# New Radio-Loud QSOs at the end of the Re-ionisation Epoch

L. Ighina1,2,3, A. Caccianiga1, A. Moretti1, S. Belladitta1,2, J. W.
Broderick3, G. Drouart3, J. K. Leung4,5,6, N. Seymour3
1INAF, Osservatorio Astronomico di Brera, via Brera 28, 20121, Milano, Italy
2DiSAT, Università degli Studi dell’Insubria, via Valleggio 11, 22100 Como,
Italy
3 International Centre for Radio Astronomy Research, Curtin University, 1
Turner Avenue, Bentley, WA, 6102, Australia
4 Sydney Institute for Astronomy, School of Physics, University of Sydney, NSW
2006, Australia
5 CSIRO Space and Astronomy, PO Box 76, Epping, NSW, 1710, Australia
6 ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav),
Hawthorn, VIC 3122, Australia E-mail<EMAIL_ADDRESS>
(Accepted XXX. Received YYY; in original form ZZZ)

###### Abstract

We present the selection of high-redshift ($z\gtrsim 5.7$) radio-loud (RL)
quasi-stellar object (QSO) candidates from the combination of the radio Rapid
ASKAP Continuum Survey (RACS; at 888 MHz) and the optical/near-infrared Dark
Energy Survey (DES). In particular, we selected six candidates brighter than
$S_{\rm 888MHz}>1$ mJy beam-1 and ${\rm mag}(z_{\mathrm{{DES}}})<21.3$ using
the dropout technique (in the $i$-band). From this sample, we were able to
confirm the high-$z$ nature ($z\sim 6.1$) of two sources, which are now among
the highest-redshift RL QSOs currently known. Based on our Gemini-South/GMOS
observations, neither object shows a prominent Ly$\alpha$ emission line. This
suggests that both sources are likely to be weak emission-line QSOs hosting
radio jets and would therefore further strengthen the potential increase of
the fraction of weak emission-line QSOs recently found in the literature.
However, further multiwavelength observations are needed to constrain the
properties of these QSOs and of their relativistic jets. From the discovery of
these two sources, we estimated the space density of RL QSOs in the redshift
range $5.9<z<6.4$ to be 0.13${}^{+0.18}_{-0.09}$ and found it to be consistent
with the expectations based on our current knowledge of the blazar population
up to $z\sim 5$.

###### keywords:

galaxies: active - galaxies: nuclei – galaxies: high-redshift - (galaxies:)
quasars: general

††pubyear: 2015††pagerange: New Radio-Loud QSOs at the end of the Re-
ionisation Epoch–New Radio-Loud QSOs at the end of the Re-ionisation Epoch

##  1 Introduction

The recent advent of wide-area optical and near-infrared (NIR) surveys, such
as the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS;
Chambers et al., 2016) and the Dark Energy Survey (DES; DES Collaboration et
al., 2018), has led to the discovery of several hundreds of $z>6$ quasi-
stellar objects (QSOs) (e.g. Morganson et al., 2012; Bañados et al., 2014,
2016; Jiang et al., 2016; Reed et al., 2017, 2019; Wang et al., 2019, 2021;
Yang et al., 2020). The systematic study of these high-$z$ systems in the
optical and infrared (IR) has enabled the characterisation of their super
massive black hole (SMBH; Mazzucchelli et al. 2017; Shen et al. 2019; Farina
et al. 2022), their host galaxy (e.g. Venemans et al., 2020; Decarli et al.,
2022) and their environment (e.g. Balmaverde et al., 2017; Ota et al., 2018;
Chen et al., 2022). However, the properties of their relativistic jets are
still poorly constrained. This is mainly due to the fact that the number of
high-$z$ QSOs detected in the radio band is still small (see e.g. Liu et al.
2021) and, as a consequence, the number of high-redshift radio-loud (RL, i.e.
hosting powerful relativistic jets111Following the literature, we define a
source to be RL if its rest-frame radio loudness $R=S_{\rm 5GHz}$/${S_{\rm
4400\text{\AA}}}>10$. Even though it has been shown that this classification
might be too simplistic for the definition of QSOs with relativistic jets, it
should include all the most powerful radio jets (e.g., Padovani, 2017).) QSOs
currently known is even smaller. Indeed, only $\sim$10 QSOs are currently
known to be RL at $z>6$. While five of them have been discovered in different
wide-area radio surveys (J0309+2717 at $z=6.10$, Belladitta et al. 2020,
2022b; J1427+3312 at $z=6.12$, McGreer et al. 2006; J1429+5447 at $z=6.18$,
Willott et al. 2010a; J2318$-$3113 at $z=6.44$, Decarli et al. 2018; Ighina et
al. 2021; J172.3556+18.7734 at $z=6.82$, Bañados et al. 2021) the recent
release of the LOFAR Two-metre Sky Survey (LoTSS, at 144 MHz; Shimwell et al.,
2017, 2019, 2022), allowed the discovery of several new RL QSOs at $z>6$
(between 3 and 5, depending on the assumed radio spectral indices; Gloudemans
et al., 2021, 2022). Similarly, with the upcoming large-area radio surveys
planned with the square kilometre array
(SKA222https://www.skatelescope.org/wp-content/uploads/2011/03/SKA-
Astophysics-Vol1.pdf; Braun et al. 2019) and its precursors, which will reach
sensitivities of tens of µJy at $\sim$1 GHz (e.g. Norris et al., 2011, 2021),
many new RL active galactic nuclei (AGNs), both QSOs and radio galaxies, are
expected to be found at $z\gtrsim 6.5$. For example, Endsley et al. (2022a, b)
very recently discovered the most distant radio galaxy currently known (at
$z=6.85$) in a 1.5 deg2 region, which implies the existence of many more
similar systems over the entire sky. These new surveys will allow us to build
complete samples of RL AGNs, from which to derive conclusive statistical
properties on the entire population, well beyond the current redshift limit
($z\sim 5.5$, e.g. Caccianiga et al., 2019). Having a large sample of RL AGNs
at high-$z$ will allow for a series of important scientific results, such as
constraining the cosmic evolution of SMBHs hosted in systems with powerful
jets directly after their formation (e.g. Volonteri et al., 2011; Diana et
al., 2022) and studying the intergalactic medium (IGM) during the epoch of re-
ionisation trough neutral hydrogen (HI) absorption at low radio frequencies
(e.g. Carilli et al., 2002; Vrbanec et al., 2020).

