# Discovery of optically emitting circumgalactic nebulae around the majority
of UV-luminous quasars at intermediate redshift

Sean D. Johnson Department of Astronomy, University of Michigan, 1085 S.
University, Ann Arbor, MI 48109, USA Sean D. Johnson<EMAIL_ADDRESS>Zhuoqi
(Will) Liu Department of Astronomy, University of Michigan, 1085 S.
University, Ann Arbor, MI 48109, USA  Jennifer I-Hsiu Li Department of
Astronomy, University of Michigan, 1085 S. University, Ann Arbor, MI 48109,
USA  Michigan Institute for Data Science, University of Michigan, Ann Arbor,
MI 48109, USA  Joop Schaye Leiden Observatory, Leiden University, PO Box 9513,
NL-2300 RA Leiden, The Netherlands  Jenny E. Greene Department of
Astrophysical Sciences, Princeton University, 4 Ivy Lane, Princeton, NJ 08544,
USA  Sebastiano Cantalupo Department of Physics, University of Milan Bicocca,
Piazza della Scienza 3, I-20126 Milano, Italy  Gwen C. Rudie The Observatories
of the Carnegie Institution for Science, 813 Santa Barbara Street, Pasadena,
CA 91101, USA  Zhijie Qu Department of Astronomy & Astrophysics, The
University of Chicago, Chicago, IL 60637, USA  Hsiao-Wen Chen Department of
Astronomy & Astrophysics, The University of Chicago, Chicago, IL 60637, USA
Marc Rafelski Space Telescope Science Institute, 3700 San Martin Drive,
Baltimore, MD 21218, USA  Department of Physics and Astronomy, Johns Hopkins
University, Baltimore, MD 21218, USA  Sowgat Muzahid Inter-University Centre
for Astronomy and Astrophysics (IUCAA), Post Bag 4, Ganeshkhind, Pune 411007,
India  Mandy C. Chen Department of Astronomy & Astrophysics, The University of
Chicago, Chicago, IL 60637, USA  Thierry Contini  Institut de Recherche en
Astrophysique et Planétologie (IRAP), Université de Toulouse, CNRS, UPS,
F–31400 Toulouse, France  Wolfram Kollatschny Institut für Astrophysik und
Geophysik, Universität Göttingen, Friedrich-Hund Platz 1, D-37077 Göttingen,
Germany  Nishant Mishra Department of Astronomy, University of Michigan, 1085
S. University, Ann Arbor, MI 48109, USA  Patrick Petitjean  Institut
d’Astrophysique de Paris 98bis Boulevard Arago, 75014, Paris, France  Michael
Rauch The Observatories of the Carnegie Institution for Science, 813 Santa
Barbara Street, Pasadena, CA 91101, USA  Fakhri S. Zahedy The Observatories of
the Carnegie Institution for Science, 813 Santa Barbara Street, Pasadena, CA
91101, USA

(Received ?; Revised ?; Accepted ?)

###### Abstract

We report the discovery of large ionized, [O II] emitting circumgalactic
nebulae around the majority of thirty UV luminous quasars at $z=0.4-1.4$
observed with deep, wide-field integral field spectroscopy (IFS) with the
Multi-Unit Spectroscopy Explorer (MUSE) by the Cosmic Ultraviolet Baryon
Survey (CUBS) and MUSE Quasar Blind Emitters Survey (MUSEQuBES). Among the 30
quasars, seven (23%) exhibit [O II] emitting nebulae with major axis sizes
greater than 100 kpc, twenty greater than 50 kpc (67%), and 27 (90%) greater
than 20 kpc. Such large, optically emitting nebulae indicate that cool, dense,
and metal-enriched circumgalactic gas is common in the halos of luminous
quasars at intermediate redshift. Several of the largest nebulae exhibit
morphologies that suggest interaction-related origins. We detect no
correlation between the sizes and cosmological dimming corrected surface
brightnesses of the nebulae and quasar redshift, luminosity, black hole mass,
or radio-loudness, but find a tentative correlation between the nebulae and
rest-frame [O II] equivalent width in the quasar spectra. This potential trend
suggests a relationship between ISM content and gas reservoirs on CGM scales.
The [O II]-emitting nebulae around the $z\approx 1$ quasars are smaller and
less common than Ly$\alpha$ nebulae around $z\approx 3$ quasars. These smaller
sizes can be explained if the outer regions of the Ly$\alpha$ halos arise from
scattering in more neutral gas, by evolution in the cool CGM content of quasar
host halos, by lower-than-expected metallicities on $\gtrsim 50$ kpc scales
around $z\approx 1$ quasars, or by changes in quasar episodic lifetimes
between $z=3$ and $1$.

††journal: ApJ

##  1 Introduction

Surveys of the star formation rates and available interstellar gas reservoirs
(Kennicutt & Evans, 2012; Tacconi et al., 2013) as well as the metallicity
distribution of stars (Hayden et al., 2015; Greener et al., 2021) in the Milky
Way and distant, massive galaxies are inconsistent with simple “closed-box”
models (Tinsley, 1974) of galaxy evolution. These discrepancies can be
resolved if galaxies accrete fresh material from external reservoirs as they
evolve (for a review, see Putman, 2017). At the same time, outflows driven by
supernovae (e.g. Dekel & Silk, 1986) and Active Galactic Nuclei (e.g. Silk &
Rees, 1998) are required in state-of-the-art cosmological simulations to
reproduce the observed stellar masses of galaxies (e.g. Vogelsberger et al.,
2014; Schaye et al., 2015). These same feedback mechanisms eject heavy
elements produced by stars and supernovae out of the interstellar medium and
into intergalactic space to reproduce the mass-metallicity relation (e.g.
Tremonti et al., 2004; Ma et al., 2016). Consequently, improving our empirical
understanding of the rich set of physical processes that regulate gas exchange
between galaxies and their surrounding cosmic ecosystems (for a reviews, see
Donahue & Voit, 2022; Faucher-Giguere & Oh, 2023) represents a key priority of
extragalactic astrophysics (National Academies of Sciences, 2021).

Direct observations of the intergalactic and circumgalactic medium (IGM/CGM)
are critical to developing a more complete understanding of galaxy evolution.
However, the diffuse nature and cool-warm temperature range ($T\approx
10^{4}-10^{6}$ K) of the IGM/CGM around typical galaxies make emission
observations difficult with current facilities except in the very local
Universe (e.g., Chynoweth et al., 2008; Lokhorst et al., 2022) and rare cases
with unusually high surface brightness. Consequently, emission observations of
the IGM/CGM often rely on deep, coadded observations averaging over hundreds
to thousands of galaxies (e.g. Wisotzki et al., 2016; Dutta et al., 2023a; Guo
et al., 2023a, b).

Much of our knowledge of the physical nature and extent of the CGM relies on
sensitive absorption spectroscopy of UV bright background sightlines that pass
through the halos of foreground galaxies (for a review, see Tumlinson et al.,
2017). These observations enable constraints on the densities, temperatures,
and metallicities of the CGM around a wide variety of galaxies and across
cosmic time (e.g., Zahedy et al., 2019; Rudie et al., 2019; Cooper et al.,
2021; Zahedy et al., 2021; Qu et al., 2022). However, the lack of
morphological information in absorption-line studies of the IGM/CGM
necessitates non-trivial model assumptions when attributing the gas to
inflows, outflows, or other origins (see Gauthier & Chen, 2012; Ho et al.,
2017; Schroetter et al., 2019; Zabl et al., 2019) except in rare cases where
multiple sightlines enable velocity shear measurements (e.g., Chen et al.,
2014; Zahedy et al., 2016; Lopez et al., 2018, 2020).

While direct emission studies of the CGM and IGM are not generally feasible
for individual galaxies beyond the local Universe, IGM/CGM emission can be
detected in some rare or extreme cases. In particular, systems like quasars
that produce a significantly elevated ionizing radiation background increase
the equilibrium recombination rate and temperature of the surrounding IGM/CGM,
resulting in increased emission in recombination and collisionally excited
lines (e.g., Chelouche et al., 2008). Consequently, observers have invested
time on large ground-based telescopes toward identifying and following up
systems where the IGM/CGM is expected to be observable in emission with state-
of-the-art wide-field Integral Field Spectrographs (IFS; Bacon et al., 2010;
Martin et al., 2010; Mateo et al., 2022). Despite cosmological surface
brightness dimming, the first successful surveys of individual CGM emission
were conducted at $z=2-4$ in H I Ly$\alpha$ emission due to the inherent
strength of the line and its redshifting into the optical window. These
surveys demonstrated that giant, $\approx 100$ kpc CGM nebulae are nearly
ubiquitous around luminous quasars at $z=2-4$ (Borisova et al., 2016; Cai et
al., 2019; O’Sullivan et al., 2020; Fossati et al., 2021; Mackenzie et al.,
2021), and some extend to hundreds of kpc in projection (e.g. Cantalupo et
al., 2014; Cai et al., 2018).

Recent IFS-enabled discoveries at lower redshifts of $z<1.5$ revealed the
potential for studies of $\approx 30-100$ kpc-scale CGM nebulae observed in
emission in non-resonant, rest-frame optical lines (e.g., [O II], H$\beta$,
and [O III]) and resonant NUV lines (e.g., Mg II) around quasars (Johnson et
al., 2018; Helton et al., 2021; Johnson et al., 2022; Dutta et al., 2023a;
Epinat et al., 2023; Liu et al., 2024), galaxy groups (Epinat et al., 2018;
Chen et al., 2019; Leclercq et al., 2022; Dutta et al., 2023a; Epinat et al.,
2023), and galaxies with evidence of recent bursts of star formation (Rupke et
al., 2019; Burchett et al., 2021; Zabl et al., 2021). The joint morphological
and kinematic analysis of the nebulae combined with deep galaxy surveys
enabled by IFS provide direct insights into the origins of the gas, including
interactions, accretion, and outflows. Furthermore, statistical analysis of
velocity-structure functions around the largest of these nebulae provide new
insights into halo-scale turbulence (Chen et al., 2023a, b). However, while
such giant, relatively high surface brightness optically emitting nebulae have
led to significant insights in individual case studies, their incidence rate,
and properties around quasars have yet to be quantified in statistical
samples.

