# Luck of the Irish? A companion of the Cloverleaf connected by a bridge of
molecular gas

H. R. Stacey1 and F. Arrigoni Battaia1
1Max Planck Institute for Astrophysics, Karl-Schwarzschild Str. 1, D-85748
Garching bei München, Germany
E-mail<EMAIL_ADDRESS>
(Accepted 2022 September 1. Received 2022 August 31; in original form 2022
July 27)

###### Abstract

We present deep observations of CO (3–2) from the Cloverleaf lensed quasar-
starburst at $z=2.56$. We discover a 4–5 times less massive companion at a
projected distance of 33 kpc from the Cloverleaf host galaxy. The galaxies are
connected by a bridge of CO emission, indicating that they are interacting and
that the companion is being stripped by the Cloverleaf. We also find evidence
for fast molecular gas in the spectral line of the Cloverleaf that may be an
outflow induced by stellar or quasar feedback. All of these features may be
ubiquitous among quasars and only detected here with the help of gravitational
lensing and the sensitivity of the data. Overall, these findings agree with
galaxy formation scenarios that predict gas-rich mergers play a key role in
quasar triggering, starburst triggering and the formation of compact
spheroids.

###### keywords:

galaxies: interactions – galaxies: evolution – submillimetre: ISM – quasars:
general – gravitational lensing: strong

††pubyear: 2022††pagerange: Luck of the Irish? A companion of the Cloverleaf
connected by a bridge of molecular gas–Luck of the Irish? A companion of the
Cloverleaf connected by a bridge of molecular gas

##  1 Introduction

Hierarchical galaxy formation models predict massive elliptical galaxies
formed as a result of gas-rich mergers (Kauffmann & Haehnelt, 2000; Hopkins et
al., 2008a, b). The very high central stellar densities of elliptical galaxies
can be produced if gas is funnelled into the centre of the galaxy faster than
the star formation rate, generating a compact starburst and rapid stellar
growth (Mihos & Hernquist, 1996; Valentino et al., 2020). At the same time,
rapid accretion of gas onto the central supermassive black hole creates an
active galactic nucleus (AGN) that injects energy and momentum into the
interstellar medium in the form of radiation pressure (Fabian et al., 2008),
relativistic winds (King, 2010) or radio jets (Mukherjee et al., 2018). These
processes can explain the scaling relations between galaxy properties which
seem to already be in place by $z\sim 2$ (Förster Schreiber & Wuyts, 2020).

Hierarchical models naturally predict that quasars (the most energetic AGN)
have many companion galaxies, the number and properties of which can probe the
strength of feedback on the surrounding environment (Costa et al., 2019).
Previous studies have found that some quasars at $z>2$ have dust-obscured
companions (Decarli et al., 2017; Trakhtenbrot et al., 2017; Banerji et al.,
2018; Neeleman et al., 2019; Chen et al., 2021; Bischetti et al., 2021). Sub-
mm observations of dust-continuum of $z\sim 2$ quasars suggests that they
reside in over-densities, although most studies lack spectroscopic information
to confirm a physical association (Silva et al., 2015; Hatziminaoglou et al.,
2018). Depending on the properties of the companions, they may be faint in UV
and/or the sub-mm emission, and only detected in spectral line emission (Drake
et al., 2020).

If quasars are triggered as a result of mergers, we should expect to see the
imprint of the galaxy interactions on the cold interstellar medium of their
host galaxies. Neeleman et al. (2019) and Drake et al. (2020) found clear
evidence for this by detecting diffuse [CII] emission between a quasar and its
dusty starburst companion at $z>4$. Simulations suggest we should also expect
to observe the effect of AGN feedback in the form of multi-phase outflows
(Costa et al., 2020). The molecular gas kinematics of quasar host galaxies
have not been studied in detail at $z\sim 2\\--4$ beyond resolving apparently
coherent velocity structure (Stacey et al., 2021; Banerji et al., 2021),
although Feruglio et al. (2017) find evidence for a component of fast
molecular gas in the CO (4–3) line emission from gravitationally lensed hyper-
luminous quasar-starburst APM 08279+5255 at $z=4$, and Spingola et al. (2020)
found disturbed CO (1–0) gas kinematics for two lensed quasar hosts at $z=2$
and 3. Furthermore, Stacey et al. (2022) detect quasar-driven molecular
outflows from dust-reddened quasars, but not unobscured quasars, suggesting
that they may only be detected in a molecular phase for a fraction of the
quasar lifetime.

