# Evolution of cold gas at $2<z<5$: a blind search for H i and OH absorption
lines towards mid-infrared color selected radio-loud AGNs

N. Gupta  Inter-University Centre for Astronomy and Astrophysics, Post Bag 4,
Ganeshkhind, Pune 411 007, India  R. Srianand  Inter-University Centre for
Astronomy and Astrophysics, Post Bag 4, Ganeshkhind, Pune 411 007, India  G.
Shukla  Inter-University Centre for Astronomy and Astrophysics, Post Bag 4,
Ganeshkhind, Pune 411 007, India  J-.K. Krogager  Institut d’astrophysique de
Paris, UMR 7095, CNRS-SU, 98bis bd Arago, 75014 Paris, France  Department of
Astronomy, University of Geneva, Chemin Pegasi 51, 1290 Versoix, Switzerland
P. Noterdaeme  Institut d’astrophysique de Paris, UMR 7095, CNRS-SU, 98bis bd
Arago, 75014 Paris, France  F. Combes  Observatoire de Paris, Collège de
France, PSL University, Sorbonne University, CNRS, LERMA, Paris, France  R.
Dutta  Dipartimento di Fisica G. Occhialini, Università degli Studi di Milano-
Bicocca, Piazza della Scienza 3, 20126 Milano, Italy  J. P. U. Fynbo  Cosmic
Dawn Center (DAWN), University of Copenhagen, Jagtvej 128, DK-2200, Copenhagen
N, Denmark  Niels Bohr Institute, University of Copenhagen, Jagtvej 128,
DK-2200, Copenhagen N, Denmark  M. Hilton  Astrophysics Research Centre and
School of Mathematics, Statistics and Computer Science, UKZN, Durban 4041,
South Africa  E. Momjian  National Radio Astronomy Observatory, Socorro, NM
87801, USA  K. Moodley  Astrophysics Research Centre and School of
Mathematics, Statistics and Computer Science, UKZN, Durban 4041, South Africa
P. Petitjean  Institut d’astrophysique de Paris, UMR 7095, CNRS-SU, 98bis bd
Arago, 75014 Paris, France

###### Abstract

We present results from a spectroscopically blind search for H i 21-cm and OH
18-cm absorption lines at $2\leq z\leq 5$ towards 88 AGNs with 1.4 GHz
spectral luminosity of 27 - 29.3 W Hz-1. One 21-cm absorber, which is
associated with M1540$-$1453 at $z_{\rm abs}$$=$ 2.1139, is detected. This is
only the fourth known associated absorption at $z>2$. The detection rate
($1.6^{+3.8}_{-1.4}$%) suggests low covering factor of cold neutral medium
(CNM; T$\sim$100 K) associated with these powerful AGNs. The intervening H i
21-cm and OH 18-cm absorption searches, with a sensitivity to detect CNM in
damped Ly$\alpha$ systems (DLAs), have comoving absorption path lengths of
$\Delta$X = 130.1 and 167.7, respectively. Using these we estimate the number
of H i and OH absorber per unit comoving path lengths to be $\leq$0.014 and
$\leq$0.011, respectively. The former is at least 4.5 times lower than that of
DLAs and consistent with the CNM cross-section estimated using H2 and C i
absorbers at $z>2$. The AGNs in our sample selected using mid-infrared colors
are optically fainter compared to the optical- and radio-selected quasars used
to search for DLAs. In our optical spectra obtained using SALT and NOT, we
detect 5 DLAs (redshift path $\sim 9.3$) and 2 proximate DLAs (within 3000 km
s-1 of the AGN redshift). This is slightly excessive compared to the
statistics based on optically selected quasars. The non-detection of H i 21-cm
absorption from these DLAs suggests small CNM covering fraction around
galaxies at $z>2$.

quasars: absorption lines — interstellar medium

††journal: ApJ††facilities: SALT, uGMRT††software: ARTIP, Astropy, CASA and
Matplotlib

,

,

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##  1 Introduction

H i 21-cm absorption lines in the spectra of radio sources can provide
valuable insights into the cold atomic gas ($T\sim$100 K) associated with
active galactic nuclei (AGNs) and intervening galaxies along the line of
sight. In the former, the matter may be associated with the AGN, its host
galaxy, a nearby companion galaxy, outflows driven by its feedback or
infalling material. In the latter, the absorbing gas corresponds to the
interstellar or circumgalactic medium of an intervening galaxy or intragroup
medium. The strength of absorption signal does not depend on the distance to
the observer. The H i 21-cm absorption line could thus be an important probe
of the properties of cold gas in distant galaxies and investigating its role
in the cosmic evolution of star formation rate (SFR) density and that of the
luminosity density of AGNs, both of which peak at $z\simeq 2$.

For a long time radio telescopes have receivers capable of observing the H i
21-cm line up to arbitrarily high redshifts. Indeed, H i 21-cm absorption has
been searched in AGNs as distant as $z\sim 5.2$ (e.g., Carilli et al., 2007).
But technical limitations imposed by narrow bandwidths, hostile radio
frequency environment and limited number of known bright radio AGNs at
high-$z$ have prevented large unbiased radio absorption line surveys.
Consequently, to date, the majority of H i 21-cm absorption line observations
and detections have been based on optically selected samples of AGNs.

For associated absorption, AGNs with known redshifts and, preferably with
compact radio morphology, have been observed to study the circumnuclear gas
which may be fueling the radio activity or impacted by the AGN feedback (e.g.,
Vermeulen et al., 2003; Gupta et al., 2006; Curran et al., 2013; Geréb et al.,
2015; Aditya et al., 2016; Dutta et al., 2019; Grasha et al., 2019). Although
more than 500 AGNs have been searched for H i 21-cm absorption the vast
majority of observations are at $z<2$ and most of the detections at $z<1$ (see
Morganti & Oosterloo, 2018, for a review). Only 3 detections at $z>2$ are
known, the highest redshift being 3.53 (Aditya et al., 2021). Overall, the
bulk of detections are towards compact radio sources (detection rate $\sim
30-50\%$) associated with galaxies having mid-infrared (MIR) colors suggesting
gas and dust rich environment (Glowacki et al., 2017; Chandola et al., 2020).
Among detections associated with more powerful AGNs (radio luminosity,
log($L_{\rm 1.4\,GHz}/({\rm W\,Hz}^{-1})>24$), the H i absorption profiles
often show signatures of radio jet-ISM interaction in the form of blue-shifted
components representing outflowing gas (Maccagni et al., 2017).