In this work we present a sample of six $z\gtrsim 5.7$ RL QSO candidates built
from the the cross-match of the second data release of the DES in the
optical/NIR and the first data release of the low-band Rapid ASKAP Continuum
Survey (RACS; McConnell et al., 2020; Hale et al., 2021), the first large-area
radio survey performed with the Australian Square Kilometre Array Pathfinder
(ASKAP; Hotan et al. 2021). From this sample, we have already identified two
new RL QSOs at $z>6$, DES J032021.431$-$352104.11 ($z=6.13$; DES J0320$-$35
hereafter) and DES J032214.541$-$184118.15 ($z=6.09$; DES J0322$-$18
hereafter).

This work is structured as follows: in Sec. 2 we outline the optical/NIR and
radio criteria adopted for the selection of high-$z$ RL QSO candidates; in
Sec. 3 we present the spectroscopic confirmation of the two most promising
candidates and their archival optical/NIR and radio data; in Sec. 4 we discuss
the number of RL QSOs expected to be found at high redshift with future
surveys; in Sec. 5 we summarise the conclusions of the work.

Throughout the paper we assume a flat $\Lambda$CDM cosmology with $H_{0}$=70
km s-1 Mpc-1, $\Omega_{m}$=0.3 and $\Omega_{\Lambda}$=0.7. Spectral indices
are given assuming S${}_{\nu}\propto\nu^{-\alpha}$ and all errors are reported
at a 68% confidence level, unless otherwise specified.

##  2 Selection Criteria

In order to efficiently select high-$z$ RL QSOs, we started from the
combination of the second data release of the DES and the first data release
of the low-band RACS (simply RACS hereafter) survey and adopted the Lyman-
break dropout technique (see e.g. Bañados et al. 2016; Wang et al. 2017, 2019;
Belladitta et al. 2019, 2020, 2022a).

###  2.1 Optical and near/mid-infrared criteria

DES covers $\sim$5000 deg2 mostly in the southern extragalactic sky and it is
one of the deepest wide-area optical/NIR surveys currently available. The
limiting magnitudes at a signal-to-noise (S/N) level of 10 in its second data
release are: $g_{\mathrm{DES}}$=24.7, $r_{\mathrm{DES}}$=24.4,
$i_{\mathrm{DES}}$=23.8, $z_{\mathrm{DES}}$=23.1 and $Y_{\mathrm{DES}}$=21.7
mag (DES Collaboration et al., 2021). In order to select bona-fide high-$z$
QSO candidates, we accessed the products of DES through the dedicated SQL
portal333Available at: https://des.ncsa.illinois.edu/desaccess/home.. To be as
complete as possible, we set our criteria to recover almost all the
$5.7<z<6.4$ QSOs (both radio loud and radio quiet) with a magnitude in the
$z_{\rm DES}$-band $<21.3$ already discovered in the DES area (28 in total),
even if they were originally selected from another survey (i.e. with different
filter sets). We adopted the following criteria for the candidate selection:

  * •

mag_auto_z $<21.3$,

  * •

class_star_z $>0.85$,

  * •

magerr_auto_z $<0.3$ and magerr_auto_y $<0.3$,

  * •

mag_auto_i $-$ mag_auto_z $>1$,

  * •

non detection in the $g-$band444To assure a reliable non-detection in this
band we firstly considered only candidates with mag_auto_g $>23.5$ and
magerr_auto_g $>0.3$. Then, after having applied all the other selection
criteria, we checked the single $g$-band images and discarded all the sources
where a S/N $\gtrsim 3$ signal was present.,

where all the magnitudes are in the AB system. These criteria are meant to
select primarily compact stellar-like objects with a very faint emission in
the $g$-band and an $i-z$ dropout due to the Ly$\alpha$ absorption at high
redshift and not to an artefact in the images. The total number of sources
selected with these optical/NIR criteria (without the inspection of the
$g$-band images) and with a radio association (see next subsection) is 25.
After filtering the DES catalogue with the criteria reported above, we also
considered mid-IR data from the catalogue Wide-field Infrared Survey Explorer
(catWISE; Eisenhardt et al., 2020). In particular, by using a cross-match
radius of 2″, we only kept candidates with
$2.5<\texttt{mag\\_auto\\_z}-\texttt{mag\\_W2}<6$ (where the WISE magnitude is
in the Vega system). This criterion should help us avoid absorbed QSOs and a
subset of elliptical galaxies at low redshift (e.g. Carnall et al., 2015). In
Fig. 1 we show the $i-z$ vs $z-W2$ (left panel) and the $i-z$ vs $z-Y$ (right
panel) colours for the $z>5.7$ QSOs currently known in the DES area as well as
for the selected candidates. Based on the colour cuts applied above, we
recovered all but one of the already known QSOs at $z>5.9$ (1 out of 20
sources not included, due to the class_star_z criterion). Whereas, three
$5.7<z<5.9$ QSOs were missed due to their small $i-z$ colour value (3 out of
8, see Fig. 1). This is because at these redshifts, the Ly$\alpha$ emission
line ($\sim$8150–8400 Å in the observed frame) and its dropout shift to
wavelengths covered by the $i$-band filter ($\sim$7100–8500 Å). A systematic
selection focused on the specific $5<z<5.9$ redshift range will be presented
in a future work.

Figure 1: mag($i$) - mag($z$) as a function of the mag($z$) -mag($W2$) (AB and
Vega system respectively; left panel) and as a function of the mag($z$)
-mag($Y$) one (right panel) for the $z>5.7$ QSOs currently known in the DES
area (colour scale based on their redshift) together with the candidates
selected with the criteria described in the text (grey pentagons). The brown
squares indicate all the sources selected from the DES catalogue (i.e., not
including the catWISE data and before the inspection of the $g$-band images).
The blue line represents the expected colours of a typical QSO between
redshift $5-6.5$ and with a weak Ly$\alpha$ emission line (given by the
combination of the composite spectrum derived in Bañados et al. 2016, see
right panel of Fig. 3 for example, and the QSO template described in Polletta
et al. 2007). Similarly, the grey line represents the expected colours for a
low-redshift ($z=0-3.5$) elliptical galaxy (based on the template of Polletta
et al. 2007). The limits used for the selection of bona-fide high-$z$
candidates are shown with the horizontal and vertical black dashed lines.