Here, we report the first IFS survey constraining the incidence of optically
emitting circumgalactic nebulae around 30 UV-luminous, unobscured quasars at
$z=0.4-1.4$ using deep observations from the Multi Unit Spectroscopic Explorer
(MUSE; Bacon et al., 2010) collected by the MUSE Quasar Blind Emitters Survey
(MUSEQuBES; e.g., Dutta et al., 2023b) and the Cosmic Ultraviolet Baryon
Surveys (CUBS; Chen et al., 2020). The paper proceeds as follows: In section
2, we describe the observations and data reduction steps. In section 3, we
characterize the properties of the MUSEQuBES and CUBS quasars. In section 4,
we present the discovery of large nebulae in the quasar fields. In section 5,
we discuss the implication of our findings, search for correlations between
the nebulae and quasar properties, and compare with circum-quasar nebulae at
higher redshift. Finally, we summarize our results and make concluding remarks
in Section 6.

Throughout, we adopt a flat $\Lambda$ cosmology with $\Omega_{\rm m}=0.3$,
$\Omega_{\Lambda}=0.7$, and a Hubble constant of $H_{0}=70$ ${\rm
km\,s}^{-1}$. Unless otherwise stated, all magnitudes are given in the AB
system (Oke & Gunn, 1983), all wavelengths are given in vacuum, and all
distances are proper.

##  2 Observations, data reduction, and processing

Both the CUBS and MUSEQuBES surveys were designed to study the CGM and IGM
around galaxies at $z\approx 0.1-1.4$ using deep and highly complete galaxy
redshift surveys with MUSE in the fields of UV-bright background quasars with
high-quality UV absorption spectra from the Cosmic Origins Spectrograph (Green
et al., 2012) aboard the Hubble Space Telescope. The broad spectral coverage
($4700-9350$ Å), wide field-of-view ($1^{\prime}\times 1^{\prime}$), and high
throughput (peaking at 35% at $7000$ Å) of MUSE Bacon et al. (2010)
collectively enable searches for low surface brightness nebulae in the
environments of the CUBS and MUSEQuBES background quasars at $z=0.4-1.4$
(Johnson et al., 2018, 2022; Chen et al., 2023a; Liu et al., 2024) at the same
time. The MUSEQuBES MUSE observations (PI: J. Schaye) were conducted in the
MUSE wide-field mode under natural seeing conditions characterized by a full
width at half maximum of ${\rm FWHM}=0.5-1.0^{\prime\prime}$ with two to ten
hours of integration. The CUBS MUSE observations (PI: H.-W. Chen) have
integration times ranging from 1.4 to 3.4 hours and were conducted in wide-
field mode with ground-layer adaptive optics (Kolb et al., 2016; Madec et al.,
2018). As a result, the CUBS datacubes exhibit improved image quality of ${\rm
FWHM}=0.5-0.7^{\prime\prime}$.

Table 1: Summary of quasar properties and MUSE observations, sorted by quasar redshift. |  | quasar properties | MUSE data
---|---|---|---
| R.A.a | Decl.b |  |  | $\log\lambda L_{\rm\lambda}^{e}$ | $\log L_{\rm bol}^{f}$ | $\log M_{\rm BH}^{g}$ |  | $W_{\rm r}([{\rm O\,II]})^{i}$ | $t_{\rm exp}^{j}$ | seeingk
quasar name | (deg) | (deg) |  $z$c | lined | $/{\rm erg\,s^{-1}}$ | $/{\rm erg\,s^{-1}}$ | $/{\rm M}_{\odot}$ | $\log RL^{h}$ | (Å) | (hr) | (arcsec)
HE 0435$-$5304 | $69.2118$ | $-52.9800$ | $0.4279$ | [O II] | $44.9$ | $45.8$ | $8.4$ | $<1.0$ | $-0.78\pm 0.04$ | $2.0$ | $1.0$
HE 0153$-$4520 | $28.8050$ | $-45.1032$ | $0.4532$ | H$\beta$ | $45.5$ | $46.5$ | $8.9$ | $<0.9$ | $-0.01\pm 0.01$ | $2.0$ | $0.7$
HE 0226$-$4110 | $37.0635$ | $-40.9541$ | $0.4936$ | [O II] | $46.0$ | $47.0$ | $9.0$ | $<0.1$ | $-0.17\pm 0.01$ | $8.0$ | $0.5$
PKS 0405$-$123 | $61.9518$ | $-12.1935$ | $0.5731$ | [O II] | $46.4$ | $47.3$ | $9.4$ | $3.1$ | $-0.36\pm 0.01$ | $9.8$ | $1.0$
HE 0238$-$1904 | $40.1357$ | $-18.8642$ | $0.6282$ | [O II] | $46.2$ | $47.2$ | $9.8$ | $<0.3$ | $-0.34\pm 0.01$ | $10.0$ | $0.7$
3C 57 | $30.4882$ | $-11.5426$ | $0.6718$ | [O II] | $45.8$ | $46.8$ | $8.9$ | $3.7$ | $-0.55\pm 0.02$ | $2.0$ | $0.7$
PKS 0552$-$640 | $88.1020$ | $-64.0363$ | $0.6824$ | [O II] | $46.4$ | $47.4$ | $9.7$ | $2.3$ | $-0.32\pm 0.02$ | $2.0$ | $0.7$
J0110$-$1648 | $17.6479$ | $-16.8077$ | $0.7822$ | [O II] | $46.2$ | $46.9$ | $9.1$ | $2.4$ | $-0.23\pm 0.02$ | $1.4$ | $0.6$
J0454$-$6116 | $73.5664$ | $-61.2740$ | $0.7864$ | [O II] | $46.2$ | $46.9$ | $9.4$ | $<0.9$ | $-0.84\pm 0.02$ | $1.4$ | $0.7$
J2135$-$5316 | $323.9716$ | $-53.2821$ | $0.8123$ | [O II] | $46.6$ | $47.3$ | $9.4$ | $<0.6$ | $-0.22\pm 0.01$ | $1.9$ | $0.5$
J0119$-$2010 | $19.9837$ | $-20.1729$ | $0.8160$ | [O II] | $46.4$ | $47.1$ | $9.3$ | $<0.9$ | $-0.27\pm 0.01$ | $1.4$ | $0.5$
HE 0246$-$4101 | $42.0261$ | $-40.8093$ | $0.8840$ | [O II] | $46.7$ | $47.4$ | $9.7$ | $<0.7$ | $-0.22\pm 0.03$ | $2.1$ | $0.7$
J0028$-$3305 | $7.1266$ | $-33.0970$ | $0.8915$ | [O II] | $46.3$ | $47.0$ | $9.4$ | $<1.0$ | $-0.13\pm 0.01$ | $1.4$ | $0.5$
HE 0419$-$5657 | $65.2246$ | $-56.8455$ | $0.9481$ | [O II] | $46.1$ | $46.8$ | $9.2$ | $<1.2$ | $-0.27\pm 0.05$ | $1.4$ | $0.6$
Q0107$-$025 | $17.5677$ | $-2.3141$ | $0.9545$ | [O II] | $46.2$ | $46.9$ | $9.4$ | $<1.1$ | $-0.67\pm 0.10$ | $2.0$ | $0.9$
Q0107$-$0235 | $17.5548$ | $-2.3313$ | $0.9574$ | [O II] | $46.1$ | $46.8$ | $9.6$ | $3.2$ | $-1.36\pm 0.05$ | $2.0$ | $0.8$
PKS 2242$-$498 | $341.2508$ | $-49.5301$ | $1.0011$ | [O II] | $46.3$ | $47.0$ | $9.6$ | $3.4$ | $-0.83\pm 0.03$ | $1.4$ | $0.6$
PKS 0355$-$483 | $59.3413$ | $-48.2042$ | $1.0128$ | [O II] | $46.3$ | $47.0$ | $9.6$ | $2.9$ | $-0.84\pm 0.02$ | $2.6$ | $0.5$
HE 0112$-$4145 | $18.5921$ | $-41.4964$ | $1.0238$ | [O II] | $46.2$ | $46.9$ | $9.5$ | $1.2$ | $-2.04\pm 0.05$ | $1.4$ | $0.7$
HE 0439$-$5254 | $70.0502$ | $-52.8048$ | $1.0530$ | Mg II | $46.3$ | $47.0$ | $9.3$ | $<1.1$ | $-0.03\pm 0.04$ | $2.5$ | $0.7$
HE 2305$-$5315 | $347.1574$ | $-52.9802$ | $1.0733$ | H$\beta$ | $46.5$ | $47.2$ | $9.5$ | $<1.0$ | $-0.03\pm 0.02$ | $1.9$ | $0.6$
HE 1003$+$0149 | $151.3968$ | $+1.5793$ | $1.0807$ | [O II] | $46.3$ | $47.0$ | $9.3$ | $1.5$ | $-0.62\pm 0.03$ | $2.0$ | $0.9$
HE 0331$-$4112 | $53.2794$ | $-41.0336$ | $1.1153$ | [O III] | $46.8$ | $47.5$ | $9.9$ | $<0.8$ | $-0.11\pm 0.03$ | $2.6$ | $0.6$
TXS 0206$-$048 | $32.3781$ | $-4.6406$ | $1.1317$ | [O II] | $46.4$ | $47.2$ | $9.8$ | $3.0$ | $-1.65\pm 0.03$ | $8.0$ | $0.6$
Q1354$+$048 | $209.3594$ | $+4.5948$ | $1.2335$ | [O II] | $46.5$ | $47.2$ | $9.5$ | $<1.1$ | $-0.67\pm 0.04$ | $2.0$ | $0.5$
J0154$-$0712 | $28.7278$ | $-7.2061$ | $1.2957$ | [O II] | $46.8$ | $47.5$ | $9.5$ | $<0.9$ | $-0.11\pm 0.01$ | $3.4$ | $0.5$
Q1435$-$0134 | $219.4511$ | $-1.7863$ | $1.3117$ | [O II] | $47.1$ | $47.8$ | $10.0$ | $1.8$ | $-0.41\pm 0.02$ | $6.1$ | $0.6$
PG 1522$+$101 | $231.1021$ | $+9.9747$ | $1.3302$ | [O II] | $47.0$ | $47.7$ | $10.1$ | $<0.7$ | $-0.18\pm 0.02$ | $2.0$ | $0.5$
HE 2336$-$5540 | $354.8050$ | $-55.3974$ | $1.3531$ | [O II] | $47.1$ | $47.8$ | $10.0$ | $2.5$ | $-0.23\pm 0.01$ | $2.6$ | $0.6$
PKS 0232$-$04 | $38.7805$ | $-4.0348$ | $1.4450$ | [O II] | $46.7$ | $47.4$ | $9.7$ | $4.0$ | $-0.70\pm 0.12$ | $10.0$ | $0.7$
aRight ascension of the quasar in J2000 coordinates from the Gaia survey Data
Release 3 (Gaia Collaboration et al., 2023).
bDeclination of the quasar in J2000 coordinates from the Gaia survey Data
Release 3 (Gaia Collaboration et al., 2023).
cQuasar systemic redshift measured as described in Section 3.
dEmission line used to measure the quasar systemic redshift. Systematic
redshift uncertainties correspond to $\approx 20,50,$ and
$130$ ${\rm km\,s}^{-1}$ for [O II], [O III], and H$\beta$/Mg II-based
redshifts, respectively.
eMonochromatic luminosity measured at rest-frame $\lambda=5100$ and $3000$ Å
for quasars at $z<0.7$ and $z>0.7$, respectively.
Systematic uncertainties in the monochromatic luminosities are dominated by
$\approx 10\%$ flux calibration errors.
fBolometric luminosity of the quasar estimated as described in Section 3.
Systematic uncertainty in bolometric luminosities are
dominated by population scatter in the bolometric corrections of $\approx 0.1$
dex (Richards et al., 2006).
gSupermassive black hole mass estimated with a single-epoch virial theorem
approach described in Section 3. Systematic
uncertainties in these black hole mass estimates are $\approx 0.4$ dex (Shen
et al., 2023).
hRadio-loudness based on flux at rest-frame wavelengths of 6 cm and 2500 Å.
Systematic uncertainties in $\log RL$ are $\approx 0.1$ dex.
iRest-frame [O II] equivalent widths measured in the MUSE quasar spectra
within the seeing disk corresponding to $\lesssim 5$ kpc.
jTotal exposure time of the MUSE observations.
kFWHM seeing measured in the MUSE white-light image of the quasar.
Figure 1: Summary of the properties of the CUBS and MUSEQuBES quasars. The
main panels show bolometric luminosity (bottom), black hole mass (middle), and
radio-loudness (top) of the CUBS and MUSEQuBES quasars versus redshift as
black points. Quasars not detected in radio data are shown as open symbols
marking the $3\sigma$ upper limits on their radio-loudness. Uncertainties are
dominated by systematics and visualized as error bars in top left of each
panel. The median bolometric luminosity and black hole mass for SDSS quasars
(Shen et al., 2011) is shown as a function of redshift with a red line in the
bottom and middle panels, with a faded red band marking 68% population scatter
at each redshift. The small panels display histograms of the redshifts (top),
bolometric luminosities (bottom right), black hole masses (middle right), and
radio-loudnesses (top right). By selection, the CUBS and MUSEQuBES quasars are
more luminous and have higher inferred black hole masses than typical SDSS
quasars at the same redshifts. The sample is evenly split between radio-quiet
and loud systems.