For intervening H i 21-cm absorption line studies the targets have been sight
lines towards quasars, the most powerful AGNs, selected from the large optical
spectroscopic surveys such as the Sloan Digital Sky Survey (SDSS; York et al.,
2000). Generally, sight lines with indications of large H i column densities
($N$(H i)) along the sight line suggested by the presence of a damped ${\rm
Ly}\alpha$ system (DLAs; $N$(H i)$>2\times 10^{20}$cm-2; e.g., Srianand et
al., 2012; Kanekar et al., 2014), a strong Mg ii absorption (rest equivalent
width, $W_{\rm r}>1\AA$; e.g., Gupta et al., 2012; Dutta et al., 2017a), or a
galaxy at small impact parameter (typially $<$30 kpc; e.g., Carilli & van
Gorkom, 1992; Gupta et al., 2010; Borthakur et al., 2010; Reeves et al., 2016;
Dutta et al., 2017c) are selected. The vast majority of the observations are
sensitive to detecting cold neutral medium (CNM; $T\sim$100 K) in
$N(\mbox{H\,{\sc i}})>5\times 10^{19}$ cm-2. The detection rates are typically
10-50%, depending crucially on the sample selection criteria (see, for
example, Dutta et al., 2017b). Although the highest redshift detection is at
$z\sim$3.38 (Kanekar et al., 2007), the bulk of the reported H i 21-cm
detections are associated with gas rich galaxies at $z<2$. These studies also
suggest that the gas traced by DLAs at $z>2$ is predominantly warm ($T>$1000
K).

It is reasonable to expect optically selected samples of AGNs to be affected
by dust-bias. Since cold gas is accompanied by dust, the bias is particularly
relevant for H i 21-cm absorption line searches. In the case of associated
absorption, the dust intrinsic to AGN may remove objects with certain
orientation (Type ii) or going through the very early stages of evolution. In
the case of intervening gas, it can substantially affect our ability to use
optically selected samples of DLAs to detect translucent and dense phases of
the ISM (Krogager et al., 2016; Geier et al., 2019), and influence the
measurements of H i and metal mass densities (Krogager et al., 2019).

The limitations due to dust obscuration can be overcome by selecting AGNs
without resorting to any optical color selection scheme or carry out blind
searches of H i 21-cm absorption. The latter is becoming possible with various
precursor and pathfinder telescopes of Square Kilometre Array (SKA) equipped
with wideband receivers. Especially, the upcoming large H i 21-cm absorption
line surveys such as the MeerKAT Absorption Line Survey (MALS; Gupta et al.,
2017) and First Large Absorption Survey in H i (FLASH; Allison et al., 2017)
will characterize the evolution of cold gas without possible selection effects
due to dust-bias or from the choice of different methods used to select sight
lines in different redshift ranges (see also Grasha et al., 2020). These will
also simultaneously search OH 18-cm main lines, providing additional
constraints on the evolution of diffuse molecular gas in the ISM (Gupta et
al., 2018a; Balashev et al., 2020).

In this paper, we present a spectroscopically blind search of H i 21-cm
absorption at $z>2$ based on a sample of AGNs selected using the mid-infrared
(MIR) colors from Wide-field Infrared Survey Explorer (WISE; Wright et al.,
2010; Cutri & et al., 2014) and having spectroscopically confirmed redshifts
using the Southern African Large Telescope (SALT; 180 hrs) and the Nordic
Optical Telescope (NOT; 3 nights; Krogager et al., 2018). Note that similar to
the radio waveband the infrared wavelengths are also unaffected by dust
obscuration. These AGNs are being observed as part of MALS, which is a large
project at the MeerKAT array in South Africa, to search H i 21-cm and OH 18-cm
lines at $z<2$. The upgraded Giant Metrewave Radio Telescope (uGMRT) survey
presented here covers $2<z<5.1$.

The paper is laid out as follows. In Section 2, we present the sample
definition and its properties in the context of previous radio-selected
samples to search for DLAs. The details of uGMRT observations and data
analysis to obtain the radio spectra and spectral line catalog are presented
in Section 3. We provide the details of H i 21-cm absorber detected from the
survey in Section 4. In sections 5 and 6, we compute the incidences of
intervening and associated H i 21-cm absorption lines, respectively. In
Section 5, we apply the same formalism to also derive the incidence of
intervening OH absorption. The availability of SALT-NOT spectra allows us to
examine the properties of gas along the sight line using ${\rm Ly}\alpha$ and
various metal absorption lines. In particular, for a subset of uGMRT targets
($z_{e}>2.7$) through deeper SALT observations we have discovered 6 DLAs and 1
candidate proximate DLA (PDLA i.e., DLA within 3000 km s-1 of $z_{q}$). In
Section 5, we also present the properties of these ${\rm Ly}\alpha$ and metal
line absorbers and discuss the nature of multi-phase ISM in the context of
uGMRT survey results. The results and future prospects are summarized in
Section 7.

Throughout this paper we use the $\Lambda$CDM cosmology with
$\Omega_{m}$=0.27, $\Omega_{\Lambda}$=0.73 and Ho=71 km s-1 Mpc-1.

##  2 Sample

###  2.1 Definition and properties

Figure 1:  Redshift and flux density (1.4 GHz) distributions for our MIR
selected sample. The vertical dashed lines mark the median for each
distribution.

The targets for the uGMRT survey are drawn from the SALT-NOT sample of 299
AGNs constructed for MALS. The SALT-NOT sample is selected on the basis of MIR
colors from WISE. We defined the following color wedge based on the first
three bands of WISE i.e., $W1$ (3.4 $\mu$m), $W2$ (4.6 $\mu$m), $W3$ (12
$\mu$m),

$\begin{array}[]{c}W_{1}-W_{2}<1.3\times(W_{2}-W_{3})-3.04;\\\ \\\
W_{1}-W_{2}>0.6.\end{array}$ (1)

As shown in Fig. 1 of Krogager et al. (2018), the MIR-wedge defined above is
optimised towards identifying most powerful AGNs (i.e., quasars) at $z>1.4$.

The details of SALT-NOT target selection process will be presented in a future
paper. In short, we cross-correlated AllWISE catalog (Cutri & et al., 2014)
and radio sources brighter than $200$ mJy in the NRAO VLA Sky Survey (NVSS;
Condon et al., 1998), to identify 2011 high-probability quasar candidates
satisfying the MIR wedge (Equation 1). We restricted the sample to declination
$<+20^{\circ}$ to ensure reasonable observability with the MeerKAT telescope.
A search radius of $10^{\prime\prime}$ for WISE-NVSS cross-matching was used
but all the coincidences were verified using higher spatial resolution quick
look radio images at 3 GHz from the Very Large Array Sky Survey (VLASS; Lacy
et al., 2020). These quick look images have a spatial resolution of $\sim
2.5^{\prime\prime}$ and the positional accuracy is limited to $\sim
0.5^{\prime\prime}$. Consequently, our sample selects preferentially compact
core-dominated AGNs. We observed 299 candidates using SALT and NOT to measure
redshifts and confirm the AGN nature. This optical spectroscopic campaign has
led to a sample of AGNs which can be split into following three categories:
(i) with emission lines in the optical spectrum (250 objects with confirmed
redshifts at $0.1<z<5.1$), (ii) with no emission lines in the optical spectrum
(25), and (iii) empty fields i.e., radio continuum peak coincides with the MIR
source but neither emission line nor a continuum source are detected in
optical spectra and images.