###  2.2 Radio association

In order to only select radio-bright candidates, we cross-matched the
optically-selected sources with the first data release of RACS. This survey
covers the entire sky south of +41o in declination with a median RMS of
$\sim$0.25 mJy beam-1 and is also expected to perform more scans of the same
area in the future also at higher frequencies (RACS-mid at $\sim$1.36 GHz and
RACS-high at $\sim$1.65 GHz). We restricted our selection only to objects with
a peak flux density $S_{\mathrm{peak}}$>1 mJy beam-1 and with a
$S_{\mathrm{int}}$/$S_{\mathrm{peak}}$<1.5 in order to select unresolved
sources only, and therefore, reducing the position uncertainty of the radio
counterpart. This last criterion is satisfied by all the 13 $z>5$ RL QSOs
already known in RACS area. Given the typical positional errors of RACS (about
2–3″, see section 3.4.3 in McConnell et al. 2020), we adopted a search radius
of 3″ between the optical and the radio counterparts. We note that for this
first search, we made use of the source lists derived from the images
described in McConnell et al. (2020)555Available from the CASDA website:
https://data.csiro.au/domain/casdaObservation?redirected=true. since the final
catalogue of RACS-low survey built by Hale et al. (2021), where all the images
have been combined together after being convolved to a common resolution of
25″, would further increase the position uncertainty, especially for faint
sources. Nevertheless, in the following, to ensure a reliable flux density
measurement when presenting a radio flux density of a source from RACS, we
will consider the values reported in the Hale et al. (2021) catalogue.

We also cross-matched the optically selected candidates with the catalogue
derived from the ‘convolved’ images of the pilot evolutionary map of the
Universe (EMU) survey (Norris et al., 2021), which is at a similar frequency
(944 MHz), it has a similar angular resolution ($\sim$18″), but it is
significantly deeper (RMS $\sim 25-30$ µJy beam-1 over $\sim$270 deg2). By
adopting a radio-optical association of 1.5″ (which is enough to account for
both the accuracy and precision uncertainties of the sources in the catalogue,
see Norris et al. 2021) and the same optical/NIR and radio criteria reported
above, we select one more $>1$ mJy beam-1 candidate.

Finally, for sources at dec $>-40^{\rm o}$ and bright enough to be detected in
the VLA Sky Survey (VLASS; resolution $\sim 2.5^{\prime\prime}$ at 3 GHz; Lacy
et al. 2020), we also checked their radio images in this survey and only kept
the candidates if a $>$3$\times$RMS signal is present within 1.2″ from the
optical/NIR counterpart. In this way we discarded 3 sources out of the 11
covered by VLASS.

The total number of candidates selected with the optical/IR and radio criteria
described above is six and we report in Tab. 1 their main properties used for
the selection. In the following we focus on two of them, which have already
been confirmed spectroscopically, while the remaining candidates will be part
of a future publication.

NAME | RA | Dec | $S_{\rm 888MHz}$ | $z_{\mathrm{DES}}$ | $(i-z)_{\rm DES}$ | $z_{\mathrm{DES}}-W2$ | distancero | radio survey
---|---|---|---|---|---|---|---|---
| (deg) | (deg) | (mJy beam-1) | (mag AB) |  | (mag AB – Vega) | (arcsec) |
DES J0249$-$28 | 42.311270 |  $-$28.855045 | 5.06$\pm$0.27 | 20.83$\pm$0.03 | 1.23 | 3.09 | 2.01 | RACS
DES J0320$-$35 $\checkmark$ | 50.089335 |  $-$35.351148 | 3.21$\pm$0.28 | 20.59$\pm$0.03 | 3.11 | 3.28 | 1.32 | RACS
DES J0322$-$18 $\checkmark$ | 50.560622 |  $-$18.688189 | 1.64$\pm$0.19 | 20.94$\pm$0.04 | 3.11 | 3.44 | 2.59 | RACS
DES J0427$-$53 | 66.930894 |  $-$53.39384 | 3.61$\pm$0.33 | 20.04$\pm$0.01 | 1.08 | 3.20 | 1.47 | RACS
DES J0616$-$48 | 94.199469 |  $-$48.347094 | 2.13$\pm$0.42 | 21.24$\pm$0.03 | 1.39 | 3.37 | 2.76 | RACS
DES J2020$-$62 | 305.170194 |  $-$62.252554 | 1.09$\pm$0.06* | 19.23$\pm$0.01 | 1.00 | 4.67 | 0.80 | EMU
Table 1: Col. (1, 2, 3): name and optical coordinates of the source. The
check-mark beside the name indicate the sources that have already been
confirmed; Col. (4): peak flux densities (with the corresponding error) at 888
MHz from the RACS catalogue derived by Hale et al. (2021) (* indicates flux
densities at 944 MHz from EMU, whose uncertainty corresponds to the RMS of the
image); Col. (5): $z$-band magnitude from DES with the corresponding error;
Col. (6): dropout value ($i-z$); Col. (7): $z_{\mathrm{DES}}-W2$ colour, where
$z_{\mathrm{DES}}$ is in the AB system and $W2$ in the Vega system; Col. (8):
radio-to-optical distance; Col. (9): radio survey from which the candidate was
selected.

###  2.3 Completeness and contamination

With the optical/IR thresholds reported above, we recover about 95 per cent of
the already discovered $z>5.9$ QSOs, while the remaining 5 per cent (one
object) does not satisfy the point-like criterion. We note that this value
does not take into account the actual completeness of the literature sources
in the DES field. Several works showed that the high-z QSO selections based on
colour cuts (such as ours) can be considered almost complete for relatively
bright high-$z$ QSOs (magnitudes $<22$), see, for example, fig. 3 in Willott
et al. (2010b) and fig. 10 in Kim et al. (2022). Thanks to the radio
counterpart requirement, our selection criteria are less strict compared to
the usual cuts adopted in other works based on the optical/IR colours only,
both in terms of the dropout value and in the colours of the continuum (i.e.
the $z-Y$ colour; see e.g. Venemans et al. 2013; Bañados et al. 2016; Reed et
al. 2017, 2019). Therefore, in the following, we adopt the 95 per cent value
as the best estimate of the completeness of our selection, noting that only
significantly different values (completeness $<40$ per cent) would change our
results in sec. 4, given the limited statistics.