We reduced the MUSEQuBES data with the MUSE Guaranteed Time Observations (GTO)
team pipeline as described in Bacon et al. (2017), which includes standard
reduction steps in the MUSE ESO pipeline (Weilbacher et al., 2020) but with
post-processing that improves image quality via self-calibration and principle
component-based sky subtraction (Soto et al., 2016). We also reduced the data
using the CubEx post-processing pipeline described in Borisova et al. (2016)
and Cantalupo et al. (2019). For MUSEQuBES fields, we performed the analysis
in this paper on both the GTO and CubEx reduction products and found
consistent results in all cases.

We reduced the CUBS datacubes with the ESO pipeline both with and without
CubEx post-processing as described in Chen et al. (2020) and Chen et al.
(2023a). For all of the CUBS and MUSEQuBES fields, we also retrieved the
reduced, coadded MUSE-DEEP datacubes from the ESO archive. Compared to the GTO
and CubEx data products, MUSE-DEEP reductions exhibit more artifacts in
continuum images and larger spectral residuals near sky lines. However, for
the narrow-band, continuum-subtracted datacubes and surface brightness maps
used in this work, we obtained similar results with the ESO-DEEP products
except when near a night sky emission line, in which case the CubEx and GTO
reductions exhibit significantly improved residuals. After reducing the data,
we converted the air wavelengths delivered by the MUSE pipelines to vacuum for
convenience. The total MUSE exposure time available and seeing FWHM measured
in the final stacked datacubes are listed for each quasar field in Table 1.

The ESO, GTO, and CubEx pipelines all share common wavelength and flux
calibration approaches. MUSE wavelength calibration frames are taken with
three built-in lamps (HgCd, Ne, and Xe) by ESO Science Operations staff in the
morning, after the science exposures are acquired. The observed locations of
the arc lines on the detectors are fit with outlier-resistant 2D polynomials
to simultaneously solve for the dispersion, tilt, and curvature of the
spectral lines in the arc frames. Comparisons of arc lamp spectra taken over
the course of a night and observations of telluric absorption features in MUSE
spectra of bright stars indicate that systematic uncertainties in the
wavelength calibration are typically less than $1$ ${\rm km\,s}^{-1}$ (Kamann
et al., 2018; Weilbacher et al., 2020).

MUSE flux calibrations are taken with spectrophotometric standards nightly.
Systematic uncertainties in the spectrophotometry as a function of position in
the datacube and wavelength are typically less than 5% for data take under
photometric conditions (Weilbacher et al., 2020), though others have reported
systematic errors in the flux calibration of up to 25% (e.g. Péroux et al.,
2019). To quantify the level of systematic errors in the CUBS and MUSEQuBES
flux calibration, we identified bright stars and galaxies in each MUSE
datacube to serve as references and calculated synthetic photometry in the
ACS$+$F814W and Dark Energy Camera $i$-bands. We then compared these synthetic
magnitudes with those measured in archival HST ACS$+$F814W images of the
MUSEQuBES fields and public Dark Energy Survey Data Release 2 (DES; Abbott et
al., 2021) photometry for the CUBS fields. Based on comparisons of the
synthetic MUSE photometry and reference photometry from HST and DES, we
estimate a $1\sigma$ uncertainty of 10%.

One of the primary differences in the ESO, GTO, and CubEx pipelines is their
approach to illumination corrections. The ESO pipeline performs illumination
corrections using a combination of flats taken with the calibration unit (Kelz
et al., 2012) and twilight flats. However, subtle differences in the true
illumination pattern during the science observations and those taken with the
calibration unit and twilight sky correlate with IFU slice, resulting in
artificial grid patterns in narrow-band images and emission line maps (see
discussion in Bacon et al., 2015; Borisova et al., 2016). To improve the
illumination correction and remove these artificial patterns from MUSE data,
both the GTO and CubEx pipelines perform “self-calibration” by producing
illumination corrections based on the night sky continuum and emission lines
observed in science frames after masking continuum sources. These self-
calibration steps effectively remove the grid pattern present in ESO cubes and
reduce residuals in synthetic images formed from the data cubes by a factor of
approximately three (Lofthouse et al., 2020).

The other major differences in the ESO, GTO, and CubEx pipeline products
results from their night-sky subtraction approaches. All three pipelines begin
by masking continuum sources to identify spaxels where the detected flux is
dominated by sky emission, but their modeling and subtraction of the night sky
emission differ significantly. The ESO sky subtraction pipeline adopts a model
of the night sky-line emission based on a fixed set of 5100 cataloged emission
lines (Cosby et al., 2006) in the MUSE wavelength range. These are categorized
into 52 groups, with lines in each group sharing the same excited state. The
night-sky line emission model fixes the relative intensity of lines in each of
these groups and fits the observed global sky spectrum, allowing the total
intensity of each group to vary and convolving with the line-spread function
(LSF). After this, the best-fitting global sky model is subtracted and the
residuals in source-free spaxels are used to estimate the continuous component
of the night sky emission for each slice (Streicher et al., 2011).

To reduce night sky residuals present after the ESO sky subtraction, the GTO
pipeline employs post-processed sky subtraction with principal component
analysis using the Zurich Atmosphere Purge package (ZAP; Soto et al., 2016).
On the other hand, the CubEx pipeline operates with post-processing on ESO
data products with the ESO sky subtraction module turned off. It then uses the
CubeSharp (Borisova et al., 2016; Cantalupo et al., 2019) routine to produce
an empirical model of the LSF on a line-by-line basis (or groups of lines if
they are close in wavelength). One key advantage of the CubEx sky subtraction
approach is that the LSF can be finely adjusted to improve sky subtraction
while also conserving flux. Both the GTO and CubEx pipelines improve sky line
residuals significantly, resulting in a factor of approximately three
reduction in the standard deviation of flux in source-free spaxels (Lofthouse
et al., 2020).