We have tested these predictions by analysing deep CO (3–2) observations of
the Cloverleaf (H 1413$+$117), a quasar at $z=2.56$ with an obscured host
galaxy that is gravitationally lensed by an intervening massive elliptical
galaxy (Magain et al., 1988; Riechers et al., 2011b; Stacey et al., 2021). The
galaxy contains an Eddington-limited starburst and the strong X-ray radiation
field may contribute to the molecular gas heating (Bradford et al., 2009;
Riechers et al., 2011a; Uzgil et al., 2016). In Section 2, we discuss the
observations and data reduction. In Section 3, we detail our analysis of the
line profile and spectral cube. Finally, in Section 4, we consider
implications for hierarchical formation models and future work.

##  2 Data

The Cloverleaf was observed with ALMA under project code 2017.1.01232.S (PI:
Nishimura) in 9 epochs on 21 Dec 2017, 29 Dec 2017, 9 Jan 2018 and 11 Jan
2018. The observations were taken in an antenna configuration with maximum
baseline 2.5 km. The data were correlated in both linear polarisations (XX,YY)
in four spectral windows, with one spectral window centred on the redshifted
rest-frequency of the CO (3–2) line (97.19267587 GHz).

The calibrated visibilities were produced using the ALMA pipeline in CASA
(McMullin et al., 2007). The data were inspected to confirm the quality of the
calibration and that no further flagging was required. The spectral line data
were prepared by subtracting the continuum from the visibility data using the
task $uvcontsub$ to fit a model to the line-free spectral windows. The line
profile of the Cloverleaf and its companion are shown in Fig. 1, each
extracted from a circular aperture around the object to maximise signal-to-
noise. Moment maps of the deconvolved image cubes are shown in Fig. 2 with
natural weighting of the visibilities. We measure an off-source rms noise of
$1\times 10^{-4}~{}{\rm Jy\,km\,s^{-1}\,beam^{-1}}$ in the image cube for a
spectral resolution of 15 km s-1 with natural weighting. The line emission
integrated around the line velocity of the companion with $uv$-taper
equivalent to 0.7 arcsec is shown in Fig. 3.

Figure 1: Line profiles of the Cloverleaf (top) and the companion (bottom)
with 15 km s-1 spectral resolution. The spectral line emission from Cloverleaf
is fit by three Gaussian components, including a broad line of FWHM $740\pm
70$ km s-1. The assumed systemic redshift is $z=2.55784$ from (Stacey et al.,
2021).

Figure 2: Moment maps of CO (3–2) emission from the Cloverleaf and its neighbour (four arcsec to the South-East). Top row is naturally weighted imaging (0.5 arcsec resolution); bottom row is with a $uv$ taper equivalent to 1 arcsec (1.2 arcsec angular resolution). Panels show velocity integrated line intensity, velocity field, velocity dispersion. The contours on the moment 0 image are signal-to-noise of $-3$, 3, 6, 12, 24.. etc. The contours in the moment 1 and 2 are steps of 100 km s-1. The synthesised beam effective FWHM is shown in the bottom-left corner.  Figure 3: The CO (3–2) line emission at 1 arcsec angular resolution, integrated over the velocity channels of the companion line profile (Fig. 1) and normalised to the rms noise. The contours are 334 GHz continuum surface brightness at 0.3 and 3 mJy beam-1, where the third object (6 arcsec North) is also seen. The synthesised beams are shown in the lower-left corner. Table 1: CO line properties for the two sources. For the calculation of the molecular gas mass of the Cloverleaf, we assume the average magnification of the CO (3–2) is $\mu=11$ as was found for CO (9–8) reported by Stacey et al. (2021). All other given properties are uncorrected for lensing magnification. The total line emission is measured directly from the moment 0 image (hence, the smaller uncertainties). The central velocity of the Gaussian components are relative to the systemic velocity reported by Stacey et al. (2021). |  | $v_{\rm CO}$ | FWHM | $I_{\rm CO}$ | $\mu L_{\rm CO}$ | $\mu L^{{}^{\prime}}_{\rm CO}$ | $M_{\rm mol}$
---|---|---|---|---|---|---|---
|  | ( km s-1) | ( km s-1) | ( Jy km s-1 beam-1) | ( L⊙) | ( K km s-1 pc2) | ( M⊙)
Cloverleaf | component 1 | $110\pm 20$ | $320\pm 30$ | $7.5\pm 1.2$ | $3.5(\pm 0.6)\times 10^{8}$ | $2.6(\pm 0.4)\times 10^{11}$ | $1.9(\pm 0.3)\times 10^{10}$
component 2 | $-100\pm 20$ | $280\pm 20$ | $5.9\pm 1.3$ | $2.8(\pm 0.6)\times 10^{8}$ | $2.1(\pm 0.5)\times 10^{11}$ | $1.5(\pm 0.3)\times 10^{10}$
component 3 | $-110\pm 40$ | $740\pm 70$ | $3.1\pm 0.8$ | $1.5(\pm 0.4)\times 10^{8}$ | $1.1(\pm 0.3)\times 10^{11}$ | $0.8(\pm 0.2)\times 10^{10}$
| total |  |  | $15.8\pm 0.1$ | $7.4(\pm 0.1)\times 10^{8}$ | $5.0(\pm 0.1)\times 10^{11}$ | $4.0(\pm 0.1)\times 10^{10}$
Companion |  | $34\pm 6$ | $200\pm 15$ | $0.31\pm 0.04$ | $1.5(\pm 0.2)\times 10^{7}$ | $1.1(\pm 0.1)\times 10^{10}$ | $0.9(\pm 0.1)\times 10^{10}$