In order to estimate the completeness related to the radio criteria adopted
for RACS, we considered the ASKAP observations of the 23rd GAlaxy Mass
Assembly (G23; Driver et al. 2011) field described in Gürkan et al. (2022).
This field covers 83 deg2 in the southern hemisphere (centred at RA = 23h and
Dec = $-$32o) and was observed in the radio band with the ASKAP telescope as
part of the EMU Early Science programme. These observations are similar to
RACS both in terms of central frequency and resolution (888 MHz and $\sim$10″,
respectively), but they are about an order of magnitude deeper (RMS$\sim$38
µJy beam-1). From the catalogue derived by Gürkan et al. (2022), we considered
all the sources with Sint/S${}_{\mathrm{peak}}<1.5$, to ensure the selection
of point-like objects only, and with $1<$ S${}_{\mathrm{peak}}<30$ mJy beam-1,
that is, with radio flux densities similar to our selected high-$z$ QSO
candidates, and evaluated the fraction of these objects detected in RACS. In
the top panel of Fig. 2 we report the flux density distribution of all the
point-like $1<$ S${}_{\mathrm{peak}}<30$ mJy beam-1 G23 objects (empty
histogram) and the subset detected in the RACS catalogue (filled histogram).
In the lower panels we report the fraction of sources detected in RACS per bin
of flux density (central panel) and their cumulative completeness fraction
above a given flux density (bottom panel). Based on the detection fraction in
RACS, we expect our overall radio selection to be about 75 per cent complete
for S${}_{\mathrm{888MHz}}>1$ mJy beam-1. In particular, while for relatively
large radio flux densities (S${}_{\mathrm{888MHz}}>3$ mJy beam-1) the
detection fraction is almost one ($\sim$94 per cent) the detection fraction
drops to about 65 per cent for faint radio sources ($1<$
S${}_{\mathrm{888MHz}}<3$ mJy beam-1). These values are similar to the ones
derived by Hale et al. (2021) for the RACS catalogue after combining the
different images to a common resolution (see their fig. 15, left panel).
Based on these values, and the total number of candidates selected from RACS
(see Tab 1), in our selection we expect to be missing about two candidates
($\sim$2.04). Moreover, if we consider the overall area of the DES ($\sim$5000
deg2), we would expect to find $\sim$0.4 sources in the region covered by the
EMU pilot survey ($\sim$270 deg2), which is consistent with the selection of
one candidate in this area.

Figure 2: Fraction of point-like radio sources present in the G23 field with
$1<$ S${}_{\mathrm{888MHz}}<30$ mJy beam-1 detected in RACS. Top: flux density
distribution of all the $1<$ S${}_{\mathrm{888MHz}}<30$ mJy beam-1 point-like
sources present in the G23 field (empty histogram), together with the
distribution of the ones detected in RACS (filled histogram); Centre:
detection fraction in RACS per flux density bin; Bottom: cumulative fraction
of sources detected in RACS as a function of flux density.

Finally, in order to determine the number of spurious associations we can
expect from our selection, we cross-matched the DES catalogue with the RACS
and EMU ones by applying a rigid relative shift in their positions and then
applied the same selection criteria used for the selection of our candidates.
In this way, we found that the expected number of spurious associations is
$1.6\pm 0.5$.

##  3 Discovery of two new $z>6$ RL QSOs

While the spectroscopic observations of the majority of the sample are
currently ongoing, we confirmed the high-$z$ nature of two candidates: DES
J0320$-$35 and DES J0322$-$18\. These two sources were selected as the first
targets to be observed because they had the highest dropout in our sample
($i-z>3$, see Fig. 1) as well as a further radio association (<0.5″ with S/N
$>3$) in the quick-look images of VLASS at 3 GHz. In the left panels of Fig.
3, we report the optical (DES, orange cross) and radio positions (RACS, dashed
red circle corresponding to the radio positional uncertainty of each source)
as well as the VLASS contours overlaid on the optical images in the
$z_{\mathrm{DES}}$ filter of both QSOs.

Figure 3: Left: 30″$\times$30″cutout around DES J0320$-$35 (top) and DES
J0322$-$18 (bottom) in the $z_{\rm DES}$ filter. The optical and radio
positions are shown with an orange cross and a dashed red circle (uncertainty
reported in the RACS catalogue), respectively, while the VLASS radio (3 GHz)
contours at 3,4,5 $\times$ RMS are displayed in cyan. Right: Optical/NIR
spectra of DES J0320$-$35 (at $z=6.13\pm 0.05$; top) and DES J0318$-$18 (at
$z=6.09\pm 0.05$; bottom) obtained with the GMOS instrument on the Gemini-
South telescope at 3Å resolution. The dashed black lines are the spectra
smoothed to a 20Å resolution. The red and blue vertical lines indicate the
expected wavelength of the Ly$\alpha$ emission line. The solid magenta line is
the composite spectrum derived by Bañados et al. (2016) for $5.6<z<6.5$ QSOs
with a weak Ly$\alpha$ emission line (like our sources), also smoothed to a
20Å resolution.

###  3.1 Spectroscopic confirmation and optical/NIR properties

We confirmed both candidates to be high-$z$ QSOs with the Gemini-South
telescope, DES J0320$-$35 on 2021 August 09 and DES J0322$-$18 on 2021 October
14-15 (program ID: GS-2021A-DD-112; P.I. L. Ighina). The observations were
carried out with the Gemini Multi-Object Spectrograph (GMOS; Hook et al. 2004)
instrument in long-slit mode (1″ aperture) and with the R400 grating. The mean
airmass during the observations was 1.03 and 1.33, while the seeing was
$\sim$0.9″ and $\sim$0.7″ for DES J0320$-$35 and DES J0322$-$18, respectively.
Both targets were observed for a total of eight exposures, half with the
central wavelength of the grism at 9000 Å and half at 9100 Å, in order to
cover the spectral gap in the detector. For DES J0320$-$35, each segment
lasted 450 s (3600 s in total), while for DES J0322$-$18, each segment lasted
800 s (6400 s in total). We also observed the spectro-photometric standard
star LP 995$-$86 in order to correct the spectra of the two targets for the
response of the instrument.

The data were then reduced using the dedicated IRAF Gemini package and
following the instructions reported in the GMOS Data Reduction Cookbook
(Version 1.1; Tucson, AZ: National Optical Astronomy Observatory; Shaw, R. A.
2016)666Available at: http://ast.noao.edu/sites/default/files/GMOS_Cookbook..
The RMS reached during the wavelength calibrations was $\sim$0.18Å for both
observations. Finally, since the nights of the observations were not in
photometric conditions, in order to have an absolute flux calibration, the two
spectra were normalised to the $z$ and $Y$ photometric data available from
different surveys, by considering the magnitudes corrected by the Galaxy
absorption as reported in the corresponding catalogue.