The uGMRT Band-3 covers 250 - 500 MHz which is suitable to search for H i
21-cm absorption over $1.9<z<4.7$. It nicely complements the MALS coverage of
$z<1.4$. For the uGMRT survey presented here we selected all the 98 objects at
$z>2$ from the SALT-NOT sample. In the allocated observing time we observed 88
of these which are listed in Table LABEL:tab:wisesamp. The redshift
(median$\sim$2.5) and 1.4 GHz flux density (median$\sim$288 mJy) distributions
are presented in Fig. 1. The 1.4 GHz spectral luminosities are in the range of
log $L_{\rm 1.4\,GHz}\simeq 27-29.3$ W Hz-1. The lower end of luminosity is
well above the radio cut-off that separates FRI and FRII radio sources, and
the upper end corresponds to the most luminous radio-loud AGN at $z>5$
discovered from the SALT-NOT survey. All except one are spectroscopically
confirmed quasars. The details of radio galaxy M1540-1453 are presented by
Shukla et al. (2021).

With the right sample, it is possible to determine the evolution of cold gas
in a dust-unbiased way. Therefore, next we examine the efficacy of our sample
selection strategy by comparing it with samples of DLAs from radio-selected
quasars.

###  2.2 Comparison with radio-selected DLA samples

The three notable DLA samples based on radio-selected quasars are: (i) the
Complete Optical and Radio Absorption Line System (CORALS) survey of 66 QSOs
($z_{em}>2.2$) by Ellison et al. (2001), (ii) the University of California San
Diego (UCSD) survey of 53 QSOs ($z_{em}>2.0$) for DLAs by Jorgenson et al.
(2006), and (iii) the survey of 45 QSOs ($z_{em}>2.4$) selected from the Texas
radio survey (Ellison et al., 2008). These surveys revealed 19, 7 and 9 DLAs
over a redshift path, $\Delta z$ of 57.16, 41.15 and 38.79, respectively. The
number of DLAs per unit redshift, $n_{\rm DLA}$, are estimated to be
$0.31^{+0.09}_{-0.08}$, $0.17^{+0.08}_{-0.07}$ and $0.23^{+0.11}_{-0.07}$,
respectively. The CORALS survey found a slightly higher incidence of DLAs and
suggested that optically-selected DLA samples may be affected by dust-bias.
But overall none of the surveys uncovered a population of dusty DLAs.

Figure 2:  Comparison of optical properties of the high-redshift ($z\gtrsim
2$), radio-selected quasar surveys: SALT-NOT $z>2$ (this work), Ellison et al.
(2008), UCSD (Jorgenson et al., 2006), and CORALS (Ellison et al., 2001). The
black line indicates the cumulative distribution of $i$-band magnitudes in the
respective surveys, and the blue line shows the modelled distribution taking
into account the survey radio flux limit and spectroscopic follow-up criterion
of $B<22$ mag (see text). The median $i$-band magnitude of each sample is
given in the upper left corner.

Targets for these three surveys have been selected at different radio
frequencies and to different radio flux limits. While such differences might
be subtle, they may still affect the optical properties of the quasars and
hence the resulting statistics of DLAs. The CORALS survey has been selected at
2.7 GHz down to a flux density limit of 250 mJy; the UCSD sample has been
selected at 4.9 GHz to a flux limit of 350 mJy; and lastly, the survey by
Ellison et al. (2008) has been selected at 356 MHz down to a flux density
limit of 400 mJy.

In order to compare the effects of the radio-selection, we generate a mock
distribution of $i$-band magnitudes for the three samples as well as for the
SALT-NOT sample presented in this work. The intrinsic ultraviolet luminosity
function is assumed to be the same in all cases and is taken from the work by
Manti et al. (2017). We assume a fixed distribution of the optical-to-radio
flux ratio, $R_{i}$, following Baloković et al. (2012) as well as a fixed
radio slope of $\alpha_{\nu}=-0.8$, in order to scale the various survey
limits to the same frequency (1.4 GHz) as used in our survey and as used by
Baloković et al. (2012). Since all the surveys impose roughly the same optical
follow-up strategy in order to detect DLAs in low-resolution spectra, we
impose a final cut on $B<22$ mag. For this cut, we use an average color
correction for high-redshift QSOs: $B=i+0.3$ with a scatter of 0.1 mag (see
color relations by Krogager et al., 2019). The resulting mock magnitude
distribution is shown in Fig. 2 (blue curve) compared to the respective survey
data (in black). While all surveys span a wide range of magnitudes, our survey
more closely samples the underlying luminosity function and hence introduces a
minimal bias in the optical properties of the sample. This is a direct
consequence of the fact that SALT-NOT survey has targeted optically fainter
quasars (refer to median $i$-band mags in Fig. 2). An analysis of dust-bias in
the sample using optical-infrared colors will be presented in a future paper.

Table 1: Sample of $z>2$ MIR-selected radio sources (88) observed with uGMRT.

