.578285 | 42.994688 | 0.400479 | 23.901838 | 0.001067 | 0.000915
9 | 199.580623 | 43.009469 | 1.088066 | 22.449223 | 0.003727 | 0.003195
10 | 199.582184 | 42.998333 | 2.298028 | 21.562593 | 0.009444 | 0.008095
11 | 199.584588 | 43.000609 | 0.749572 | 23.984762 | 0.001136 | 0.000973