To study faint line emission near the bright quasars, we performed quasar
light subtraction as described in Johnson et al. (2018) and Helton et al.
(2021), which effectively removes the spatially unresolved continuum, broad-
line emission, and narrow-line emission from the nucleus. Finally, to search
for line-emitting nebulae, we produced continuum-subtracted emission-line maps
around the expected wavelength of the [O II] doublet at the redshift of each
quasar. To do so, we first identified suitable continuum regions on the blue
and red sides of the potential emission line. The continuum regions are
typically $3000{-}4000$ ${\rm km\,s}^{-1}$ in width but tailored for each
quasar based on proximity to night-sky emission lines. We then fit a low-order
polynomial to the continuum regions in each spaxel of the datacube and
subtracted the best-fit model to produce continuum-subtracted datacubes using
the MUSE Python Data Analysis Framework (MPDAF Bacon et al., 2016).

##  3 Quasar Properties

To characterize the properties of the CUBS and MUSEQuBES quasars, we extracted
their optical MUSE spectra with MPDAF (Bacon et al., 2016; Piqueras et al.,
2017) prior to quasar and continuum subtraction with apertures chosen to
include $>95\%$ of the quasar light. We measured precise, narrow-line
redshifts for the quasars using the [O II] $\lambda\lambda 3727,3729$ doublet
whenever possible, assuming an effective centroid of $\lambda 3728.6$ expected
for the $0.8{:}1$ to $0.9{:}1$ doublet ratio typical of quasar narrow-line
emission (Hewett & Wild, 2010). At the same time, we also measured the rest-
frame equivalent width of the [O II] doublet in the quasar spectra extracted
within a seeing disk, $W_{\rm r}([{\rm O\,II}])$. The seeing disks correspond
to $\lesssim 5$ kpc from the quasars, ensuring that the [O II] equivalent
widths capture emission from the narrow-line region rather than more extended
nebulae. Quasar redshifts based on [O II] emission are characterized by
typical uncertainties of $20-30$ ${\rm km\,s}^{-1}$ (Boroson, 2005; Hewett &
Wild, 2010). Four quasars do not exhibit detectable [O II] despite the high
S/N available in the MUSE spectra, necessitating the use of other emission
line observed in the near-IR by Sulentic et al. (2004). For HE 0331$-$4112, we
estimated the systemic redshift using the [O III] $\lambda 5008$ emission-line
peak. Quasar redshifts measured with the peak of the [O III] line are
characterized by typical systematic uncertainties of $\approx 50$ ${\rm
km\,s}^{-1}$ when compared to stellar absorption-based redshifts (see
discussion in Shen, 2016). For the remaining three quasars, narrow emission
lines are not detectable, necessitating the use of broad H$\beta$ (HE
0153$-$4520 and HE 2305$-$5315) or Mg II (HE 0439-5254). We estimated the
systemic redshifts of these three quasars by cross-correlating the observed
quasar spectra with the template from Hewett & Wild (2010) using methods
described in that work. As discussed in Hewett & Wild (2010), the resulting
broad H$\beta$ and Mg II emission redshifts exhibit uncertainties of $\approx
130$ ${\rm km\,s}^{-1}$ when compared to more precise narrow-line redshifts.
The quasar redshifts and the emission lines driving them are listed in Table
1.

To estimate the luminosities and black hole masses of the quasars, we used
PyQSOFit (Guo et al., 2018, 2019) to fit the quasar continuum with a power-law
and Fe II templates (Boroson & Green, 1992; Vestergaard & Wilkes, 2001) and
multiple Gaussians for broad and narrow emission lines. To enable estimates of
the bolometric luminosities and black hole masses of the quasars, we measured
the monochromatic luminosity at $5100$ Å and line-width of broad H$\beta$ for
systems at $z<0.7$, and the monochromatic luminosity at $3000$ Å and line-
width of broad Mg II for those at $z>0.7$. For four CUBS quasars (HE
0331$-$4112, J2135$-$5316, PKS 2242$-$498, and HE 2305$-$5315), the Mg II
emission-line falls near or in the MUSE Sodium laser gap necessitated by the
adaptive optics system. For these, we obtained supplementary spectra with the
Magellan Echellete (MagE) spectrograph (Marshall et al., 2008) to enable Mg II
line-width measurements (for details of the MagE observations and reductions,
see Li et al. (2024)). We then combined the monochromatic luminosities and
line-widths to infer supermassive black hole masses ($M_{\rm BH}$) using the
single-epoch virial theorem relations described in Shen et al. (2011). Typical
uncertainties in single-epoch black hole mass estimates are $0.4$ dex, though
uncertainties may be higher for the CUBS and MUSEQuBES quasars because they
are higher in luminosity than typical reverberation mapping quasar samples
used to calibrate the relations (see discussion in Shen et al., 2023).

To enable comparisons of the luminosities of the quasars across redshifts, we
converted the monochromatic luminosity measurements to bolometric luminosity
estimates ($L_{\rm bol}$) using bolometric corrections from Richards et al.
(2006) of $10.3\times$ and $5.6\times$ for monochromatic luminosities at
$5100$ Å and $3000$ Å, respectively. Uncertainties in the bolometric
luminosities are dominated by $\approx 0.1$ dex intrinsic scatter in
bolometric corrections (Richards et al., 2006). Finally, we estimated the
radio-loudness (RL), defined as the flux density ratio at rest-frame 6 cm vs.
2500 Å, of each quasar using radio observations from the Rapid Australian
Square Kilometre Array Pathfinder (ASKAP) Continuum Survey (RACS) Data Release
1 (Hale et al., 2021) as described in Li et al. (2024). In the case of non-
detections, we calculated $3\sigma$ upper limits on the radio flux and
converted these to upper limits on radio-loudness. The RACS radio observations
and MUSE rest-NUV observations are separated by a time period of several
years, resulting in non-negligible systematic uncertainty of 0.1 dex due to
potential NUV variability on these timescales (Welsh et al., 2011), though we
caution that rare but significant radio variability could result in
significantly larger errors in the radio-loudness (Nyland et al., 2020). The
monochromatic luminosities, bolometric luminosities, supermassive black hole
mass estimates, radio-loudness measurements, and [O II] rest-frame equivalent
widths for each quasar are reported in Table 1.

To contextualize the properties of the CUBS and MUSEQuBES quasars, we plot
their luminosities, black hole masses, and radio-loudnesses vs redshift and
compare with quasars in the SDSS sample from Shen et al. (2011) in Figure 1.
The CUBS and MUSEQuBES quasars were selected to be sufficiently UV bright for
high-quality absorption spectroscopy with COS. As a result, the CUBS and
MUSEQuBES quasars are among the most luminous quasars in the $z<1.4$ Universe,
with luminosities typically $\approx 1.5$ dex higher than the median SDSS
quasar at the same redshift. Similarly, the inferred SMBH masses of the CUBS
and MUSEQuBES quasars are $\approx 1$ dex above the SDSS median at similar
redshifts (Shen et al., 2011). The 30 quasars are approximately evenly split
between radio-loud and radio-quiet with a division between the two classes of
$\log RL\approx 1$.