Source name | F${}_{1.4\,GHz}$ | $z_{em}$ | Obs. run | Beam | F${}_{\rm p,420\,MHz}$ | F${}_{\rm 420\,MHz}$ | F${}_{\rm p,420\,MHz}$/F${}_{\rm 420\,MHz}$ | $\alpha^{1.4}_{0.4}$ | $\alpha_{inband}$ |  $\Delta$F | Ncand
---|---|---|---|---|---|---|---|---|---|---|---
| (mJy) |  |  |  | (mJy b-1) | (mJy) |  |  |  | (mJy b-1) |
(1) | (2) | (3) | (4) | (5) | (6) | (7) | (8) | (9) | (10) | (11) | (12)
M004243.06$+$124657.6 | 635.0 | 2.150 | 16SEP | $7.0^{\prime\prime}\times 6.3^{\prime\prime},-12.0^{\circ}$ | 1628.8 | 1882.5 | 0.87 |  $-$0.87 |  $-$0.86 | 1.9 | -
M005315.65$-$070233.4 | 248.2 | 2.130 | 16SEP | $7.3^{\prime\prime}\times 6.5^{\prime\prime},+17.0^{\circ}$ | 546.1 | 536.5 | 1.02 |  $-$0.62 |  $-$0.61 | 2.8 | -
M013047.38$-$172505.6 | 250.3 | 2.528 | 14SEP | $9.9^{\prime\prime}\times 6.9^{\prime\prime},+40.0^{\circ}$ | 532.4 | 560.2 | 0.95 |  $-$0.64 |  $-$0.66 | 1.5 | -
M021231.86$-$382256.6 | 244.5 | 2.260 | 14SEP | $17.5^{\prime\prime}\times 6.8^{\prime\prime},+33.0^{\circ}$ | 495.7 | 605.4 | 0.82 |  $-$0.72 |  $-$0.71 | 1.6 | 1
M022613.72$+$093726.3 | 374.6 | 2.605 | 14SEP | $8.9^{\prime\prime}\times 7.0^{\prime\prime},+87.0^{\circ}$ | 435.1 | 436.6 | 1.0 |  $-$0.12 |  $-$0.02 | 2.1 | -
M022639.92$+$194110.1 | 209.8 | 2.190 | 14SEP | $10.4^{\prime\prime}\times 6.7^{\prime\prime},-83.0^{\circ}$ | 390.5 | 381.7 | 1.02 |  $-$0.48 |  $-$0.41 | 1.9 | -
M024939.93$+$044028.9 | 420.5 | 2.008 | 17SEP | $9.1^{\prime\prime}\times 7.6^{\prime\prime},+89.0^{\circ}$ | 927.7 | 992.7 | 0.93 |  $-$0.69 |  $-$0.67 | 1.6 | -
M025035.54$-$262743.1 | 389.2 | 2.918 | 17SEP | $11.0^{\prime\prime}\times 6.7^{\prime\prime},+25.0^{\circ}$ | 389.6 | 419.0 | 0.93 |  $-$0.06 |  $+$0.01 | 1.7 | 1
M032808.59$-$015220.2 | 221.9 | 2.679 | 17SEP | $12.0^{\prime\prime}\times 8.0^{\prime\prime},+72.0^{\circ}$ | 370.2 | 527.3 | 0.70 |  $-$0.69 |  $-$0.63 | 1.3 | -
M041620.54$-$333931.3 | 264.1 | 3.045 | 08SEP | $17.5^{\prime\prime}\times 9.0^{\prime\prime},-42.0^{\circ}$ | 130.0 | 117.8 | 1.1 |  $+$0.64 |  $+$0.51 | 1.7 | -
M042248.53$-$203456.6 | 224.3 | 2.582 | 08SEP | $11.8^{\prime\prime}\times 8.4^{\prime\prime},-58.0^{\circ}$ | 187.3 | 192.1 | 0.98 |  $+$0.12 |  $+$0.16 | 1.7 | 1
M044849.48$-$093531.3 | 240.9 | 2.079 | 14SEP | $9.3^{\prime\prime}\times 7.2^{\prime\prime},+46.0^{\circ}$ | 152.5 | 147.3 | 1.04 |  $+$0.39 |  $+$0.39 | 1.6 | -
M050725.04$-$362442.9 | 212.4 | 2.930 | 08SEP | $15.9^{\prime\prime}\times 9.6^{\prime\prime},-35.0^{\circ}$ | 494.5 | 474.5 | 1.04 |  $-$0.64 |  $-$0.51 | 1.6 | -
M051240.99$+$151723.8 | 966.5 | 2.568 | 07SEP | $6.8^{\prime\prime}\times 6.5^{\prime\prime},-79.0^{\circ}$ | 560.1 | 595.9 | 0.94 |  $+$0.39 |  $+$0.27 | 1.8 | -
M051340.03$+$010023.6 | 447.0 | 2.673 | 07SEP | $7.7^{\prime\prime}\times 6.6^{\prime\prime},+64.0^{\circ}$ | 342.5 | 349.7 | 0.98 |  $+$0.20 |  $-$0.04 | 2.1 | 4
M051511.18$-$012002.4 | 288.8 | 2.287 | 07SEP | $8.9^{\prime\prime}\times 6.4^{\prime\prime},+59.0^{\circ}$ | 412.6 | 641.5 | 0.64 |  $-$0.64 |  $-$0.68 | 1.6 | 1
M051656.35$+$073252.7 | 231.7 | 2.594 | 07SEP | $10.6^{\prime\prime}\times 6.9^{\prime\prime},+90.0^{\circ}$ | 44.2 | 44.1 | 1.0 |  $+$1.32 |  $+$1.10 | 1.9 | -
M052318.55$-$261409.6 | 1354.9 | 3.110 | 08SEP | $12.0^{\prime\prime}\times 9.1^{\prime\prime},-52.0^{\circ}$ | 477.7 | 451.8 | 1.06 |  $+$0.88 |  $+$1.08 | 2.2 | -
M061038.80$-$230145.6 | 360.2 | 2.829 | 14SEP | $11.0^{\prime\prime}\times 7.0^{\prime\prime},+31.0^{\circ}$ | 130.5 | 129.0 | 1.01 |  $+$0.82 |  $+$0.89 | 1.7 | -
M061856.02$-$315835.2 | 346.1 | 2.134 | 09SEP | $10.8^{\prime\prime}\times 6.3^{\prime\prime},+0.0^{\circ}$ | 877.1 | 828.6 | 1.06 |  $-$0.70 |  $-$0.35 | 2.0 | 1
M063602.28$-$311312.5 | 262.1 | 2.654 | 09SEP | $19.2^{\prime\prime}\times 11.1^{\prime\prime},+33.0^{\circ}$ | 178.7 | 162.4 | 1.1 |  $+$0.38 |  $+$0.42 | 3.1 | -
M063613.53$-$310646.3 | 208.0 | 2.757 | 09SEP | $11.6^{\prime\prime}\times 6.9^{\prime\prime},+17.0^{\circ}$ | 436.2 | 474.2 | 0.92 |  $-$0.66 |  $-$0.73 | 2.1 | -
M065254.73$-$323022.6† | 322.1 | 2.239 | 08SEP | $11.4^{\prime\prime}\times 10.1^{\prime\prime},+5.0^{\circ}$ | 475.9 | 611.1 | 0.78 |  $-$0.85 |  $-$0.95 | 2.3 | -
|  |  |  |  | 279.0 | 328.2 | 0.85 |  |  | 2.3 | -
M070249.30$-$330205.0 | 314.6 | 2.410 | 08SEP | $12.3^{\prime\prime}\times 9.8^{\prime\prime},+33.0^{\circ}$ | 574.9 | 599.8 | 0.96 |  $-$0.52 |  $-$0.52 | 2.8 | 1
M073159.01$+$143336.3 | 316.5 | 2.632 | 16SEP | $8.8^{\prime\prime}\times 8.1^{\prime\prime},-49.0^{\circ}$ | 180.0 | 185.4 | 0.97 |  $+$0.43 |  $+$0.40 | 7.8 | -
M073714.60$-$382841.9 | 219.3 | 2.107 | 14SEP | $14.5^{\prime\prime}\times 6.6^{\prime\prime},+18.0^{\circ}$ | 498.9 | 515.3 | 0.97 |  $-$0.68 |  $-$0.67 | 2.4 | -
M080804.34$+$005708.2 | 317.0 | 3.133 | 08SEP | $14.3^{\prime\prime}\times 7.1^{\prime\prime},-73.0^{\circ}$ | 434.8 | 450.7 | 0.96 |  $-$0.28 |  $-$0.26 | 2.1 | -
M081936.62$-$063047.9 | 280.0 | 2.507 | 08SEP | $11.9^{\prime\prime}\times 6.7^{\prime\prime},-73.0^{\circ}$ | 339.0 | 354.9 | 0.96 |  $-$0.19 |  $-$0.08 | 2.3 | -