##  3 Results

We clearly detect a companion in the CO (3–2) line emission at a distance of 4
arcsec ($\approx 33$ kpc) South-East of the Cloverleaf (J2000 RA: 14:15:46.49,
Dec: 11.29.41.8). The gravitational lensing magnification at this projected
distance is negligible. The companion is close to the systemic velocity of the
Cloverleaf in CO (3–2) and CO (9–8) line emission ($z=2.558$; Stacey et al.
2021), slightly redshifted by $34\pm 6$ km s-1, although this is within
uncertainties of the Cloverleaf systemic velocity. The companion has an
integrated line intensity of $0.31\pm 0.04$ Jy km s-1 beam-1, relating to a
luminosity of $L^{\prime}_{\rm CO}=1.1(\pm 0.1)\times 10^{10}~{}{\rm K\ km\
s^{-1}\ pc^{2}}$. Assuming a conversion factor of $0.8\,{\rm(K\ km\ s^{-1}\
pc^{2})^{-1}}$, typically adopted for high-redshift starbursts (Greve et al.,
2014), this companion galaxy has a molecular gas reservoir of $0.9(\pm
0.1)\times 10^{9}$ M⊙. The galaxy is marginally resolved in the direction of
the Cloverleaf.

In contrast, the Cloverleaf has an integrated line luminosity of $15.8(\pm
0.1)$ Jy km s-1 beam-1. Assuming a magnification of $\approx 11$, as found for
both the CO (9–8) and sub-mm dust emission by Stacey et al. (2021), we infer a
molecular gas reservoir of $4.0(\pm 0.1)\times 10^{10}$ M⊙, suggesting that
the companion is 4–5 times less massive than the Cloverleaf from the
perspective of its molecular gas content (Table 1). The molecular gas mass we
infer for the Cloverleaf is larger than the $3.1\times 10^{10}$ M⊙ found by
Riechers et al. (2011b) with large-velocity gradient modelling of the CO
spectral line energy distribution (although no uncertainties were given) and
consistent with the 0.2–$5\times 10^{10}$ M⊙ inferred by Bradford et al.
(2009) with a similar approach.

We searched for evidence of the companion in archival ALMA observations of the
Cloverleaf and detect it at $0.5\pm 0.1$ mJy in 334 GHz (rest-frame 250
$\upmu$m) continuum (2012.1.00175.S; PI: van der Werf) where it is unresolved
with $\approx 0.2$ arcsec angular resolution. The companion is about 6 times
fainter than the intrinsic flux density of the Cloverleaf at similar
frequencies. We did not detect the companion in other CO lines, H2O or [CI]
(1–0), probably due to the sensitivity of the observations. Although we note
that there is a slight extension towards the location of the companion in low-
angular-resolution observations of [CI] (2–1) by Weiß et al. (2003). We did
not find any association for the companion in R, I or H-band HST imaging,
suggesting that the companion is also dust-obscured.

We resolve an extension of molecular gas connecting the Cloverleaf and its
companion at the systemic velocities of the companion. This can be seen
clearly in the moment maps where we have applied a $uv$-taper to increase the
surface brightness sensitivity (Fig. 3). This intergalactic bridge indicates
that gas is being stripped by ram pressure or a tidal interaction. The gas in
the companion and in the bridge is close to the systemic velocity, suggesting
that the direction of motion is perpendicular to the line-of-sight.