##  4 Discovery of optically emitting circumgalactic nebulae

Figure 2: [O II] surface brightness maps for each quasar field ordered by redshift. The surface brightness maps are displayed after quasar light subtraction, continuum subtraction, and 3D extraction, as described in the text. Each panel is 30”$\times$30” on a side, and a blue star at the center of each image marks the quasar’s position. A scale bar corresponding to $50$ proper kpc at the redshift of the quasar is shown in the bottom left of each panel. The names and redshifts of the quasars are shown in the top right of each panel. The contour line in each panel corresponds to the $3\sigma$ detection limit averaged over one square arcsecond based on the variance in each image and is labeled in units of $10^{-17}\ {\rm erg\,s^{-1}\,cm^{-2}\,arcsec^{-2}}$. The contours are included to visualize the extent of the nebulae and are distinct from the $5\sigma$ significance requirement used to define a detection. Table 2: Summary of the [O II] emitting nebulae around the CUBS and MUSEQuBES quasars, ordered by redshift. |  |  | FWHMc | sized | areae |  | [O II] luminosityg | ${\rm SB}_{10-30}(1+z)^{4\,h}$
---|---|---|---|---|---|---|---|---
quasar |  $z$a | flagb | (arcsec) | (kpc) | (kpc2) | SNRf | ($\rm erg\,s^{-1}$) | ($\rm erg\,cm^{-2}\,s^{-1}\,arcsec^{-2}\,$)
HE 0435$-$5304 | 0.4279 | $=$ | 2.6 | 55 | 1170 | 35.3 | $9.6{\times}10^{41}$ | $5.6{\times}10^{-17}$
HE 0153$-$4520 | 0.4532 | $<$ | 0.6 | 7 | 35 | 2.4 | $1.9{\times}10^{40}$ | $1.4{\times}10^{-18}$
HE 0226$-$4110 | 0.4936 | $=$ | 3.9 | 84 | 2740 | 156.3 | $1.1{\times}10^{42}$ | $3.9{\times}10^{-17}$
PKS 0405$-$123i | 0.5731 | $=$ | 9.1 | 129 | 8180 | 134.8 | $2.7{\times}10^{42}$ | $2.2{\times}10^{-17}$
HE 0238$-$1904j | 0.6282 | $=$ | 4.9 | 103 | 4820 | 187.9 | $2.3{\times}10^{42}$ | $9.0{\times}10^{-17}$
3C 57 | 0.6718 | $=$ | 3.6 | 71 | 1930 | 71.3 | $1.2{\times}10^{42}$ | $5.6{\times}10^{-17}$
PKS 0552$-$640 | 0.6824 | $=$ | 6.2 | 153 | 5180 | 119.6 | $3.4{\times}10^{42}$ | $1.1{\times}10^{-16}$
J0110$-$1648 | 0.7822 | $=$ | 1.8 | 29 | 530 | 20.3 | $4.1{\times}10^{41}$ | $1.3{\times}10^{-17}$
J0454$-$6116k | 0.7864 | $=$ | 3.5 | 102 | 4350 | 234.0 | $4.8{\times}10^{42}$ | $1.3{\times}10^{-16}$
J2135$-$5316k | 0.8123 | $=$ | 3.8 | 83 | 2250 | 47.9 | $2.1{\times}10^{42}$ | $9.4{\times}10^{-17}$
J0119$-$2010 | 0.8160 | $=$ | 4.2 | 96 | 3090 | 60.1 | $2.5{\times}10^{42}$ | $1.4{\times}10^{-16}$
HE 0246$-$4101 | 0.8840 | $=$ | 3.8 | 74 | 2280 | 44.3 | $1.0{\times}10^{42}$ | $4.6{\times}10^{-17}$
J0028$-$3305 | 0.8915 | $=$ | 2.1 | 42 | 500 | 23.2 | $2.9{\times}10^{41}$ | $1.1{\times}10^{-17}$
HE 0419$-$5657 | 0.9481 | $=$ | 2.1 | 35 | 620 | 23.2 | $5.6{\times}10^{41}$ | $2.5{\times}10^{-17}$
Q0107$-$025 | 0.9545 | $=$ | 1.9 | 28 | 410 | 13.9 | $2.0{\times}10^{41}$ | $1.4{\times}10^{-17}$
Q0107$-$0235 | 0.9574 | $=$ | 3.1 | 90 | 2130 | 144.8 | $3.7{\times}10^{42}$ | $1.3{\times}10^{-16}$
PKS 2242$-$498 | 1.0011 | $=$ | 3.4 | 71 | 2260 | 54.6 | $2.3{\times}10^{42}$ | $1.2{\times}10^{-16}$
PKS 0355$-$483 | 1.0128 | $=$ | 2.8 | 50 | 1310 | 70.5 | $1.0{\times}10^{42}$ | $6.2{\times}10^{-17}$
HE 0112$-$4145 | 1.0238 | $=$ | 1.8 | 38 | 890 | 70.8 | $2.7{\times}10^{42}$ | $7.8{\times}10^{-17}$
HE 0439$-$5254 | 1.0530 | $=$ | 2.5 | 47 | 970 | 34.3 | $9.1{\times}10^{41}$ | $5.6{\times}10^{-17}$
HE 2305$-$5315 | 1.0733 | $<$ | 0.6 | 15 | 100 | 3.9 | $9.6{\times}10^{40}$ | $1.1{\times}10^{-17}$
HE 1003$+$0149 | 1.0807 | $=$ | 2.8 | 53 | 1660 | 94.8 | $7.0{\times}10^{42}$ | $3.9{\times}10^{-16}$
HE 0331$-$4112 | 1.1153 | $=$ | 1.8 | 32 | 520 | 25.9 | $6.9{\times}10^{41}$ | $3.1{\times}10^{-17}$
TXS 0206$-$048l | 1.1317 | $=$ | 5.0 | 200 | 10800 | 257.1 | $9.7{\times}10^{42}$ | $3.0{\times}10^{-16}$
Q1354$+$048 | 1.2335 | $=$ | 6.4 | 126 | 2860 | 66.5 | $2.9{\times}10^{42}$ | $9.9{\times}10^{-17}$
J0154$-$0712 | 1.2957 | $=$ | 2.8 | 63 | 1340 | 41.9 | $2.2{\times}10^{42}$ | $1.2{\times}10^{-16}$
Q1435$-$0134 | 1.3117 | $=$ | 2.7 | 63 | 2010 | 76.0 | $7.3{\times}10^{42}$ | $2.6{\times}10^{-16}$
PG 1522$+$101 | 1.3302 | $=$ | 2.9 | 50 | 1220 | 28.3 | $2.5{\times}10^{42}$ | $1.6{\times}10^{-16}$
HE 2336$-$5540 | 1.3531 | $<$ | 0.4 | 7 | 30 | 3.6 | $5.0{\times}10^{40}$ | $4.3{\times}10^{-17}$
PKS 0232$-$04 | 1.4450 | $=$ | 4.0 | 116 | 3670 | 97.1 | $7.8{\times}10^{42}$ | $3.6{\times}10^{-16}$
aQuasar systemic redshift repeated from Table 1 for convenience.
bNebula detection flag with “=” indicating a detection and “$<$” indicating a
non-detection in which case all nebula
measurements represent upper limits.
cFWHM of the [O II] surface brightness profiles, which have systematic
uncertainties of $\approx 10\%$.
dProjected linear sizes of the [O II] nebula defined as the major axis
diameter, which have systematic uncertainties of $15\%$.
eProjected areas of the [O II] nebula, which have systematic uncertainties of
$\approx 20\%$.
fTotal signal-to-noise ratios of the [O II] nebula.
gTotal luminosities of the [O II] nebula, which have systematic uncertainties
of $\approx 25\%$.
hMean cosmological dimming corrected surface brightness measured in an annulus
with inner and outer radii of $10$ and $30$
kpc, which have systematic uncertainties of $\approx 25\%$.
i For more details on this nebula and its environment, see Johnson et al.
(2018).
j For more details on this nebula and its environment, see Zhao & Wang (2023)
and Liu et al. (2024).
k For more details on these nebulae and their environments, see Chen et al.
(2023a)
l For more details on this nebula and its environment, see Johnson et al.
(2022).

To identify large nebulae around the CUBS and MUSEQuBES quasars, we first
visually inspected the quasar light and continuum-subtracted MUSE datacubes by
searching for extended emission within $|\Delta v|<2000$ ${\rm km\,s}^{-1}$ of
the expected wavelength of the [O II] doublet. We chose this large velocity
interval to be sufficient to include emission from bound gas in even massive
galaxy clusters. Furthermore, the velocity interval is sufficient to account
for potential systematic uncertainty in the quasar systemic redshifts, even
for those with redshifts based on Mg II or broad H$\beta$. We focused the
search on the [O II] doublet because the MUSE spectral range covers it for all
of the CUBS and MUSEQuBES quasars. Moreover, detailed studies of individual
nebulae around quasars at $z\approx 0.5-0.6$, where a more comprehensive suite
of emission lines is available, suggest that [O II] dominated regions of
circum-quasar nebulae are often more extended than [O III] dominated areas
(e.g. Chen et al., 2023a; Liu et al., 2024). Our initial inspection revealed
that most of the CUBS and MUSEQuBES quasars exhibit extended [O II] emission
on $\gtrsim 20-30$ kpc scales.

To enable quantitative detection and analysis of the nebulae, we performed
thresholded detection and segmentation in three dimensions (3D; 2 spatial + 1
spectral) following the approach often used in studies of Ly$\alpha$ nebulae
at $z=2-3$ (e.g. Borisova et al., 2016; Arrigoni Battaia et al., 2019b;
Sanderson et al., 2021). In particular, we formed a signal-to-noise ratio
(SNR) datacube and then smoothed it in both the spatial and spectral
dimensions using a Gaussian kernel with $\sigma\approx 1.5$ pixels, though
with a somewhat larger $\sigma$ in cases with elevated noise due to night sky
lines. We then performed 3D source detection and segmentation on the smoothed
SNR maps with a threshold of ${\rm SNR}\approx 1.5$, though with a slightly
higher threshold in cases where the night sky line residuals are elevated
relative to the variance map. We then connected adjacent pixels above this
threshold and required a minimum of ten connected pixels and a total ${\rm
SNR}\gtrsim 5$ to define a detection. To uniformly visualize and characterize
these nebulae, we produced [O II] surface brightness maps by integrating the
unsmoothed flux in each spaxel over the wavelength interval defined by the 3D
segmentation. For spaxels that do not include any detected nebulae, the
background and noise properties correspond to those expected of three spectral
pixels at the wavelength where the mean SNR per pixel of the nebula is
highest. The resulting [O II] surface brightness maps for each quasar are
shown in Figure 2.

These 3D extractions have the advantage of visualizing the full extent of
detected emission while reducing noise for kinematically quiescent or
undetected regions compared to surface brightness maps defined by fixed
velocity intervals. However, they also exhibit complicated, spatial varying
noise characteristics (see discussion in Borisova et al., 2016; Arrigoni
Battaia et al., 2019a; Mackenzie et al., 2021). To quantify systematic
uncertainty in the measured properties of the nebulae, we also produced
surface brightness maps integrated over fixed velocity intervals chosen for
each quasar field to include 95% of the detected [O II] nebular emission in
the quasar-light and continuum subtracted datacubes. The resulting narrow-band
surface brightness maps are broadly consistent with those obtained from 3D
segmentation. However, the SNR in the 3D segmentation-based maps is noticeably
improved compared to synthetic narrow-band images in cases where the nebular
emission is spread over a wide velocity range. To ensure the robustness of the
results, we performed all of the analysis in this paper on the surface
brightness maps produced by 3D segmentation and synthetic narrow bands. We
confirmed that the conclusions of the paper hold with both approaches.