M085826.92$-$260721.0 | 404.5 | 2.036 | 09SEP | $17.3^{\prime\prime}\times 8.5^{\prime\prime},-19.0^{\circ}$ | 561.0 | 523.9 | 1.07 |  $-$0.21 |  $-$0.28 | 2.6 | -
M090910.66$-$163753.8 | 340.1 | 2.475 | 09SEP | $9.0^{\prime\prime}\times 7.3^{\prime\prime},-11.0^{\circ}$ | 776.2 | 909.7 | 0.85 |  $-$0.79 |  $-$0.94 | 1.7 | -
M091051.01$-$052626.8† | 337.9 | 2.395 | 16SEP | $9.6^{\prime\prime}\times 7.2^{\prime\prime},-44.0^{\circ}$ | 151.4 | 166.3 | 0.91 |  $+$0.20 |  $+$0.29 | 1.6 | -
|  |  |  |  | 80.0 | 97.3 | 0.82 |  |  | 1.6 | -
M095231.66$-$245349.1 | 209.5 | 2.626 | 14SEP | $10.4^{\prime\prime}\times 7.0^{\prime\prime},+5.0^{\circ}$ | 224.2 | 210.3 | 1.07 |  $-$0.00 |  $-$0.17 | 1.8 | -
M100715.18$-$124746.7 | 381.1 | 2.113 | 16SEP | $7.7^{\prime\prime}\times 6.2^{\prime\prime},-24.0^{\circ}$ | 476.6 | 470.0 | 1.01 |  $-$0.17 |  $-$0.21 | 2.1 | -
M101313.10$-$254654.7 | 248.8 | 2.965 | 14SEP | $10.6^{\prime\prime}\times 6.9^{\prime\prime},+10.0^{\circ}$ | 216.7 | 218.6 | 0.99 |  $+$0.10 |  $-$0.0 | 2.1 | -
M102548.76$-$042933.0 | 363.5 | 2.292 | 16SEP | $6.9^{\prime\prime}\times 6.7^{\prime\prime},+89.0^{\circ}$ | 356.3 | 391.6 | 0.91 |  $-$0.06 |  $-$0.03 | 2.5 | -
M104314.53$-$232317.5 | 212.1 | 2.881 | 07SEP | $8.9^{\prime\prime}\times 6.5^{\prime\prime},-10.0^{\circ}$ | 460.6 | 505.8 | 0.91 |  $-$0.69 |  $-$0.79 | 2.2 | -
M111820.61$-$305459.0 | 233.2 | 2.352 | 08SEP | $18.5^{\prime\prime}\times 8.4^{\prime\prime},-51.0^{\circ}$ | 64.8 | 70.0 | 0.93 |  $+$0.96 |  $+$0.10 | 3.0 | -
M111917.36$-$052707.9 | 1174.4 | 2.651 | 07SEP | $7.1^{\prime\prime}\times 6.3^{\prime\prime},-48.0^{\circ}$ | 1696.6 | 1793.2 | 0.95 |  $-$0.34 |  $-$0.47 | 3.6 | -
M112402.56$-$150159.1 | 261.9 | 2.551 | 07SEP | $9.0^{\prime\prime}\times 6.9^{\prime\prime},-23.0^{\circ}$ | 194.7 | 196.0 | 0.99 |  $+$0.23 |  $+$0.01 | 2.6 | -
M114226.58$-$263313.7 | 294.7 | 3.237 | 08SEP | $15.5^{\prime\prime}\times 8.3^{\prime\prime},-56.0^{\circ}$ | 293.6 | 340.7 | 0.86 |  $-$0.35 |  $-$0.31 | 4.0 | -
M115222.04$-$270126.3 | 238.2 | 2.703 | 08SEP | $13.3^{\prime\prime}\times 8.6^{\prime\prime},-60.0^{\circ}$ | 410.4 | 441.4 | 0.93 |  $-$0.49 |  $-$0.64 | 2.5 | -
M115306.72$-$044254.5† | 684.8 | 2.591 | 16SEP | $7.0^{\prime\prime}\times 6.5^{\prime\prime},+86.0^{\circ}$ | 929.3 | 899.1 | 1.03 |  $-$0.78 |  $-$0.89 | 2.7 | -
|  |  |  |  | 980.1 | 912.2 | 1.07 |  |  | 2.7 | -
M120632.23$-$071452.6 | 698.8 | 2.263 | 16SEP | $8.0^{\prime\prime}\times 6.2^{\prime\prime},-9.0^{\circ}$ | 1402.8 | 1267.9 | 1.10 |  $-$0.48 |  $-$0.60 | 2.7 | -
M121514.42$-$062803.5 | 360.4 | 3.218 | 16SEP | $9.2^{\prime\prime}\times 6.6^{\prime\prime},-8.0^{\circ}$ | 511.1 | 461.5 | 1.11 |  $-$0.20 |  $-$0.31 | 2.8 | -
M123150.30$-$123637.5 | 276.0 | 2.106 | 07SEP | $7.2^{\prime\prime}\times 6.5^{\prime\prime},-17.0^{\circ}$ | 159.7 | 205.1 | 0.78 |  $+$0.24 |  $+$0.36 | 1.8 | -
M123410.08$-$332638.5 | 297.9 | 2.820 | 08SEP | $15.8^{\prime\prime}\times 9.7^{\prime\prime},-57.0^{\circ}$ | 665.4 | 710.4 | 0.94 |  $-$0.69 |  $-$0.72 | 3.4 | -
M124448.99$-$044610.2 | 384.9 | 3.104 | 16SEP | $9.1^{\prime\prime}\times 7.7^{\prime\prime},+28.0^{\circ}$ | 677.2 | 616.1 | 1.1 |  $-$0.38 |  $-$0.40 | 2.3 | 1
M125442.98$-$383356.4 | 219.2 | 2.776 | 17SEP | $14.7^{\prime\prime}\times 7.0^{\prime\prime},+7.0^{\circ}$ | 297.4 | 300.9 | 0.99 |  $-$0.25 |  $-$0.25 | 2.3 | -
M125611.49$-$214411.7 | 260.7 | 2.178 | 16SEP | $10.3^{\prime\prime}\times 6.5^{\prime\prime},+13.0^{\circ}$ | 446.8 | 455.4 | 0.98 |  $-$0.45 |  $-$0.15 | 1.7 | -
M131207.86$-$202652.4 | 778.1 | 5.064 | 16SEP | $10.5^{\prime\prime}\times 6.7^{\prime\prime},+21.0^{\circ}$ | 1727.9 | 1721.9 | 1.0 |  $-$0.63 |  $-$0.62 | 2.0 | 1
|  |  | 21APR‡ | $15.8^{\prime\prime}\times 13.8^{\prime\prime},-38.0^{\circ{\ddagger}}$ | - | 2577.0‡ | - | - | - | - | -
M132657.20$-$280831.4 | 404.5 | 2.238 | 16SEP | $15.0^{\prime\prime}\times 7.0^{\prime\prime},+20.0^{\circ}$ | 671.3 | 874.8 | 0.77 |  $-$0.62 |  $-$0.83 | 2.7 | -
M135131.98$-$101932.9 | 726.1 | 2.999 | 16SEP | $9.2^{\prime\prime}\times 7.0^{\prime\prime},+47.0^{\circ}$ | 1338.3 | 1886.5 | 0.71 |  $-$0.76 |  $-$0.79 | 2.0 | -
M141327.20$-$342235.1 | 274.7 | 2.812 | 09SEP | $20.9^{\prime\prime}\times 6.7^{\prime\prime},-42.0^{\circ}$ | 78.2 | 76.4 | 1.02 |  $+$1.02 |  $+$0.81 | 3.9 | -
M143709.04$-$294718.5 | 273.8 | 2.331 | 09SEP | $14.6^{\prime\prime}\times 7.0^{\prime\prime},-44.0^{\circ}$ | 333.0 | 456.1 | 0.73 |  $-$0.41 |  $-$0.49 | 2.5 | -
M144851.10$-$112215.6 | 455.5 | 2.630 | 07SEP | $7.3^{\prime\prime}\times 6.0^{\prime\prime},-48.0^{\circ}$ | 967.0 | 896.8 | 1.08 |  $-$0.54 |  $-$0.71 | 3.1 | 1
M145342.95$-$132735.2 | 254.5 | 2.370 | 07SEP | $7.6^{\prime\prime}\times 6.2^{\prime\prime},-33.0^{\circ}$ | 477.3 | 634.7 | 0.75 |  $-$0.73 |  $-$0.83 | 2.2 | -
M145502.84$-$170014.2 | 294.7 | 2.291 | 07SEP | $8.3^{\prime\prime}\times 6.4^{\prime\prime},-13.0^{\circ}$ | 345.8 | 352.2 | 0.98 |  $-$0.14 |  $-$0.23 | 2.3 | -