In both cases, the presence of a high drop in the continuum spectrum at about
$\sim$8700Å confirms the high-$z$ nature of the candidates. However, due to
the lack of evident spectral lines in the observed range, we used the observed
wavelength of the drop to estimate the best-fit redshift for both sources. In
particular, we considered the composite spectrum derived by Bañados et al.
(2016) from a sample of 16 QSOs at $5.6<z<6.5$ with a weak Ly$\alpha$ emission
line (rest-frame equivalent width, REW, $<15.4$Å) and, after smoothing it to a
20Å resolution, we used it to perform a fit to the observed spectrum, also
smoothed to 20Å, in the wavelength range 8000–9500Å, where the only free
parameters were the redshift and the normalisation. The best-fit redshift
values we obtained are the following: $z=6.13\pm 0.05$ for DES J0320$-$35 and
$z=6.09\pm 0.05$ for DES J0320$-$18\. As error on these estimates we consider
$3\times 20\text{\AA}$ ($\sigma_{z}=\pm 0.05$), which roughly corresponds to
the width of the observed drops. Nevertheless, further observations with a
larger wavelength range are needed to accurately determine their redshift.

The reduced spectra together with the high-$z$ QSOs composite spectra are
reported in the right panels of Fig. 3. In Tab. 2 we list the magnitudes of
both QSOs (AB system777See http://casu.ast.cam.ac.uk/surveys-
projects/vista/technical/filter-set and
https://wise2.ipac.caltech.edu/docs/release/allsky/expsup/sec4_4h.html for the
conversion factors from Vega to AB system for the VHS/VIKING and WISE filters,
respectively.) in the optical and the IR available from the following surveys:
DES, Pan-STARRS, the VISTA Kilo-degree Infrared Galaxy Public Survey (VIKING;
Edge et al. 2013), the VISTA Hemisphere Survey (VHS; McMahon et al., 2013) and
the catWISE. In order to compute the optical spectral index of the two
sources, we considered the power-law that best describes the continuum
obtained from all the NIR photometric data points, that is, the filters $Y$,
$J$, $H$, $K_{s}$, also from different surveys when available. The resulting
spectral indices are as follows:
$\alpha_{\mathrm{o}}^{\mathrm{\lambda}}=1.72\pm 0.03$
($\alpha_{\mathrm{o}}^{\mathrm{\nu}}=0.28$; DES J0320$-$35) and
$\alpha_{\mathrm{o}}^{\mathrm{\lambda}}=1.60\pm 0.04$
($\alpha_{\mathrm{o}}^{\mathrm{\nu}}=0.40$; DES J0322$-$18), which are roughly
consistent with the typical values found in QSOs (e.g. Vanden Berk et al.,
2001).

Table 2: Optical and IR magnitudes (AB system) of DES J0320$-$35 and DES J0322$-$18 available from the DES, VIKING, VHS and catWISE surveys. For DES J0322$-$18 we report the magnitudes from PanSTARRS in brackets. Upper limits are given at a 5$\sigma$ significance level. We also report the slope of the continuum and the rest-frame luminosities at 2500 and 4400 Å. Filter | DES J0320$-$35 | DES J0322$-$18
---|---|---
$r_{\mathrm{{DES}}}$ ($r_{\mathrm{{PS}}}$) | >24.56 | >24.56 (>23.20)
$i_{\mathrm{{DES}}}$ ($i_{\mathrm{{PS}}}$) | 23.70$\pm$0.24 | >23.96 (>23.10)
$z_{\mathrm{{DES}}}$ ($z_{\mathrm{{PS}}}$) | 20.59$\pm$0.03 | 20.94$\pm$0.04 (21.77$\pm$0.07)
$Y_{\mathrm{{DES}}}$ ($Y_{\mathrm{{PS}}}$) | 20.36$\pm$0.08 | 20.86$\pm$0.11 (20.77$\pm$0.12)
$z_{\mathrm{{Vista}}}$ | 20.88$\pm$0.05 | –
$Y_{\mathrm{{Vista}}}$ | 20.33$\pm$0.07 | –
$J_{\mathrm{{Vista}}}$ | 20.24$\pm$0.08 | 20.55$\pm$0.12
$H_{\mathrm{{Vista}}}$ | 20.20$\pm$0.11 | –
$K_{\mathrm{{Vista}}}$ | 20.11$\pm$0.17 | 20.68$\pm$0.39
$W_{\mathrm{{1}}}$ | 20.01$\pm$0.06 | 20.28$\pm$0.08
$W_{\mathrm{{2}}}$ | 20.27$\pm$0.12 | 20.84$\pm$0.23
$\alpha_{\mathrm{o}}^{\mathrm{\lambda}}$ | 1.72$\pm$0.03 | 1.60$\pm$0.04
L${}_{\mathrm{2500\text{\AA}}}$ (erg s-1 Hz-1) | 1.80${}^{+0.21}_{-0.19}\,\times 10^{31}$ | 1.20${}^{+0.19}_{-0.16}\,\times 10^{31}$
L${}_{\mathrm{4400\text{\AA}}}$ (erg s-1 Hz-1) | 2.11${}^{+0.24}_{-0.22}\,\times 10^{31}$ | 1.52${}^{+0.24}_{-0.21}\,\times 10^{31}$

###  3.2 Archival Radio Data

To characterise the radio properties of these two newly discovered QSOs, we
searched for archival data from past and current radio surveys. As already
mentioned, both QSOs were first selected from RACS, where they were detected
with a high S/N (>11 for DES J0320$-$35, off-source RMS$\sim$0.28 mJy beam-1,
and >8 for DES J0322$-$18, off-source RMS$\sim$0.19 mJy beam-1, according to
the catalogue derived by Hale et al. 2021). Furthermore, their radio
association was also confirmed by the VLASS quick-look images, which have an
higher angular resolution ($\sim$2.5″). As the VLASS quick-look images might
be unreliable at low flux densities, we added a further 10 per cent to the
uncertainty of their flux densities in this work (for details, see Gordon et
al. 2021).