To characterize the size and flux of the nebulae around the CUBS and MUSEQuBES
quasars relative to the seeing and noise level, we used the source measurement
functions of Photutils (Bradley et al., 2016) to measure the FWHM of the
nebulae in the surface brightness maps. We integrated the 3D segmentation to
estimate their total [O II] flux. We define extended nebulae to be detections
if their FWHM exceeds the FWHM seeing measured in the datacube and if their
total SNR is greater than five. With these criteria, all but three of the
quasars have detected [O II] emitting, extended nebulae surrounding them. One
of these, HE 2305$-$5315, does exhibit a small nebula offset from the quasar
centroid, but its FWHM is comparable to the seeing, so we do not consider it
further in this paper. In all 27 cases with detected, extended nebulae, the
measured FWHM of the nebulae exceed the seeing FWHM by a factor of $>2.4$.
This ensures that the effect of seeing on measured properties of the nebulae
are minimal. For example, the contributions of the seeing to the measured FWHM
of the nebulae are $<10\%$ when considered in quadrature.

To estimate the size of the nebulae, we measured the major axis length-scale
determined by the maximum projected separation between pixels contained within
the SNR-defined segmentation map corresponding to each nebula. We adopted this
definition of the size of the nebulae to enable comparison with studies of
$z\approx 3$ Ly$\alpha$ nebulae around quasars such as Borisova et al. (2016)
and Mackenzie et al. (2021) which used a similar approach. In two cases, PKS
0405$-$123 (Johnson et al., 2018) and TXS 0206$-$048 (Johnson et al., 2022),
multiple large nebulae are detected in the quasar environment, some of which
are well offset from the quasars themselves. For these, we report the
projected size for the largest contiguous nebula as defined in the 3D
segmentation.

Several of the nebulae are quite elongated along one axis, meaning that the
major axis size does not fully encapsulate the extent of the systems. For this
reason, we also report the area in square kpc defined by the projection of the
3D segmentation of each nebula. In cases with multiple nebulae, we report the
sum of their areas. Finally, to quantify the surface brightness of the
nebulae, we computed the redshift dimming corrected surface brightness
measured in an annulus centered on the quasar with an inner radius
corresponding to $10$ kpc and an outer radius corresponding to $30$ kpc, which
we denote as ${\rm SB}_{10-30}(1+z)^{4}$. The outer radius of $30$ kpc is
chosen to be half of the median major axis full size of the nebulae around the
CUBS and MUSEQuBES quasars. The inner radius is chosen to be large enough to
avoid residuals from quasar light subtraction and to exceed the effective
radii expected in stellar continuum images of late-type and early-type massive
galaxies at intermediate redshift (e.g. van der Wel et al., 2014). The FWHM,
major axis size, projected area, total SNR, total [O II] luminosity, and $\rm
SB_{10-30}(1+z)^{4}$ measured for the nebulae are reported in Table 2. To
quantify the systematic uncertainty in these quantities, we remeasured them
using the synthetic narrow-band images of the [O II] nebulae. While the
systematic differences inferred from the median ratios between the quantities
measured with the 3D segmentation approach versus the synthetic narrow-band
images are minimal, there is a non-negligible scatter corresponding to 10% for
FWHM, 15% for size, 20% for area, and 25% for both luminosity and ${\rm
SB_{10-30}}(1+z)^{4}$.

While half of the 30 quasars in this work have HST images available in the
archive, the images were acquired with long exposures to enable study of the
morphologies of faint foreground galaxies and cross-correlate with CGM/IGM
absorption. As a result, the bright quasars are saturated, preventing studies
of stellar continuum from the host galaxies. However, the mean effective
radius of the stellar component of massive late- and early-type galaxies at
these redshifts is $R_{\rm eff}\approx 9-10$ kpc (e.g. van der Wel et al.,
2014), corresponding to a diameter of $\approx 20$ kpc. If quasar hosts
represent $2\sigma$ outliers in the mass-size relation, we expect their
effective radii to be $\approx 15$ kpc, corresponding to a diameter of 30 kpc.

To gain insights into the extent of the nebulae in comparison to the expected
extents of stellar components of their host galaxies, we compute the fraction
of systems with nebulae larger than 100, 50, 30, and 20 kpc in the following.
Among the thirty quasars, seven ($23^{+9}_{-6}\%$) have detected [O II]
nebulae with major axis sizes greater than $100$ kpc, twenty
($67^{+7}_{-9}\%$) greater than 50 kpc, 25 ($83^{+5}_{-9}\%$) greater than 30
kpc, and 27 ($90^{+3}_{-8}\%$) greater than 20 kpc. The median size of the [O
II] nebulae detected around the CUBS and MUSEQuBES quasars is $60$ kpc. The
sizes of the [O II] nebulae are therefore larger than the expected sizes of
the stellar component of their host galaxies, indicating that the majority of
UV luminous quasars at $z=0.4-1.4$ are surrounded by [O II] emitting, CGM-
scale nebulae.

Interestingly, the smaller $20-60$ kpc nebulae are generally well centered on
the quasars and relatively symmetric, but the larger ones often exhibit
irregular morphology and may not be well centered (e.g. PKS 0405$-$123 and
Q1354+048). In some cases, the morphologies of the nebulae are coincident with
nearby galaxies with redshifts similar to the quasars as previously reported
for PKS 0405$-$123 (Johnson et al., 2018), TXS 0206$-$048 (Johnson et al.,
2022), and HE 0238$-$1904 (Liu et al., 2024). This can be explained if the
larger nebulae are often result from ram pressure and tidal stripping
experienced during galaxy interactions as previously suggested by Stockton &
MacKenty (1987). The nebulae do not exhibit obvious coincidences with the
radio lobe and jet orientation of the radio-loud quasars, disfavoring jet-
driven outflow origin (though see Fu & Stockton, 2009). Further investigation
of the coincidences between the extended nebulae and interacting galaxies in
the quasar host environment will require detailed investigation of the group
environments like those in Johnson et al. (2018, 2022); Helton et al. (2021),
and Liu et al. (2024).

##  5 Discussion

Figure 3: Projected linear sizes (top panels) and dimming corrected surface
brightness measured in an annulus of $10-30$ kpc radius (bottom panels), ${\rm
SB}_{10-30}(1+z)^{4}$, of the [O II] emitting nebulae detected around the CUBS
and MUSeQuBES quasars vs quasar properties including (from left to right)
redshift, bolometric luminosity, black hole mass, radio-loudness, and rest-
frame [O II] equivalent width. In all panels, quasars with detections of
extended, [O II] emitting quasars are shown in black, while upper limits from
non-detections are shown in grey. Quasars with non-detections in the radio
data are shown as open triangles marking the upper limit on their radio-
loudness. For reference, the $5\sigma$ surface brightness detection limit in
${\rm SB}_{10-30}(1+z)^{4}$ for the quasars are shown in the bottom left panel
as small open symbols. Systematic uncertainties are visualized with error bars
in the top left of each panel. Statistical uncertainty in [O II] equivalent
width are shown as error bars, though most are smaller than the data points.
While most of the panels exhibit significant scatter and little evidence of a
correlation, the sizes and surface brightnesses of the nebulae are tentatively
correlated with rest-frame [O II] equivalent width with a generalized Kendall
$\tau$ test returning correlation coefficients of $-0.3$ and $p$-values of
$0.01$.

Empirical characterizations of cool CGM around quasars have advanced
significantly in the last decade, thanks to a combination of large ground-
based surveys like SDSS that enable studies of Mg II absorption at $z\lesssim
2$ (e.g. Bowen et al., 2006; Farina et al., 2014; Johnson et al., 2015; Chen
et al., 2023c), dedicated absorption surveys with large telescopes at $z=2-4$
in rest UV absorption (e.g. Hennawi et al., 2006; Prochaska et al., 2013; Lau
et al., 2016), and observations of Ly$\alpha$ emission nebulae at $z>2$ (e.g.
Borisova et al., 2016; Cai et al., 2019; O’Sullivan et al., 2020; Fossati et
al., 2021; Mackenzie et al., 2021). The discovery of rest-frame optical
emission from $>20-30$ kpc ionized nebulae around the majority of UV-luminous
quasars at $z=0.4-1.4$ represents a new opportunity to study the relationship
between cool CGM and the galactic systems they surround while also opening a
new set of physical diagnostics based on non-resonant emission lines.

###  5.1 Possible relationships between nebulae and quasar properties

Previous studies of the cool, absorbing CGM around quasars at $z<2$
demonstrated that strong Mg II absorbers, which typically trace cool
($T\approx 10^{4}$ K), high column density ($\log N({\rm H\,I})/{\rm
cm^{-2}}\gtrsim 17$) enriched gas, are common around quasar host galaxies
(Bowen et al., 2006; Farina et al., 2014; Johnson et al., 2015). The Mg II
covering factors observed around quasar hosts is significantly greater than
those found around luminous red galaxies (e.g. Huang et al., 2016) and
comparable to or higher than those of massive star-forming galaxies (e.g.
Huang et al., 2021). Moreover, Johnson et al. (2015) identified a strong
correlation between Mg II absorption covering fractions at $d<200$ kpc and
quasar luminosity with luminous quasars ($\log L_{\rm bol}/{\rm
erg\,s^{-1}}>45.5$) exhibiting Mg II covering fractions approximately double
those of lower luminosity quasars. Early studies of large-scale optical
emission around quasars based on a combination of narrow-band [O III] imaging
and follow-up long slit spectroscopy suggested that emitting nebulae on scales
of tens of kpc are common around radio-loud quasars and uncommon around radio-
quiet ones (e.g. Boroson et al., 1985; Stockton & MacKenty, 1987). This
correlation between extended nebulosity and radio properties led to
suggestions that the nebulae arise from radio-mode feedback (e.g. Fu &
Stockton, 2009).