M145625.83$+$045645.2 | 287.9 | 2.134 | 09SEP | $7.1^{\prime\prime}\times 6.8^{\prime\prime},+87.0^{\circ}$ | 800.8 | 813.2 | 0.98 |  $-$0.83 |  $-$0.82 | 2.6 | -
M145908.92$-$164542.3 | 378.9 | 2.006 | 07SEP | $8.5^{\prime\prime}\times 6.6^{\prime\prime},-9.0^{\circ}$ | 853.1 | 909.6 | 0.94 |  $-$0.70 |  $-$0.85 | 2.1 | -
M150425.30$+$081858.6 | 210.8 | 2.035 | 09SEP | $8.4^{\prime\prime}\times 7.6^{\prime\prime},-32.0^{\circ}$ | 122.1 | 138.5 | 0.88 |  $+$0.34 |  $+$0.27 | 4.8 | -
M151129.01$-$072255.3 | 326.3 | 2.582 | 17SEP | $7.8^{\prime\prime}\times 6.8^{\prime\prime},-85.0^{\circ}$ | 672.7 | 624.9 | 1.08 |  $-$0.52 |  $-$0.71 | 2.7 | -
M151304.72$-$252439.7† | 217.6 | 3.132 | 09SEP | $9.1^{\prime\prime}\times 6.8^{\prime\prime},+1.0^{\circ}$ | 855.7 | 819.4 | 1.04 |  $-$1.26 |  $-$1.26 | 3.6 | -
|  |  |  |  | 268.5 | 242.1 | 1.11 |  |  | 3.6 | -
M151944.77$-$115144.6 | 441.0 | 2.014 | 17SEP | $9.2^{\prime\prime}\times 7.6^{\prime\prime},+84.0^{\circ}$ | 425.0 | 561.5 | 0.76 |  $-$0.19 |  $-$0.01 | 3.7 | -
M154015.23$-$145341.5 | 203.3 | 2.098 | 17SEP | $9.8^{\prime\prime}\times 7.5^{\prime\prime},+61.0^{\circ}$ | 642.4 | 595.0 | 1.08 |  $-$0.86 |  $-$0.98 | 2.6 | 1
M155825.35$-$215511.1 | 206.9 | 2.760 | 09SEP | $13.7^{\prime\prime}\times 6.7^{\prime\prime},-42.0^{\circ}$ | 209.5 | 235.8 | 0.89 |  $-$0.10 |  $+$0.09 | 3.0 | -
M161907.44$-$093952.5 | 340.3 | 2.891 | 17SEP | $8.9^{\prime\prime}\times 6.8^{\prime\prime},+66.0^{\circ}$ | 757.0 | 695.5 | 1.09 |  $-$0.57 |  $-$0.43 | 2.2 | -
M162047.94$+$003653.2 | 317.8 | 2.438 | 17SEP | $10.7^{\prime\prime}\times 7.6^{\prime\prime},+2.0^{\circ}$ | 226.9 | 234.3 | 0.97 |  $+$0.24 |  $+$0.16 | 3.3 | -
M164950.51$+$062653.3 | 389.2 | 2.144 | 17SEP | $13.6^{\prime\prime}\times 9.3^{\prime\prime},-11.0^{\circ}$ | 154.3 | 204.3 | 0.76 |  $+$0.51 |  $+$0.36 | 2.6 | -
M165038.03$-$124854.5 | 275.5 | 2.527 | 09SEP | $7.3^{\prime\prime}\times 6.2^{\prime\prime},-20.0^{\circ}$ | 729.6 | 675.1 | 1.08 |  $-$0.72 |  $-$0.52 | 5.0 | -
M165435.38$+$001719.2 | 255.3 | 2.363 | 07SEP | $9.2^{\prime\prime}\times 7.3^{\prime\prime},-14.0^{\circ}$ | 405.7 | 381.5 | 1.06 |  $-$0.32 |  $-$0.44 | 4.2 | -
M194110.28$-$300720.9 | 315.0 | 2.059 | 17SEP | $23.6^{\prime\prime}\times 8.3^{\prime\prime},+51.0^{\circ}$ | 164.0 | 227.2 | 0.72 |  $+$0.26 |  $+$0.40 | 3.6 | -
M200209.37$-$145531.8 | 620.3 | 2.192 | 17SEP | $8.6^{\prime\prime}\times 7.1^{\prime\prime},-17.0^{\circ}$ | 942.0 | 896.1 | 1.05 |  $-$0.29 |  $-$0.31 | 2.9 | -
M201708.96$-$293354.7 | 327.2 | 2.617 | 17SEP | $20.5^{\prime\prime}\times 7.3^{\prime\prime},+50.0^{\circ}$ | 1044.7 | 1201.1 | 0.87 |  $-$1.04 |  $-$1.02 | 2.7 | 1
M203425.65$-$052332.2 | 419.7 | 2.070 | 17SEP | $8.0^{\prime\prime}\times 7.3^{\prime\prime},+85.0^{\circ}$ | 366.2 | 407.2 | 0.9 |  $+$0.02 |  $+$0.10 | 4.0 | -
M204737.67$-$184141.2 | 241.7 | 2.994 | 16SEP | $13.8^{\prime\prime}\times 6.7^{\prime\prime},-49.0^{\circ}$ | 273.0 | 284.3 | 0.96 |  $-$0.13 |  $-$0.17 | 2.2 | -
M205245.03$-$223410.6 | 330.9 | 2.072 | 16SEP | $12.3^{\prime\prime}\times 6.0^{\prime\prime},-41.0^{\circ}$ | 631.8 | 608.3 | 1.04 |  $-$0.49 |  $-$0.60 | 3.5 | -
M210143.29$-$174759.2 | 959.5 | 2.803 | 16SEP | $9.5^{\prime\prime}\times 5.8^{\prime\prime},-39.0^{\circ}$ | 2554.1 | 2477.5 | 1.03 |  $-$0.76 |  $-$0.91 | 3.6 | -
M212821.83$-$150453.2 | 245.5 | 2.547 | 17SEP | $9.2^{\prime\prime}\times 6.9^{\prime\prime},+6.0^{\circ}$ | 443.4 | 460.8 | 0.96 |  $-$0.50 |  $-$0.49 | 2.0 | -
M220127.50$+$031215.6 | 300.5 | 2.181 | 17SEP | $18.3^{\prime\prime}\times 8.6^{\prime\prime},+77.0^{\circ}$ | 180.5 | 191.6 | 0.94 |  $+$0.36 |  $+$0.33 | 1.7 | -
M222332.81$-$310117.3 | 231.7 | 3.206 | 17SEP | $25.0^{\prime\prime}\times 9.2^{\prime\prime},+40.0^{\circ}$ | 230.6 | 330.2 | 0.7 |  $-$0.28 |  $-$0.40 | 2.6 | -
M223816.27$-$124036.4 | 213.6 | 2.623 | 16SEP | $12.3^{\prime\prime}\times 8.3^{\prime\prime},-50.0^{\circ}$ | 464.3 | 435.5 | 1.07 |  $-$0.57 |  $-$0.65 | 3.7 | -
M224111.48$-$244239.0 | 211.4 | 2.242 | 16SEP | $10.9^{\prime\prime}\times 6.1^{\prime\prime},-31.0^{\circ}$ | 253.0 | 254.8 | 0.99 |  $-$0.15 |  $-$0.17 | 2.2 | -
M224705.52$+$121151.4 | 223.7 | 2.185 | 14SEP | $12.1^{\prime\prime}\times 7.5^{\prime\prime},+83.0^{\circ}$ | 474.3 | 489.1 | 0.97 |  $-$0.62 |  $-$0.60 | 1.9 | -
M224950.57$-$263459.6 | 228.8 | 2.174 | 16SEP | $9.8^{\prime\prime}\times 5.7^{\prime\prime},-23.0^{\circ}$ | 568.3 | 542.1 | 1.05 |  $-$0.69 |  $-$0.59 | 2.4 | -
M230036.41$+$194002.9 | 210.4 | 2.160 | 17SEP | $27.7^{\prime\prime}\times 8.2^{\prime\prime},+79.0^{\circ}$ | 475.8 | 556.0 | 0.86 |  $-$0.78 |  $-$0.65 | 11.2 | -
M231634.61$+$042940.2 | 214.0 | 2.180 | 16SEP | $6.9^{\prime\prime}\times 6.6^{\prime\prime},+47.0^{\circ}$ | 103.3 | 97.9 | 1.06 |  $+$0.70 |  $+$0.50 | 3.0 | -
M234910.12$-$043803.2 | 206.1 | 2.240 | 16SEP | $7.7^{\prime\prime}\times 6.8^{\prime\prime},-62.0^{\circ}$ | 168.3 | 185.6 | 0.91 |  $+$0.08 |  $-$0.04 | 5.1 | -
M235722.47$-$073134.3 | 235.5 | 2.764 | 16SEP | $7.0^{\prime\prime}\times 6.0^{\prime\prime},-19.0^{\circ}$ | 372.4 | 398.2 | 0.94 |  $-$0.42 |  $-$0.40 | 2.4 | -