At the same time, we also checked the other publicly available radio surveys.
At low frequency, we did not find a counterpart in the TIFR Giant Metrewave
Radio Telescope Sky Survey (TGSS; Intema et al., 2017), at 148 MHz, nor in the
GaLactic and Extragalactic All-sky Murchison Widefield Array South Galactic
Pole (GLEAM-SGP; Franzen et al., 2021), at 216 MHz888Here we only consider the
observations in the wide-band 200–231 MHz image, which provide the strongest
constraints.. The RMS values between our two sources differ significantly due
to the presence of a very bright object near DES J0320$-$35\. Moreover, DES
J0322$-$18 is also detected in the NRAO VLA Sky Survey (NVSS; Condon et al.,
1998) at 1.4 GHz with a 4$\sigma$ significance (RMS$\sim$0.37 mJy beam-1, at a
distance <5″), even if not reported in the catalogue. Near DES J0320$-$35 a
radio signal is also present, but the corresponding emission peak is 25″ away.
Since this distance is larger than the pixel size of the NVSS images (15″) we
do not consider the corresponding flux density.

We summarise in Tab. 3 the flux density measurements of both sources in the
radio surveys described above. For both QSOs we consider the peak flux density
of each image since neither of them is resolved. In case of a non-detection,
we report the upper limit computed as 3$\times$local RMS. From both the
detections and non-detections, the power-law that best describe the two radio
spectra have spectral indices of $\alpha_{\mathrm{r}}=1.04\pm 0.07$ (DES
J0320$-$35) and $\alpha_{\mathrm{r}}=0.61\pm 0.11$ (DES J0322$-$18), obtained
with the MRMOOSE code (Drouart & Falkendal, 2018a, b). In Fig. 4 we show the
radio detections and upper limits of both QSOs together with the best-fit
power-law. These values are consistent with the ones typically observed in RL
AGNs (e.g. Bañados et al., 2021), even though they are relatively uncertain
given the few data available and the potential intrinsic variability. From the
combination of the LoTSS (144 MHz) and the Faint Images of the Radio Sky at
Twenty-Centimeters (FIRST, 1.4 GHz; Becker et al. 1995) surveys, Gloudemans et
al. (2021, 2022) found that $z>5$ QSOs have a median spectral index of
$\alpha_{\rm r}=0.3$. This difference can be attributed to two main factors:
the frequency range probed by the different works and the unprecedented
sensitivity reached by LoTSS. Indeed, while typical high-$z$ RL QSOs have
spectral indices of $\sim 0.7-1$ (e.g. Bañados et al., 2015; Spingola et al.,
2020), many of them also present a turnover in the radio spectrum at observed
frequencies of 0.5–2 GHz (e.g. Shao et al., 2022; Ighina et al., 2022b;
Belladitta et al., 2022a), which can be interpreted as a nearly flat spectrum
from a two-point spectral index with a measurement at very low frequencies.
Moreover, since the FIRST survey is shallower, compered to LoTSS, the sources
with a measurement in both surveys are likely to have a nearly flat or even
negative spectral index (see sec. 3 in Gloudemans et al. 2021).

Interestingly, the non-detection in the TGSS survey at 148 MHz might imply a
flattening of the radio spectrum or the presence of a turnover at lower
frequencies also for DES J0320$-$35\. While for DES J0322$-$18, given its
relatively flat spectrum, the upper limits at low frequencies are still
consistent with the single power law extrapolation. In both cases more radio
observations are needed to accurately constrain the spectrum and the nature of
the two RL QSOs.

Table 3: Radio data available from public surveys for DES J0320$-$35 and DES J0322$-$18\. Since the sources are not resolved, we report the peak flux density measured in each image. Upper limits are at a 3$\sigma$ significance level. We also report the best-fit spectral index, the luminosity density at 5 GHz rest frame and the radio-loudness parameter. |  | DES J0320$-$35 | DES J0322$-$18
---|---|---|---
Survey | Frequency | $S_{\nu}$ | RMS | $S_{\nu}$ | RMS
| (MHz) |  (mJy beam-1) |  (mJy beam-1)
TGSS | 148 | <12.2 | 4.1 | <6.3 | 2.1
GLEAM-SGP | 216 | <14.5 | 4.8 | <7.8 | 2.6
RACS | 888 | 3.21$\pm$0.28 | 0.28 | 1.64$\pm$0.19 | 0.19
NVSS | 1400 | – | 0.45 | 1.44$\pm$0.37 | 0.37
VLASS (1) | 3000 | 0.74$\pm$0.14 | 0.12 | 0.69$\pm$0.19 | 0.17
VLASS (2) | 3000 | 0.91$\pm$0.18 | 0.16 | 0.68$\pm$0.17 | 0.15
$\alpha_{\mathrm{r}}$ |  | 1.04$\pm$0.07 | 0.61$\pm$0.11
L5GHz (W Hz-1) |  | 2.42${}^{+0.25}_{-0.25}\times$1026 | 1.10${}^{+0.16}_{-0.15}\,\times$1026
$R$=$S_{\rm 5GHz}$/${S_{\rm 4400\text{\AA}}}$ |  | 115$\pm$18 | 73$\pm$15
Figure 4: Radio data available for DES J0320$-$35 (red squares) and DES
J0322$-$18 (blue circles). The dashed lines represent the single power law
that best fit the data of each source. The black crosses correspond to the
extrapolated flux density at the 5 GHz rest-frame frequency of each object.

###  3.3 Lack of spectral emission lines

Based on the optical/NIR spectra obtained from Gemini-GMOS, neither source
shows prominent emission lines in the observed range (namely Ly$\alpha$).
Given their RL nature, the lack of emission lines could be associated with a
BL Lacertae (BL Lac) nature (REW $<5\text{\AA}$; e.g. Padovani, 2017). If
confirmed, these would be the farthest BL Lac objects currently known, with
other three potential candidates at $z>6$ (see Koptelova & Hwang, 2022;
Gloudemans et al., 2022). In this case, we would expect to observe a nearly
flat radio spectrum; simultaneous observations in the radio band would be
needed to reliably constrain the spectral slopes of these sources. Another
possibility could be that these are weak emission-line QSOs (WLQSOs) that
happen to host two powerful relativistic jets (similarly to the $z=6.18$ RL
QSO J1429+5447, Shen et al. 2019, and the ones discussed in Gloudemans et al.
2022). As their name suggests, these are QSOs with very weak emission lines
(REW[CIV1549Å] $<15.4$Å; Diamond-Stanic et al. 2009). The physical conditions
of these sources are not fully clear. The two main scenarios proposed to
explain the lack of strong emission lines involve either a young accretion
system (e.g. Eilers et al., 2018; Andika et al., 2020, 2022), where the Broad
Line Region (BLR) has not fully formed yet (e.g. Meusinger & Balafkan, 2014),
or a softer ionising continuum, due to different accretion properties or
absorption between the disc and the BLR (e.g. Luo et al., 2015).

