Cold Molecular Gas Halo at z ∼ 6 with ngVLA
Seiji Fujimoto1,2 , Yoshiaki Ono3 , Izumi Takuma4 , Yuichi Harikane3,5 , Kotaro Kohno6,7 , and Masami Ouchi4,3,8
1

Cosmic Dawn Center (DAWN), Jagtvej 128, DK2200 Copenhagen N, Denmark
Niels Bohr Institute, University of Copenhagen, Lyngbyvej 2, DK2100 Copenhagen Ø, Denmark3
Institute for Cosmic Ray Research, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277–8582, Japan
4
National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
5
Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
6
Institute of Astronomy, Graduate School of Science, The University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan
7
Research Center for the Early Universe, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
8
Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa-shi, Chiba,277-8583, Japanfujimoto@nbi.ku.dk
2

Abstract
The Next Generation Very Large Array (ngVLA) will open the window for low-J CO line observations andprovide us with the robust measurement of the total molecular gas mass over the circum-galactic medium (CGM)
scale, where recent ALMA observations unveil the existence of the metal-enriched cold gas halo at z ∼ 6. In thismemo, we present mock observations results for the CGM-scale CO(2-1) halo at z ∼ 6 with ngVLA. We find that
the CO(2-1) halo at z ∼ 6 is sufficiently recovered up to a radius of ∼15-kpc scale with a moderate integration
time, which does not exceed the required integration time to detect the [C II] halo with ALMA more than a factor
of several. Given the limited knowledge of the low-J CO line properties at z > 5 among moderately star-forming−1
galaxies (e.g., star-formation rate <∼ 500 M yr ), it is required to increase the sample size of the low-J CO line
detection and test its realistic feasibility before the era of ngVLA. Bright Lyman-break galaxies and strongly lensed
objects at z > 5 identified in recent surveys are the promising targets to achieve it.
Key words: Galaxy evolution — Circumgalactic medium — High-redshift — Outflow

1.

Introduction

Recent ALMA studies unveil the existence of the cold
gas halo emitting [C II] 158 µm line beyond the inter-stellar
medium (ISM) of the central galaxies and spread over thecircum-galactic medium (CGM; >∼ 5–10 kpc) scale at z ∼ 6(Fujimoto et al. 2019; Fujimoto et al. 2020; Ginolfi et al.
2020; Herrera-Camus et al. 2021) (Figure 1). Given the firstgeneration of galaxies formed in high-density peaks of primordial gas, the existence of the CGM-scale metal-enriched
gas is evidence of the outflow activities in the early universe.
Studying the CGM scale metal-enriched gas in the early universe is thus an important probe to understand the outflow
mechanisms that regulate the early galaxy formation and evolution.
To understand the outflow mechanisms, the outflowing gas
mass is one of the critical quantities. Except for molecularhydrogen, the most abundant molecule, CO, has been commonly used in observations to probe the cold molecular gas(e.g., Carilli & Walter 2013 for a review). Among the rotational transition of CO, the low ground state transition, CO(10) and CO(2-1) traces the most diffuse and massive gas reservoir in and around galaxies, while higher-J transitions probe
sequentially denser environments. The observed frequenciesof these low-J CO lines cannot be covered with the current facilities (e.g., VLA) at z > 3, where the Next Generation Very
Large Array (ngVLA) will open the window for these low-J
CO lines. This indicates that ngVLA will offer us an invalu-

able opportunity to unveil the cold molecular gas view in the
CGM-scale halo and constrain the outflow gas mass even atz ∼ 6 via the low-J CO observations.

10 kpcdust cont.

[CII] 158um

Fig. 1. Extended [C II] halo structure appeared around 18 star-forming(SFR∼10–70 M yr−1 ) galaxies at z = 5 − 7 via the visibility-based
stacking (Fujimoto et al. 2019). The natural-weighted field image ofthe dust continuum and [C II] line are shown in the left and right panel,√
respectively. Contours start at ±2σ and increases by a factor of 2.
The synthesized beam is presented at the bottom left.

In this memo, we investigate the ability of ngVLA to detect the low-J CO halo around star-forming galaxies at z ∼ 6.
Section 2 outlines the simulation setup of mock ngVLA observations for the low-J CO halo at z = 6, and we report themock observation results in Section 3. In Section 4, we discuss
the time efficiency compared with ALMA and what is required
before the era of ngVLA. A summary is presented in Section5.

2[Vol. ,

2.

Simulation setup

2.1.