To search for correlations between properties of the quasars and surrounding
nebulae, Figure 3 displays plots of the sizes and dimming-corrected surface
brightnesses (${\rm SB_{10-30}}(1+z)^{4}$) of the nebulae versus quasar
redshift, bolometric luminosity, black hole mass, radio-loudness, and [O II]
rest-frame equivalent width. The panels in Figure 3 exhibit substantial
scatter with no clear trend between quasar redshifts, bolometric luminosities,
and the sizes and areas of the [O II] nebulae. One the other hand, the [O II]
emission levels in the quasar spectra (with negative values indicating
stronger emission relative to the continuum) appear to be weakly correlated
with the sizes and surface brightnesses of the nebulae, though with
substantial scatter (see the right panel of Figure 3). We quantified the
correlations and level of significance with the generalized Kendall $\tau$
test, accounting for upper limits and non-detections following Isobe et al.
(1986). Tests for correlations between the properties of the nebulae and
quasar redshifts, bolometric luminosities, black hole masses, and radio-
loudness returned no statistically significant results ($p$-values between 0.2
and 0.8), consistent with the lack of a visual trend seen in Figure 3. On the
other hand, the apparent correlation between [O II] equivalent width and the
sizes and surface brightnesses of the nebulae is marginally significant, with
a correlation coefficient of $-0.3$ (indicating a positive correlation between
the emission-line strengths in the quasar spectra and large-scale nebulae) and
a $p$-value of 0.01 (2.5$\sigma$). To demonstrate that the extended nebulae
themselves are not responsible for this correlation, we re-measured the [O II]
equivalent widths in quasar spectra extracted with an aperture radius of 0.6
arcsec (3-5 kpc at $z=0.4-1.4$) and found consistent results. The tentative
correlation between [O II] equivalent width and the nebulae suggests a general
correlation between circum-nuclear and ISM-scale gas observed in the quasar
spectra and larger-scale gas supplies in the massive halos that host UV
luminous quasars (see Li et al. (2024)). This trend is reminiscent of a
previously reported potential correlation between ISM or nuclear [O II]
emission in luminous red galaxies and Mg II absorption in their CGM at
projected distances of $d\lesssim 50$ kpc (Huang et al., 2016).

The lack of correlations between the properties of the nebulae and the
bolometric luminosities of the quasars that they surround suggests that the
presence of the emitting gas may be unrelated to quasar outflows in many
cases. Alternatively, the lack of correlations between the nebulae and quasar
luminosity could be partly due to a lack of dynamic range in the sample. The
CUBS and MUSEQuBES surveys do not include any quasars of $\log L_{\rm
bol}/{\rm erg\,s^{-1}}<45.5$. The correlation between Mg II absorption at
$d<200$ kpc and quasar luminosity reported by Johnson et al. (2015) sets in
when comparing luminous quasars of $\log L_{\rm bol}/{\rm erg\,s^{-1}}>45.5$
with less luminous ones of $\log L_{\rm bol}/{\rm erg\,s^{-1}}<45.5$, and this
study reported no trend within the more luminous quasar population.
Furthermore, previous studies of the extended narrow-line regions around
obscured AGN found a trend between luminosity and [O III]-detected narrow-line
region size with AGN of luminosity $\log L_{\rm bol}/{\rm erg\,s^{-1}}<45.5$
exhibiting narrow line region sizes of $R_{\rm NLR}\lesssim 6$ kpc and more
luminous obscured quasars exhibiting larger extended narrow-line regions with
$R_{\rm NLR}\approx 10-15$ kpc (Sun et al., 2017). We note that these narrow-
line region scales are smaller than most of the nebulae found around the CUBS
and MUSEQuBES quasars, which is likely the result of the choice of emission
line ([O II] versus [O III]) or increased sensitivity of MUSE relative to more
traditional long-slit spectrographs. Consequently, determining whether the
correlation between cool CGM and quasar luminosity extends to ionized, line-
emitting nebulae on circumgalactic scales will require samples of lower-
luminosity systems observed with wide-field IFS.

The lack of correlation between the presence, size, and surface brightness of
the [O II] emitting nebulae and radio-loudness also stands in contrast to
previous studies, which found [O III] emitting nebulae around radio-loud
quasars but not radio-quiet ones (e.g. Boroson et al., 1985; Stockton &
MacKenty, 1987). While the nebulae presented in this work are observed in [O
II] for the sake of uniformity given our lack of [O III] coverage for the
$z>0.85$ systems, the results are not substantially different for [O III]
within the lower-$z$ CUBS and MUSEQuBES quasars.

Our finding of common large nebulae around radio-quiet quasars compared to
their absence in previous surveys may be the result of the sensitivity of
MUSE. The larger telescope aperture, higher throughput, and higher spectral
resolution of MUSE result in dramatically improved surface brightness limits
compared to previous long-slit spectroscopy and narrow-band imaging surveys.
Indeed, Ly$\alpha$ nebulae around $z>2$ quasars were commonly detected around
radio-loud systems but were thought to be rare around radio-quiet ones until
new, more sensitive observations with MUSE and KCWI became available. The
increased sensitivity resulted in the discovery of ubiquitous, $100$ kpc-scale
nebulae around radio-quiet quasars at high redshift, despite non-detections
with less sensitive instruments (see discussion in Borisova et al., 2016;
Cantalupo, 2017). However, there is no correlation between surface brightness
and radio loudness within the sample of 30 quasars studied here. Reconciling
this result with previous studies that found a correlation between radio
loudness and the presence of [O III] emitting nebulae will likely require
larger samples observed with wide-field IFS data covering both [O II] and [O
III].

###  5.2 Comparison with giant Ly$\alpha$ nebulae observed around quasars at
$z=2-4$

The discovery that half of luminous quasars at $z\approx 1$ are surrounded (in
projection) by [O II] emitting nebulae with sizes greater than $60$ kpc
indicates that dense, cool CGM is common in quasar-host halos at this epoch.
However, this represents a significantly lower incidence and smaller size-
scale than findings of ubiquitous, $\gtrsim 100$ kpc H I Ly$\alpha$ emitting
nebulae around quasars at $z>2$ (Borisova et al., 2016; Cai et al., 2019;
O’Sullivan et al., 2020; Fossati et al., 2021; Mackenzie et al., 2021).
However, physical interpretation of this apparent difference requires
accounting for the expected [O II] to H I Ly$\alpha$ line ratio and the
difference in surface brightness dimming between $z\approx 1$ and $z>2$.

The radiative transfer involved in modeling Ly$\alpha$ is complex, so we opt
for an empirical approach and adopt a measured $\rm Ly\alpha/H\alpha$ ratio
observed around quasars and then combine this with model expectations for $\rm
H\alpha/[O\,II]$ to determine whether we expect the types of nebulae observed
in Ly$\alpha$ at $z=2-4$ around quasars to be observable in [O II] at
$z\approx 1$. To set expectations for $\rm H\alpha/[O\,II]$, we considered
emission models of moderately enriched, dust-free gas photoionized by an AGN
using Cloudy v17.03 (Ferland et al., 2017) with model parameters motivated by
Groves et al. (2004). In particular, we chose a power-law ionizing spectrum
with spectral slope ranging from $\alpha=-2.0$ to $-1.4$. We note that the
range of predicted H$\alpha$/[O II] ratios for AGN photoionized gas are
similar to predictions for fast shocks (e.g. Allen et al., 2008) and star-
forming galaxies (e.g. Kewley et al., 2004), indicating that our conclusions
are not sensitive to the ionization source.

One of the dominant drivers of variation in expected H$\alpha$/[O II] ratios
is gas metallicity. While the metallicity of the CGM around quasars at
$z\approx 1$ is not well studied due to observational limitations, Liu et al.
(2014) inferred approximately solar metallicity in the nebula around HE
0238$-$1904 for regions detectable in weak-line diagnostics. Furthermore, the
cool absorbing CGM around luminous quasars at $z=2-4$ from Lau et al. (2016)
and luminous red galaxies (LRGs) at $z=0.4$ from Zahedy et al. (2019) exhibit
typical metallicities of $Z\approx 0.25\,Z_{\odot}$, though with substantial
scatter in the case of LRGs. We therefore adopt a fiducial metallicity of
$Z=0.25\,Z_{\odot}$ for the AGN photoionized gas models but also explore a
range of $Z=0.1-1.0Z_{\odot}$. The presence of substantial dust in large
nebulae around quasars is disfavored in cases where Balmer line ratios are
measurable (e.g., Helton et al., 2021; Liu et al., 2024). The left axis of
Figure 4 shows the $\rm H\alpha/[O\,II]$ ratio versus $\rm[O\,III]/[O\,II]$
over the range of $\rm[O\,III]/[O\,II]$ observed in extended nebulae around
$z\approx 0.6$ quasars (Johnson et al., 2018; Helton et al., 2021; Liu et al.,
2024).

To translate this expected $\rm H\alpha/[O\,II]$ into expected $\rm
Ly\alpha/[O\,II]$, we multiply by a factor of 3.7 corresponding to the
observed $\rm Ly\alpha/H\alpha$ ratio of $3.7\pm 0.3$ observed in nebulae
around three $z\approx 3$ quasars from Langen et al. (2023). We note that
ratio is a factor of $\approx 2-3$ below that expected from pure recombination
radiation. Langen et al. (2023) and previous works (e.g. Cantalupo et al.,
2019) attribute this decreased $\rm Ly\alpha/H\alpha$ to radiative transfer
effects combined with aperture losses. Combining this empirical $\rm
Ly\alpha/[O\,II]$ ratio with the modeled $\rm H\alpha/[O\,II]$ we expect
$\log({\rm Ly\alpha/[O\,II]})\approx 0.25-1.4$ assuming AGN photoionization,
$\rm Ly\alpha$ emission arising primarily from recombination with moderate
scattering, and modest metal enrichment levels. The expected $\rm
Ly\alpha/[O\,II]$ ratios are shown as a function of $\rm[O\,III]/[O\,II]$ in
the right axis of Figure 4. The presence of dust would only increase the
strength of [O II] relative to Ly$\alpha$.