Interestingly, recent works did find an increase of the WLQSO fraction at high
redshift being $\sim$10-14% at $z\gtrsim 5.7$ (e.g. Bañados et al. 2016; Shen
et al. 2019), as opposed to $\sim$6% found by Diamond-Stanic et al. (2009)
considering $3<z<5$ QSOs. For the RL population the observed evolution is even
more pronounced, with $\sim$38% RL QSOs at $z>5$ being classified as WLQSOs
(see e.g. Gloudemans et al., 2022). Therefore, this redshift evolution can
indicate possible changes in the accretion mode of SMBHs and also in the RQ/RL
populations. However, current estimates from $z>5$ studies are still limited
by small number statistics. If the weak-line nature of both objects was
confirmed, it would further strengthen the presence of an increase of WLQSOs
as a function of redshift. However the spectroscopic identification of the
remaining candidates selected in this work is needed to have an estimate of
the weak-line fraction in our sample. Moreover, we stress once again that a
NIR spectrum covering a larger wavelength range and with a higher S/N is
needed in order to understand conditions of the accretion and of the BLR in
these two confirmed high-$z$ QSOs.

##  4 Comparison with expectations

Thanks to its wide area coverage and relatively deep radio/optical
sensitivity, the combination of RACS+DES is currently one of the most
effective tools for selecting high-$z$ RL QSOs. From our work, we can derive
an estimate of the space density of RL QSOs with $S_{\rm 888MHz}>1$ mJy beam-1
and ${\rm mag}(z_{\mathrm{{DES}}})<21.3$ at $5.9<z<6.4$, where our selection
is nearly complete ($\sim$71 per cent, considering both the optical/NIR and
radio completeness, see Sec. 2). Based on the discovery of DES J0320$-$35 and
DES J0322$-$18 over an area of 5000 deg2 (corresponding to a comoving volume
slice of 21.5 Gpc3 between $5.9<z<6.4$) the resulting space density is
0.13${}^{+0.18}_{-0.09}$ Gpc-3. As uncertainties we considered the Poissonian
error (lower) and the total number of candidates selected from RACS (5;
upper). However, a comparison with the expectations from lower redshift is not
straightforward. This is because the RL population is composed by a large
variety of sources, whose observed properties can be affected by relativistic
beaming at different degrees (depending, for example, on the orientation of
the relativistic jets) and therefore modify the observed shape of their
luminosity function (LF; e.g. Urry & Padovani 1995). For this reason, the
single classes of RL AGNs are often studied separately (e.g. Rigby et al.
2011; Diana et al. 2022). However, since it is not possible to exclude that
some degree of relativistic boosting is present in DES J0320$-$35 and DES
J0322$-$18 with the data currently available, we estimated the overall number
of RL QSOs expected to be found in a given survey using the following
approach.

We started from the radio LF derived by Mao et al. (2017) for the flat-
spectrum radio quasar (blazar hereafter) population, that is, sources with the
relativistic jet oriented close to our line of sight ($\theta_{\rm
view}<1/\Gamma$, where $\Gamma$ is the bulk Lorentz factor of the jet) and,
therefore, for which we expect the radio spectrum to be dominated by the
radiation relativistically boosted. The LF derived by Mao et al. (2017) was
built using blazars in the redshift range $z=0.5-3$ and then was confirmed up
to $z\sim 5$ by Caccianiga et al. (2019). From this LF we created a mock
sample of blazars at $z>5$ whose optical luminosities were computed based on
the optical-to-radio ratio distribution observed in the sample discussed by
Caccianiga et al. (2019), which corresponds to the highest-redshift complete
sample of blazars currently available. In this way we built a sample of
blazars for which the redshift, the radio flux density and the optical/NIR
magnitude are known (we assumed $\alpha_{\rm r}=0$, Padovani 2017, and
$\alpha_{\rm o}=0.44$, Vanden Berk et al. 2001). By considering different
radio/optical limits, we were then able to determine the corresponding number
of detectable blazars for a given radio/optical survey combination. Finally,
in order to estimate the overall number of RL QSOs starting from blazars, we
computed the number of RL QSOs with a misaligned jets whose radio flux density
is relativistically de-beamed999The (de-)amplification factor scales as
$\propto\delta^{p}$, where the Doppler factor, $\delta$, is given by
$[\Gamma(1-\beta{\rm cos}\theta_{\rm view})]^{-1}$ and $p=2+\alpha_{\rm r}$
(e.g. Cohen et al., 2007)., but still detectable in the given radio survey,
following the approach outlined in section 2 of Ghisellini & Sbarrato 2016. As
a reference, this last step resulted in an increase of a factor $\sim$10 in
the number of total RL QSOs (with respect to blazars only) for $S_{\rm
888MHz}>1$ mJy beam-1 and $5.9<z<6.4$.

In Fig. 5, we show the expected number of RL QSOs per bin of redshifts to be
detectable in different combinations of radio+optical/NIR surveys. In
particular, in the radio band we considered RACS (limit 1 mJy beam-1; Hale et
al. 2021) and the EMU survey (limit 0.1 mJy beam-1; Norris et al. 2021), while
in the optical band we considered DES (limit mag($z$-band) = 21.3, as in this
work), the Vera C. Rubin Legacy Survey of Space and Time (VRO/LSST; limit
mag($z$-band) = 23 after the first year; Ivezić et al. 2019) and the Euclid
wide survey (limit mag(NIR) = 24; Euclid Collaboration et al. 2022). Moreover,
in the case of DES and VRO/LSST, we only show the expected number of sources
up to $z=6.9$, since the wavelength coverage of these surveys will not allow
for the detection of objects at higher redshift.

Based on the extrapolations from lower redshift, the number of expected RL
QSOs with S${}_{\rm 888MHz}>1$ mJy beam-1 and ${\rm
mag}(z_{\mathrm{{DES}}})<21.3$ at $5.9<z<6.4$ is about three. This is broadly
consistent with the discovery of two new RL QSOs at $\sim 6.1$. Indeed, by
taking the completeness of our selection into account ($\sim 71$ per cent ,
see Sec. 2), the expected number of sources is $2.8_{-1.8}^{+3.7}$ (black
square in Fig. 5), where we computed the upper error by considering the total
number of high-$z$ candidates found in this work from RACS.