Model

Target

ISM component

Aside from dusty starburst populations (star-formation rate,−1
SFR >∼ 500 M yr ; e.g., Riechers et al. 2013), there has
been only one and zero reports of the CO(2-1) and CO(1-0) line
detection so far, respectively, among star-forming galaxies atz > 5. The CO(2-1) line has been detected in Hz10 at z = 5.65
with SFR ∼ 100 M yr−1 and the stellar mass Mstar of ∼3 × 1010 M (Pavesi et al. 2018). We thus target the CO(2-1)
line as the first step and adopt the same source redshift and total
CO(2-1) luminosity as Hz10 in the following analysis.
2.2.

2.3.

Observation setup

To obtain mock observation maps, we use the CASA task
SIMOBS by utilizing the ngVLA configuration file1 . We choose

the plain+core configuration with a total of 168 antennas composed of 94 core stations and 74 antennas included in the fivespiral arm pattern. At the observed frequency of the CO(21) line for our target, this configuration achieves the synthesized beam size of 0.00 15 × 0.00 13 in the natural-weighted map.
Because our goal is detecting the diffuse emission extended
over ∼10 kpc scale (∼ 200 at z = 6), there is another option tochoose the 94 core stations alone to target such diffuse emission. However, the resulting synthesized beam size with thecore stations alone is too large (∼ 3” at 30 GHz) to distinguishthe emission from the central galaxy and the halo area at z = 6.
Based on several outputs from test runs, we fix the on-source1

https://ngvla.nrao.edu/page/tools

flux [mJy]

0.2
0.1

Halo component

0.0

re, Halo = 5 kpc

-0.1
-2000

-1000
0
1000
velocity offset [km/s]

6’’ × 6’’

10 kpcFig. 2. Left: CO(2-1) line detection from Hz10 at z = 5.65 (Pavesi
et al. 2019). The CO(2-1) spectrum (yellow) is shown with the normalized [C II] spectrum (blue). The white contour in the inset panel shows
the velocity-integrated CO(2-1) intensity, where the background is thatof the [C II] line. Right: Illustration of our model for the spatial distribution of the CO(2-1) line composed of the central (re,cent = 1 kpc)
and the halo component (re,halo = 5 kpc).

Spatial distribution of CO Halo

Because of the lack of deep observation results that sufficiently provide the spatial distribution of the CO(2-1) line fromthe star-forming galaxies at z > 5, we model the surface brightness profile of the CO(2-1) line from the recent deep [C II] observation results. In Fujimoto et al. (2019), the average of 18
star-forming galaxies at z = 5 − 7 via the ALMA visibilitybased stacking shows the extended [C II] line structure up to aradius of the ∼10-kpc scale, which cannot be explained only bya single Sérsic profile. Alternatively, the two (= ISM + Halo)
component fitting reproduces the surface brightness profile, including the extended structure, very well and suggests that the
average profile has the effective radii of ∼1 kpc and 5 kpc, for
the ISM and Halo component, respectively. We assume that thesurface brightness profile of the CO(2-1) line is the same as the[C II] line that consists of the ISM and Halo components witheffective radii of re,ISM = 1 kpc and re,Halo = 5 kpc, respectively, contributing to the total brightness equally.
We note that it is still unclear that there exists a cold molecular gas halo which is emitting the low-J CO line around thestar-forming galaxies at z ∼ 6, albeit the existence of the [C II]
halo, albeit the little variation of the radial [C II] and CO line
ratio in the Milkey Way (Pineda et al. 2013). To understandhow differently the spatial distributions will be observed if the
CO halo does not exist, we also model another surface brightness profile composed only of the ISM component. In the rightpanel of Figure 2, we show our model for the surface brightness
profile of the CO(2-1) line.

re, ISM = 1 kpc

CO(2-1)

integration time of 4 hours in our mock observations. To reduce the output visibility data size in the routine of the mockobservations, we make the line channels averaged over as one
continuum channel. For the output visibility data, we conduct
TCLEAN with CASA down to the 2σ levels with the automask
mode and standard TCLEAN parameters2 . In addition to thenatural weighting, we also produce lower resolution maps torecover the diffuse emission by applying several uv-taper parameters in the imaging process.
3.

Results

In Figure 3, we summarize the CO(2-1) intensity maps averaged over the line width obtained by the mock observations.
The top and bottom panels present the surface brightness profiles composed of ISM and ISM+Halo components, respectively. From left to right, we show the high to low-resolution
maps by applying different uv-taper parameters. We identify
the Halo component, i.e., the existence of the extended CO(21) structure (bottom panels) that is distinct from the morphology only with the ISM component (top panels) especially in the
uv-tapered maps. The spatial resolution of the original map
without uv-taper is likely too high to clearly identify the extended structure.
To quantitatively evaluate the identification of the Halo component, we also derive the radial profiles of the CO(2-1) line.
In Figure 4, we summarize the radial profile with the beamprofile for each map. We find that all of the maps show theextended structure in the ISM+Halo profile, where the structure is sufficiently recovered up to the radius of ∼15 kpc scale
beyond the errors in the uv-tapered maps with the spatial resolution > 0.00 6. We thus conclude that we can detect the CGM
scale CO halo even from the star-forming galaxy at z ∼ 6 with
ngVLA, although the proper uv-taper is required.
2

https://casaguides.nrao.edu/index.php/Automasking Guide

No. ]