Figure 4: Expected line ratios of H$\alpha$/[O II] as a function of [O III]/[O
II] ratio for dust-free AGN photoionized gas of $Z=0.25Z_{\odot}$ (black
lines) and with metallicities of $Z=0.1-1.0Z_{\odot}$ (faded red band). The
gas metallicity is chosen to be consistent with typical metallicities of
absorbing CGM around quasars at $z=2-4$ (Lau et al., 2016) and luminous red
galaxies at $z=0.4$ (Zahedy et al., 2019). The two lines show the predicted
line ratio for ionizing spectrum power-law slopes of $\alpha=-1.4$ and $-2.0$
in solid and dashed black line for reference. The right axis is scaled to
$3.7\times$H$\alpha$/[O II] to approximate the expected $\rm Ly\alpha$/[O II]
ratio under the assumption that the Ly$\alpha$ emission is primarily from
recombination with modest scattering (see discussion in Langen et al., 2023).
Observations of ionized nebulae around quasars at $z=0.4-0.7$ where the [O
III] to [O II] ratio is constrained with optical data indicate that the line
ratios range from $\log\rm[O\,III]/[O\,II]=-1$ to $1$. Figure 5: left panel:
Comparison of the sizes of [O II] emitting nebulae discovered around CUBS and
MUSEQuBES quasars at $z=0.4-1.4$ from this work with Ly$\alpha$ nebulae around
$z\approx 3$ quasars Borisova et al. (2016); Mackenzie et al. (2021). The full
CUBS and MUSEQuBES [O II] sample is shown in black, while the sub-sample of
radio-quiet systems is shown in solid grey histogram. For the $z\approx 3$
Ly$\alpha$ nebulae, the size distribution of luminous quasars from Borisova et
al. (2016) is shown in red dashed line while that of less luminous quasars
from Mackenzie et al. (2021) is shown in orange dash-dotted line. The [O II]
nebulae around $z=0.4-1.4$ quasars are substantially smaller than Ly$\alpha$
nebulae at $z\approx 3$ regardless of radio properties. right panel:
Comparison of the luminosities of the CUBS and MUSEQuBES quasars with the
$z=3$ samples from Borisova et al. (2016) and Mackenzie et al. (2021). The [O
II] nebulae discovered around the CUBS and MUSEQuBES quasars are smaller and
more rare than the Ly$\alpha$ nebulae discovered around $z\approx 3$ quasars
Borisova et al. (2016) and low-luminosity quasars Mackenzie et al. (2021) even
though the luminosities of the CUBS and MUSEQuBES quasars fall within the
range spanned by the two $z=3$ samples.

Surveys of $\rm Ly\alpha$ emission around quasars at $\langle z\rangle\approx
3.3$ such as Borisova et al. (2016) and Mackenzie et al. (2021) achieve
observed-frame surface brightness limits similar to the ones enabled by the
CUBS and MUSEQuBES observations. However, the difference in surface brightness
dimming at the mean redshift of the Borisova et al. (2016) and Mackenzie et
al. (2021) studies versus $\langle z\rangle=0.95$ in this work introduces a
factor of $(1+3.3)^{4}/(1+0.95)^{4}\approx 24$ difference in rest-frame
surface brightness sensitivity. This factor of 24 difference in surface
brightness dimming is equivalent to $1.4$ dex, compensating for the
expectation that [O II] will be a weaker line than Ly$\alpha$. Therefore, if
the cool, $\rm Ly\alpha$ emitting CGM observed around quasars primarily arise
from recombination radiation with only modest scattering, if the quantities of
cool, emitting CGM around quasars do not evolve significantly between
$z\approx 3$ and $z\approx 1$, if CGM metallicities around $z\approx 1$
quasars are not substantially lower than expected from absorption surveys, and
if quasar episodic lifetimes are not smaller at intermediate redshift compared
to $z\approx 3$, we expect that the [O II] nebulae observed in this work would
be similar in size and incidence rate to $\rm Ly\alpha$ at higher redshift.

In contrast to this expectation, the [O II] emitting nebulae found around CUBS
and MUSEQuBES quasars are smaller and less common than their Ly$\alpha$
counterparts at $z\approx 3$ as shown in the left panel Figure 5. In
particular, while $100$ kpc-scale Ly$\alpha$ nebulae are fairly ubiquitous at
$z=3$, only seven of the thirty ($23^{+9}_{-6}\%$) of the CUBS and MUSEQuBES
quasars exhibit such large [O II] nebulae. The smaller sizes of the [O II]
nebulae at $z\approx 1$ are even starker when considered relative to the
estimated virial radii of the quasar host halos, which are $\approx 120$ kpc
at $z\approx 3$ (Shen et al., 2007) compared to $\approx 500$ kpc for UV
luminous quasars at $z\approx 1$ (Li et al., 2024). The fact that [O II]
emitting nebulae are smaller and less common around $z\approx 1$ quasars than
Ly$\alpha$ nebulae around $z=3$ ones can be explained if a substantial
fraction Ly$\alpha$ in the outer regions of the $z=3$ nebulae arise from
scattered Ly$\alpha$ photons in a more neutral phase of the CGM that does not
emit in [O II]. The contrast could also be explained if there is substantial
evolution in the quantities or density distribution of the cool, dense phase
of halo gas around quasars between $z\approx 3$ and $z\approx 1$ or if the
metallicities of the CGM of $z\approx 1$ quasar hosts on $\approx 50$ kpc
scales are substantially lower than $0.25Z_{\odot}$. We note, however, that
the $Z\approx 0.25Z_{\odot}$ metallicity used to estimate the [O II] to
Ly$\alpha$ ratio in Figure 4 is based on absorption measurements in massive
halos at distances of $d\approx 50-100$ kpc (e.g., Lau et al., 2016; Zahedy et
al., 2019), similar to the size-scale of the Ly$\alpha$ nebulae. Further,
Johnson et al. (2015) found little evidence for evolution in Mg II covering
factor observed in the CGM of quasars over this same cosmic period.
Alternatively, the difference in size and incidence of Ly$\alpha$ nebulae at
$z\approx 3$ and [O II] emitting nebulae at $z\approx 1$ could also be
explained if luminous quasar episodic lifetimes at $z\approx 1$ are shorter
than at $z\approx 2-3$. Further investigation of the differences in optically-
emitting [O II] nebulae vs UV emitting Ly$\alpha$ nebulae around quasars
requires multi-wavelength observations at both redshifts.

##  6 Summary and conclusions

In this paper, we presented the first comprehensive search for non-resonant,
optically emitting [O II] nebulae around $30$ UV luminous quasars at
$z=0.4-1.4$ using deep integral field spectroscopy acquired as part of the
Cosmic Ultraviolet Baryons Survey and MUSE Quasar Blind Emitters Survey. Our
findings indicate that $>60$ kpc-scale circumgalactic nebulae are common
around UV luminous quasars, with approximately half of the sample exhibiting
such large-scale emission while $\approx 80-90\%$ exhibit emission on $>20-30$
kpc scales. Within the CUBS and MUSEQuBES quasar samples, the presence and
size of the [O II] nebulae are not correlated with quasar redshift,
luminosity, supermassive black hole mass, or radio-loudness. On the other
hand, the sizes and surface brightnesses of the [O II] nebulae are tentatively
correlated ($2.5\sigma$ significance) with the rest-frame [O II] equivalent
width detected in the quasar spectra due to circumnuclear and ISM-scale
ionized gas. Further investigation of the relationship between quasars and
their surrounding cool gas supplies will require larger samples extending over
a broader range of quasar properties, particularly to lower luminosities and
black hole masses.

The [O II] emitting nebulae around quasars at $z\approx 1$ found in this work
are less common and smaller on average than Ly$\alpha$ emitting nebulae found
around quasars at $z>2$. This difference can be explained, for example, if the
dense, cool phase of the CGM around quasars evolves significantly with
redshift, if the metallicities in quasar-host CGM on $\approx 50$ kpc scales
are lower than expected from absorption surveys, if quasar episodic lifetimes
are shorter at $z\approx 1$ compared to $z\approx 3$, or if the scattering of
Ly$\alpha$ photons off of more neutral gas contributes non-negligibly to the
outer regions of Ly$\alpha$ halos around quasars at $z=2-3$. These scenarios
can be tested by observing the nebulae with broader spectral coverage using
upcoming near-IR IFS such as MIRMOS (Konidaris et al., 2020) and HARMONI
(Thatte et al., 2021), or with new UV space-based instrumentation targeting
low-$z$ systems in H I Ly$\alpha$.

## Acknowledgements

We thank the anonymous referee for constructive comments and suggestions that
significantly strengthened this paper. We are grateful to Eric Bell and Kayhan
Gültekin for insightful discussions which helped shape interpretation of the
results. SDJ acknowledges partial support from HST-GO-15280.009-A, HST-
GO-15298.007-A, HST-GO-15655.018-A, and HST-GO-15935.021-A. JIL is supported
by the Eric and Wendy Schmidt AI in Science Postdoctoral Fellowship, a Schmidt
Futures program. Based on observations from the European Organization for
Astronomical Research in the Southern Hemisphere under ESO. The paper made use
of the NASA/IPAC Extragalactic Database, the NASA Astrophysics Data System,
Astropy (Astropy Collaboration et al., 2018), and Aplpy (Robitaille &
Bressert, 2012).

## Data Availability

The raw MUSE data, associated calibrations, and ESO reduced datacubes are
publicly available in the ESO archive.

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