Note. — Column 1: source name based on right ascension and declination (J2000)
from NVSS. Column 2: 20 cm flux density from NVSS. Column 3: emission line
redshift measured from SALT-NOT survey. Column 4: observing run (see Table
LABEL:tab:obslog). Note that only M1312-2026 was also observed at Band-2.
Columns 5 - 7: synthesised beam, peak of the prominent Gaussian component and
total flux densities, respectively, from the continuum image based on 390-450
MHz range (average 420 MHz). Column 8: ratio of columns 6 and 7. Column 9:
spectral index derived using NVSS and 420 MHz flux densities. In a few cases,
which are all sources with a single component fit, this ratio marginally
($\sim$5%) exceeds 1, suggesting that the radio emission may be partially
resolved. Column 10: in-band spectral index. Column 11: observed spectral rms
at 420 MHz. Column 12: number of absorption candidates.

${\dagger}$: The radio source is double lobed in 420 MHz image (see Section
3.1 for details). ${\ddagger}$: Corresponds to Band-2.

##  3 Observations and data analysis

###  3.1 Observations

We used the recently commissioned Band-2 (120-240 MHz) and Band-3 (250-500
MHz) of uGMRT to observe redshifted associated and intervening H i 21-cm
absorption lines from the sample. The total allocated time including all
overheads for the survey observations was 90 hrs. The Band-3 observations were
split into 6 observing runs in September, 2018 (see Table LABEL:tab:obslog).
For these, we used GMRT Wideband Backend (GWB) with a baseband bandwidth of
200 MHz covering 300-500 MHz and split into 8192 frequency channels. This
corresponds to a redshift coverage of 1.84 - 3.73 for H i 21-cm line. The
channel resolution is 24.414 kHz, which at 400 MHz provides a velocity
resolution of 18.3 km s-1. Each target was observed for typically 30-45 mins.
The details of which target sources were visited in which observing run are
summarized in column 4 of Table LABEL:tab:wisesamp.

For Band-2 observations which targeted only M1312-2026, the highest redshift
quasar in the sample, the GMRT Software Backend (GSB) was used to configure a
baseband bandwidth of 4.17 MHz split into 512 spectral channels. The observing
band was centered at 234.1 MHz (resolution$\sim$10 km s-1), the redshifted H i
21-cm line frequency of the source. The total on-source time was 4.2 hrs.

Additionally, five absorption candidates identified from the Band-3 survey
observations were reobserved on December 10, 2019 and February 20, 2020. We
used GWB with a bandwidth of 6.25 MHz centered at line frequency (details in
Section 3.3) and split into 4096 channels (resolution$\sim$1 km s-1). Each
candidate was observed for 3 hrs (on-source time $\sim$2.2 hrs).