Another neighbouring galaxy was detected 6 arcsec North of the Cloverleaf in
dust emission with ALMA by Stacey et al. (2021) (see also Fig. 3). No spectral
line emission was detected in either CO (9–8) or CO (3–2) in any spectral
window, although it is much brighter in continuum emission than the close
companion, suggesting that this third object is not physically associated with
the Cloverleaf. We searched for other close companions to the Cloverleaf
within $\pm 1000$ km s-1 within the primary beam ($\approx 500$ kpc radius)
using the publicly available code LineSeeker (González-López et al., 2017),
requiring any detection to have a FWHM $<1000$ km s-1 and a signal-to-noise of
$>3$ in three adjacent channels (equivalent to $L^{\prime}_{\rm CO}\geq
10^{8.9}$ K km s-1 pc2). We did not find evidence for additional companions.

The best fit to the line profile from the Cloverleaf comprises three Gaussian
components, two of which could be consistent with gas in a disc, and an
additional broad component with a FWHM of $740\pm 70$ km s-1. The integrated
line emission between $-750$ and $-450$ km s-1 is show in Fig. 4. The location
of this excess blue emission suggests is located within the quasar host galaxy
but not coincident with the quasar (see also Figs. 2d and 4d by Stacey et al.
2021). This could be an outflow driven by star formation (Ginolfi et al.,
2020) or quasar feedback (Stacey et al., 2022). It may also be strongly
differentially lensed. A similar feature was detected in the CO (4–3) line
profile of the hyper-luminous strongly lensed quasar APM 08279+5255 (Feruglio
et al., 2017) where it is also unclear whether the feature is due to
differential lensing. The Gaussian fit suggests that the mass in the possible
outflow is about 20 percent of the molecular gas mass of the Cloverleaf and
comparable to the gas mass of the companion (Table 1), although we stress that
this is highly dependent on the CO–H2 conversion factor and local lensing
magnification.

Figure 4: The CO (3–2) line emission from the Cloverleaf integrated between
$-750$ and $-400$ km s-1with natural weighting. The contours are the total
integrated line emission, as in Fig. 2.

##  4 Discussion and conclusions

###  4.1 The companion

Cosmological hydro-dynamical simulations and semi-analytic models predict that
gas-rich mergers play a key role in forming elliptical galaxies by causing
cold gas to lose angular momentum, triggering a compact starburst (Bois et
al., 2011; Duc et al., 2011; Naab et al., 2014; Mihos & Hernquist, 1996;
Hopkins et al., 2008a). Observations of $z>2$ quasars are broadly consistent
with this model, being located within galaxy over-densities (Decarli et al.,
2017; Drake et al., 2020) some of which have disturbed gas kinematics (Litke
et al., 2019; Spingola et al., 2020; Neeleman et al., 2021). Here, we find
clear evidence for an ongoing interaction and displacement of cold gas in a
quasar-starburst, in agreement with a merger-triggering scenario. Our results
suggest Cloverleaf and its companion are likely hosted in a massive dark
matter halo (Kauffmann & Haehnelt, 2002; Costa et al., 2014).

Surveys of quasar and blank fields do not reach the limiting CO (3–2)
luminosity of these data (Decarli et al., 2017). While not much can be
inferred from our single companion detection due to Poisson uncertainties, we
can expect statistical observations with similar quality to give strong
constraints on quasar environments at $z\sim 2\\--3$.

###  4.2 The bridge

An outflow is unlikely to explain the presence of the bridge that connects the
Cloveleaf to its companion. As the velocities in the bridge are close to the
systemic velocity of the galaxies with relatively small velocity dispersion
(Fig. 2), it is not consistent with what is typically observed for molecular
outflows from quasars (e.g. Feruglio et al. 2010). An outflow must have a very
narrow opening angle that is coincidentally directed towards the companion, in
the plane of the sky.