Based on Fig. 4, if we consider instead the upcoming EMU and VRO/LSST surveys,
we expect to increase the number of RL QSOs up to $z\sim 6.5$ by more than an
order of magnitude. This is due both to the deeper sensitivities, in the radio
and in the optical/NIR bands, as well as to the much larger common area (the
entire southern sky; Ivezić et al. 2019; Norris et al. 2013). However, the
VRO/LSST, similarly to DES, will only be sensitive to sources up to $z\sim 7$,
after which we expect all the optical emission of QSOs to be redshifted at
wavelengths outside the $grizY$ filters. At the same time, other complementary
surveys such as the Euclid wide survey, with filters also in the NIR ($YJH$),
will reach a similar magnitude limit at higher wavelengths (e.g. Euclid
Collaboration et al. 2019) and therefore, together with the EMU survey, they
will be able to explore the RL QSO population up to $z\sim 8.5$.

Figure 5: Expected number of $z>5$ RL QSOs per redshift bin in different
combinations of current/future radio+optical/NIR surveys rescaled to an area
of 5000 deg2. The sensitivities adopted for the different surveys are the
following: DES mag($z$-band) = 21.3, VRO/LSST 1yr mag($z$-band) = 23,
Euclid–WIDE mag(NIR)=24,

RACS $S_{\rm 888MHz}$ = 1 mJy beam-1 and EMU $S_{\rm 944MHz}$ = 0.1 mJy
beam-1. In the redshift bin $5.9<z<6.4$ we also show the constraints derived
from the discovery of the two new RL QSOs from the RACS+DES combination
discussed in the work. The estimate has been corrected for the completeness of
the selection described in Sec. 2, while the upper error is given by the
completeness-corrected number of candidates we selected from RACS.

##  5 Summary and Conclusions

We have presented the selection of a sample of high-$z$ RL QSOs in the
southern hemisphere based on their optical/NIR colours as reported in DES and
their detection in the RACS radio survey. We set the selection criteria such
that we were able to recover almost all previously discovered QSOs in the same
area above our optical/radio flux limits. The completeness and the efficiency
of our selection, mainly driven by the radio catalogue for the association,
will significantly increase with the upcoming RACS data releases.

From the sources selected in this work, we were already able to identify two
new high-$z$ RL QSOs: DES J0320$-$35 at $z=6.13\pm 0.05$ and DES J0322$-$18 at
$z=6.09\pm 0.05$. These are now two of the most distant RL QSOs currently
known. Indeed only a few $z>6$ RL QSOs have been detected in the radio band
(e.g. Liu et al., 2021; Gloudemans et al., 2021) and even fewer are confirmed
as radio loud (e.g. Bañados et al., 2021; Gloudemans et al., 2022).
Interestingly, both sources do not present prominent broad emission lines,
suggesting that they are both WLQSOs. If confirmed by NIR further
spectroscopy, this would further strengthen the increase of the WLQSOs
fraction at high redshift recently hinted both in the RQ (e.g. Bañados et al.,
2016; Shen et al., 2019) and the RL (e.g. Gloudemans et al., 2022) QSO
populations and therefore imply an evolution of SMBH accretion
conditions/modes at high redshift.

By comparing the number of selected and confirmed candidates to the expected
number of RL QSOs at $5.9<z<6.4$ with the same flux density cuts ($S_{\rm
888MHz}>1$ mJy beam-1 and ${\rm mag}(z_{\mathrm{{DES}}})<21.3$), we found that
the discovery of DES J0320$-$35 and DES J0322$-$18 is consistent with
expectations. Moreover, we also showed how the upcoming wide-area surveys
(e.g. VRO/LSST, Euclid wide and EMU) can increase the number of RL QSOs at
$z>5$ by a factor $>10$ and potentially reach sources up to $z\sim 8.5$.
Combined with samples of high-$z$ radio galaxies selected with different
techniques (e.g. Saxena et al., 2018a, b, 2019; Drouart et al., 2020;
Broderick et al., 2022), these sources can be used to address important
questions of modern astrophysics: on the formation and growth of SMBHs (e.g.
Overzier, 2022), the evolution of relativistic jets properties (e.g. Ighina et
al., 2022a), their impact on the environment (e.g. Hardcastle & Croston, 2020)
and the properties of the IGM in the early Universe (e.g. Carilli et al.,
2004; Furlanetto et al., 2006).

## Acknowledgements

We thank the referee for their suggestions that have improved the quality of
the paper. This work is based on observations obtained at the international
Gemini Observatory, a program of NSF’s NOIRLab, which is managed by the
Association of Universities for Research in Astronomy (AURA) under a
cooperative agreement with the National Science Foundation. on behalf of the
Gemini Observatory partnership: the National Science Foundation (United
States), National Research Council (Canada), Agencia Nacional de Investigación
y Desarrollo (Chile), Ministerio de Ciencia, Tecnología e Innovación
(Argentina), Ministério da Ciência, Tecnologia, Inovações e Comunicações
(Brazil), and Korea Astronomy and Space Science Institute (Republic of Korea).
In this work we made use of the Gemini IRAF package. IRAF is distributed by
the National Optical Astronomy Observatory, which is operated by the
Association of Universities for Research in Astronomy (AURA) under a
cooperative agreement with the National Science Foundation.
We acknowledge financial contribution from the agreement ASI-INAF n.
I/037/12/0 and n.2017-14-H.0 and from INAF under PRIN SKA/CTA FORECaST. We
acknowledge financial support from INAF under the project ‘QSO jets in the
early Universe’, Ricerca Fondamentale 2022.
This research made use of Astropy (http://www.astropy.org) a community-
developed core Python package for Astronomy (Astropy Collaboration et al.,
2018). This research has made use of the VizieR catalogue access tool, CDS,
Strasbourg, France (DOI : 10.26093/cds/vizier). The original description of
the VizieR service was published in 2000, A&AS 143, 23.

## Data Availability

All of the data used in this work are publicly available as described in the
text. The DES catalogue can be retrieved from the corresponding SQL portal
(https://des.ncsa.illinois.edu/desaccess/home), while the RACS source lists
used for the radio association can be found on the CASDA portal under the
AS110 project code
(https://data.csiro.au/domain/casdaObservation?redirected=true). Reprocessed
data are also available upon reasonable request to the corresponding author.

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