30.’’15 × 0.’’13

0.’’64 × 0.’’60

0.’’25 × 0.’’22

1.’’1 × 0.’’95

5e-05

ISM only

3e-05

ISM + Halo

5e-06

[Jy/beam]

1.5e-05

1.5e-06-5e-07

Fig. 3. Mock CO(2-1) observation maps (600 × 600 ) with ngVLA for Hz10 at z = 5.65 by integrating 4 hours on source with the plain+core configuration.
Top and bottom panels show the surface brightness profiles composed of the ISM component only and the ISM+Halo components (Figure 2), respectively.
The natural weighted maps are shown from left to right without uv-taper, with uv-taper of 0.00 1 × 0.00 1, 0.00 3 × 0.00 3, and 0.00 5 × 0.00 5, respectively.
The black contour starts at 3.5 µJy/beam and increases by a factor of 2. The black ellipse denotes the synthesized beam whose FWHM is presented at the
top of the panel.

natural(0.’’15 × 0.’’13)

uv-tapered(0.’’25 × 0.’’22)

uv-tapered(0.’’64 × 0.’’60)

uv-tapered(1.’’1 × 0.’’95)

ISM only
ISM+Halo
Gaussian of beam

Fig. 4. Radial surface brightness profiles of the CO(2-1) line from the mock observations with ngVLA. The black and red circles indicate the radial
profiles composed of the ISM component only and the ISM+Halo components (Figure 2), respectively. From left to right, the panels are arranged in thesame order as Figure 3. The dashed curve denotes the Gaussian profile of the synthesized beam.

4.
4.1.

Discussion[C II] with ALMA vs. CO(2-1) with ngVLA

Our mock observation results indicate that ngVLA requires
more or less 4 hours on-source integration time to detect theCO(2-1) halo from Hz10-like object with SFR ∼130 M yr−1 .
This indicates that the required on-source integration time canexceed > 10 hours when we target less luminous objects and/or
explore more time-expensive observations such as studying thekinematic of the halo. Although the low-J CO line is nevertheless essential to have a reliable mass estimate for the molecular gas extended over the CGM scale, it is also helpful to understand which is more time-efficient: targeting CO(2-1) with
ngVLA or other bright FIR lines (e.g., [C II] 158 µm) withALMA.

To address which observation is more time-efficient, we also
perform mock observations with ALMA, targeting the [C II]
halo with the same simulation setup. We use the same target
of Hz10 whose [C II] line is also detected in previous studies(Capak et al. 2015). We model the same surface brightness
profile and adopt the configuration C43-3 which has the synthesized beam size of 0.00 68 × 0.00 64 at the observed [C II] frequency in the natural weighting, almost comparable to one of
the uv-tapered maps of ngVLA.
In Figure 5, we show the mock observation results with
ALMA. We find that 1.5-hours on-source integration observations recover the [C II] halo structure up to the radius of ∼10kpc, where the data quality is comparable to the mock observations with ngVLA. The absence of the emission from the radiusof ∼10–15 kpc scale is likely because the less sampling in the4[Vol. ,

short-baseline with ALMA, compared with the core station of
ngVLA. These results ensure the power of ngVLA, allowing
us to detect the faint low-J CO emission in the moderate integration time that does not exceed the required integration timewith ALMA for the [C II] line more than a factor of several.
However, it is also important to keep in mind that the observations for the bright FIR lines such as [C II] with ALMA could
be more time efficient for CGM-scale gas studies around starforming galaxies at z ∼ 6 even in the era of ngVLA, if theobjective of the study does not necessarily rely on the low-J
CO line (e.g., the kinematics of the CGM).
(0.’’64 × 0.’’60)

ISM + Halo

ISM only

ISM only
ISM+Halobeam-0.000027-0.000011-0.000022-0.000011

0.000011

0.000054

0.0001410.000313

0.000661

0.001349

0.002719

0.000141 3e-4
0.000313
0.000661 1.5e-3
0.001349
0.002719
0.000054
0 0.000011 5e-5
[Jy/beam]

Fig. 5. Mock [C II] observation maps (600 × 600 ) with ALMA for Hz10
at z = 5.65 composed of the the ISM component only (left top) andthe ISM+Halo components (left bottom) by integrating 1.5 hours on
source with the C43-3 configuration. In the right panel, the symbols
are assigned in the same manner as Figure 4.