Another explanation for the presence of the intergalactic bridge is that gas
in the less massive companion is being stripped during the interaction with
the Cloverleaf or the intergalactic medium. Tidal interactions and ram-
pressure stripping are potential mechanisms. Merger tails and ram-pressure-
stripped gas have been found to be dynamically cold with velocities similar to
the parent galaxy (Hibbard & van Gorkom, 1996; Wild et al., 2014), but may be
dynamically hot at larger distances ($>20$ kpc) where it transitions from a
laminar to turbulent flow (Fumagalli et al., 2014): the low velocity
dispersion in the bridge suggests that the gas from the companion is
dynamically cold. Following Mo et al. (2010), we estimate the tidal stripping
radius assuming the impulse approximation to be $\sim 1$ kpc using the ratio
of gas masses and projected distance from the Cloverleaf, suggesting that
tidal stripping is ongoing. We see no evidence of an extended tidal tail on
the opposite side of the companion that is expected due to conservation of
angular momentum, although this could be because of its very low surface
brightness. Adopting the formula to estimate ram-pressure stripping from
Domainko et al. (2006), we compute the stripping radius assuming a
perpendicular velocity equivalent to the relative systemic velocity of the
companion, and a stellar mass of $6\times 10^{9}$ M⊙ computed from the
difference between its dynamical mass (following Neeleman et al. 2021) and gas
mass. Assuming a halo density of $10^{-3}~{}{\rm cm^{-3}}$ from Nelson et al.
(2016), we find a ram-pressure-stripping radius of $\sim 10$ kpc. This
suggests ram pressure stripping is unlikely to be responsible for the
molecular bridge unless the perpendicular velocity of the companion is $\sim
10$ times larger111This would be a more realistic relative velocity and mean
that most of the companion’s motion is on the plane of the sky. But for a
velocity of $\sim 100$ times larger, the companion would not be
gravitationally bound to the halo and the halo density is $\gtrsim 100$ times
larger than found in hydrodynamical simulations. However, we stress that these
are probably no better than order-of-magnitude estimates. Detection of the
Sunyaev-Zeldovich effect (Sunyaev & Zeldovich, 1972) and/or X-ray radiating
hot gas could test whether a halo, with sufficiently high temperature and
density for ram-pressure stripping to occur, is already established around the
Cloverleaf.

Observations of tidal stripping in galaxy interactions in the local Universe
suggest that the stripped gas originates in the parent galaxy (Lisenfeld et
al., 2004, 2008). However, observations of low redshift galaxies that are
being stripped by cluster environments and tidal interactions show evidence
for molecular gas formation (Moretti et al., 2020) and star formation (de
Mello et al., 2008; Fumagalli et al., 2011; Arrigoni Battaia et al., 2012;
Fossati et al., 2016) in the stripped gas, which could also be responsible for
the CO (3–2) emission.

###  4.3 The fast molecular gas

The fast molecular gas in the Cloverleaf system could have a number of
origins, including turbulent ram-pressure-stripped gas (Fumagalli et al.,
2014), feedback from star formation or AGN feedback. There are multiple AGN
feedback mechanism that appear be ongoing within the galaxy: radiation
pressure (Stacey et al., 2022), relativistic winds (Chartas et al., 2007) and
radio jets (Peters et al. in prep), as well as an Eddington-limited starburst
(Bradford et al., 2009; Stacey et al., 2021), so both AGN and star formation
feedback are highly plausible. Future work will involve detailed lensing-
kinematic modelling (e.g. Spingola et al. 2020; Stacey et al. 2021; Rizzo et
al. 2021) to resolve the gas kinematics in the Cloverleaf host galaxy and
diagnose the nature of the high-velocity molecular gas.

Finally, we emphasise that discoveries reported in this letter are only
possible due to the combination of gravitational lensing and deep ALMA
observations. If the Cloverleaf were not lensed, it would have to be observed
for $\approx 120$ times as long ($\approx 1000$ hrs) to achieve comparable
signal-to-noise. Therefore, the fast molecular gas (and perhaps the
intergalactic bridge) may be ubiquitous among quasars at cosmic noon and
beyond, but undetectable with current instruments without the aid of
gravitational lensing. This also raises the possibility that the extended
haloes of cold gas detected around quasars $z>4$ from stacking (Bischetti et
al., 2019; Stanley et al., 2019) are actually detecting these intergalactic
structures and/or companion galaxies. Observations of gravitationally lensed
quasars with comparable quality will be required to test whether the
properties of the Cloverleaf are typical (or if we have been lucky).

## Acknowledgements

We thank Ryan Farber and Adam Schaefer for helpful discussions. Our research
used Astropy, NumPy and Matplotlib packages for Python (Astropy Collaboration
et al., 2013, 2018; Harris et al., 2020; Hunter, 2007). HRS acknowledges
funding from the European Research Council (ERC) under the European Union’s
Horizon 2020 research and innovation programme (LEDA: grant agreement No
758853). We made use of ALMA data with project codes 2017.1.01232.S and
2012.1.00175.S. ALMA is a partnership of ESO (representing its member states),
NSF (USA) and NINS (Japan), together with NRC (Canada), MOST and ASIAA
(Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of
Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ.

## Data Availability

All observations reported in this work are publicly available in the ALMA
archive (https://almascience.eso.org/aq) and all the analysis was performed
with publicly available software. The data sets generated during this study
are available from the corresponding author upon reasonable request.

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