4.2.

Before the era of ngVLA

Studying the low-J CO line for high-redshift star-forming
galaxies will be one of the main streams in the era of ngVLA(e.g., Decarli et al. 2018; Casey et al. 2018; Carilli & Shao2018; Emonts et al. 2018). In this context, it is important toenlarge the sample with the low-J CO line detection at leastfrom the ISM component and understanding the basic low-J
CO line properties and its realistic feasibility to further explore
CGM-scale low-J CO studies in the era of ngVLA.
So far, the CO(2-1) line has been detected only from Hz10
with the deep VLA observations, and none of the CO(1-0) lines
has been detected among star-forming galaxies at z > 5, except−1
for the dusty starburst population (SFR >∼ 500 M yr ; e.g.,
Riechers et al. 2013). The recent deep and wide optical survey with Subaru Hyper Sprime-Cam (HSC) has been identified> 10,000 of Lyman-break galaxies (LBGs) at z ∼ 5–7 (Onoet al. 2018) that are comparably bright as Hz10, and follow-up
observations successfully detect bright FIR lines of [C II] and[O III] (Harikane et al. 2020). These bright HSC LBGs would
be prominent targets to dramatically enlarge the sample size of
the CO(2-1) and CO(1-0) line detection and investigate the feasibility of the low-J CO lines among the star-forming galaxies
at z > 5.
We caution that the results obtained from these bright ob-

jects might be biased. In fact, the rest-frame ultra-violet (UV)luminosity of Hz10 is ∼ 5 times larger than the characteristic∗
UV luminosity at z = 6 (MUV
= −20.91; Ono et al. 2018). To
reduce the bias, another solution is the strongly lensed objects.
Fujimoto et al. 2021 report the uniquely bright [C II] line detection from a strongly lensed and multiply imaged sub-L∗ galaxy
at z = 6.072 with SFR ∼ 5 M yr−1 and Mstar ∼ 1 × 109 M .
Owing to the strong gravitational lens effect, the observed
UV and [C II] line luminosities are the brightest (mUV = 23.7
magAB , L[CII] = 4.5 × 109 L ) among the star-forming galaxies at z > 6 known to date. Khullar et al. 2021 also report theidentification of a remarkably bright (mUV = 20.47 magAB )lensed galaxy at z = 5.04. These bright properties of strongly
lensed galaxies will allow us to explore unprecedented faintlines, including the low-J CO lines, even before the era ofngVLA with the less biased view.
An important notice is that the diffuse emission is sufferedfrom the cosmic microwave background (CMB) at such highredshifts like z > 5 (e.g., da Cunha et al. 2013; Zhang et al.
2016), but it depends on the ISM properties, including the gas
density and kinematic temperature that could be constrained
via Large velocity gradient (LVG) modeling for the CO spectral line energy distribution (e.g., Weiss et al. 2007). With thecurrent facilities, the multiple CO lines from low to high-J canbe obtained with the combination of e.g., VLA and ALMA forstar-forming galaxies at z ∼ 6. The bright LBGs and strongly
lensed galaxies at z > 5 will also be helpful to constrain these
ISM parameters to understand how significantly the CMB affects the emissivity of the low-J CO line before the era ofngVLA.
5.

Summary

In this memo, we study the ability of ngVLA to detect the
low-J CO halo around star-forming galaxies at z ∼ 6 via realistic mock observations. Based on the mock observation results,
we also discuss the time efficiency compared to the [C II] observations with ALMA and what we should do before the eraof ngVLA. The major findings are summarized below:
1. The CGM (radius ∼ 10–15 kpc) scale CO(2-1) halo isclearly recovered and distinguished from the ISM-scale
emission in the proper uv-tapered maps obtained withthe plain+core configuration and 4 hours on-source integration, when we assume the CO(2-1) line properties of
Hz10 with SFR ∼ 130 M yr−1 at z = 5.65.
2. When we assume the CO(2-1) and [C II] line properties
of Hz10, we find that the [C II] line observations with
ALMA is more time efficient to detect the emission fromthe halo by a factor of ∼2–3.
3. To maximize outcomes in the era of ngVLA based onan enlarged sample with the low-J CO line detection
among star-forming galaxies at z > 5, it is promising to carry out deep CO observations towards bright
HSC LBGs (e.g., Ono et al. 2018; Harikane et al.
2020) and strongly lensed galaxies (e.g., Fujimoto et al.
2021; Khullar et al. 2021) at z > 5 with the current facilities such as VLA and ALMA, before the era of ngVLA.

No. ]
6.

Acknowledgements

This work was completed with the generous support of the
National Astronomical Observatory of Japan.
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