For all the observations, only parallel hand correlations RR and LL were
obtained. During each observing run 3C48, 3C147 and/or 3C286 were observed for
flux density and bandpass calibrations. A complex gain calibrator was also
observed for each target source.

Table 2: Details of uGMRT observations.

Run ID | Band | Date† | Duration‡
---|---|---|---
Survey observations
21APR | Band-2 | 2018 April 21 | 7
07SEP | Band-3 | 2018 September 07 | 11
08SEP | ” | 2018 September 08 | 10
09SEP | ” | 2018 September 09 | 11
14SEP | ” | 2018 September 14 | 10
16SEP | ” | 2018 September 16 | 21
17SEP | ” | 2018 September 17 | 20
Follow-up observations
10DEC | ” | 2019 December 10 | 6
20FEB | ” | 2020 February 20 | 9

Note. — ${\dagger}$: Start date as per Indian Standard Time. ${\ddagger}$: In
hours.

Figure 3:  uGMRT radio continuum (420 MHz) contours overlaid on PS1 $yig$
color composite images. For M0636-3106 and M0652-3230, the background image is
PS1 $i$-band and uGMRT 420 MHz, respectively. The contour levels are shown at
20$\times$(-1, 1, 2, 4, 8, …) mJy beam-1. The synthesized beams, shown at the
bottom-left corner of images, and the peak and total flux densities are
provided in column 5 - 7 of Table LABEL:tab:wisesamp, respectively. The
position of WISE source is marked with a cross.

###  3.2 Data analysis

All the data were edited, calibrated and imaged using the Automated Radio
Telescope Imaging Pipeline (ARTIP) following the steps described in Gupta et
al. (2021). After flagging and calibration, the spectral line processing of
wideband Band-3 data was sped up by partitioning the 200 MHz bandwidth into
four 50 or 60 MHz wide spectral windows with an overlap of 10 MHz between the
adjacent windows. These spectral windows covered: 300-360 MHz, 350-410 MHz,
400-460 MHz and 450-500 MHz. The calibrated visibilities for each spectral
window were processed separately (independently) for RFI flagging, continuum
imaging and self-calibration, and continuum subtraction. The continuum
subtracted visibilities were imaged to obtain RR and LL spectral line cubes.
For this a ‘common’ synthesized beam corresponding to the lowest frequency
spectral window was used.

The narrow band datasets from Band-2 survey and Band-3 follow-up observations
were processed as a single 4.17 or 6.25 MHz wide spectral window,
respectively.

####  3.2.1 Continuum analysis

For Band-3, the spectral window covering 390-450 MHz, hereafter identified
through the central reference frequency of 420 MHz, is least affected by RFI
resulting in best possible continuum images from the data. We used CASA task
IMFIT to model the radio continuum emission in these images as multiple
Gaussian components. The 9 cases requiring more than one Gaussian component
are shown in Fig. 3. Only in 4 cases i.e., M0652$-$3230, M0910$-$0526,
M1153$-$0442 and M1513$-$2524, does the second component contains more than
20% of the total flux density.

In columns 5 and 6 of Table LABEL:tab:wisesamp we list synthesized beams and
peak flux densities of the most prominent Gaussian component. For the above-
mentioned four sources the second component is also listed. In the remaining
cases the additional components are too faint ($<$50 mJy) to be useful for the
objectives of this paper, hence we do not list their individual properties.
The total flux density as estimated from the single or multiple component fit
is provided in column 7.

In Fig. 3, we also show optical images from Pan-STARRS1 (PS1) (Chambers et
al., 2016). Note that M0652-3230 is too south to be covered in PS1. The
location of MIR sources from WISE are also shown in the images. Owing to the
MIR-selection wedge described in Section 2, all but one radio source in our
sample are quasars. Indeed the median spectral index111Spectral index $\alpha$
is defined by the power law, $S_{\nu}\propto\nu^{\alpha}$,
$\alpha^{1.4}_{0.4}$, derived using the NVSS 1420 MHz and the uGMRT 420 MHz
total flux densities is $-0.38$ (see column 9 of Table LABEL:tab:wisesamp and
Fig. 4). As expected this is flatter than the overall radio source population
which has $\alpha\sim-0.8$. Thus, for our sample, when radio emission is
dominated by a single component, we expect AGN to be located close to the peak
of the radio emission. In case two prominent radio components are present
i.e., a compact symmetric object (CSO; Conway, 2002) morphology, the AGN is
expected to be located in between them. In all but 2 cases (M0910-0526 and
M1513-2524; details provided below), the optical / MIR counterpart is at the
location of the AGN expected from the radio morphology. As previously
mentioned we have also verified these coincidences using higher spatial
resolution 3 GHz VLASS images.

Figure 4:  Distributions of radio spectral index between 1400 and 420 MHz
($\alpha^{1.4}_{0.4}$) and 5$\sigma$ 21-cm optical depth ($\int\tau$dv). The
vertical dashed lines mark the median for each distribution. The dotted lines
correspond to $\alpha$ = -0.8 and $\tau_{21}$dv = 1.1 km s-1.

In the case of M0910-0526, the northern component could be an unrelated radio
source (Fig 3). We will exclude this component from the absorption line
statistics. M1513-2524, the only radio galaxy in the sample presented here, is
among the optically faintest ($r>23$ mag) in our survey. Among the two radio
components, one of them is closer to the MIR source (see Fig 3). We have
tentatively detected faint radio emission i.e., radio core at the location of
the MIR source. For details see higher spatial resolution radio images
presented in Shukla et al. (2021). We will consider the eastern and western
radio components as the two radio lobes of this radio galaxy.

For M1312$-$2026, the only target also observed in Band-2, the properties at
234 MHz are also provided in Table LABEL:tab:wisesamp. The associated radio
emission is compact with a deconvolved size of 15.8′′$\times$13.8′′ (position
angle = $-38.0^{\circ}$). Based on the observed flux densities, M1312-2026,
has a spectral luminosity of L(1.4 GHz) = $1.2\times 10^{29}$ W Hz-1, which is
more than three orders of magnitude higher than the radio power cut-off that
separates FRI and FRII radio sources, and greater than the luminosity of any
known radio-loud AGN at $z>5$. The multi-frequency VLA and VLBA observations
of this AGN have been obtained to investigate its radio morphology.

###  3.3 Spectral line analysis

Figure 5:  RR and LL spectra of M1206-0714 and continuum fits (dashed line).

Figure 6:  The continuum subtracted Stokes-$I$ spectrum of M0422-2034
($z_{em}$=2.582; see arrow at 396.51 MHz marking the redshifted H i 21-cm line
frequency). Shaded regions mark frequency ranges that were masked prior to any
calibration. The median spectra obtained using the full survey and only 08SEP
run are plotted at +0.042 (median-survey) and +0.020 Jy (median-08SEP),
respectively. In the spectrum of M0422-2034, the pixels flagged on the basis
of median spectra are shown in red. The error spectrum (5$\times\sigma_{\rm
rolling}$) is also shown. The dotted and dashed lines at the bottom in panels
3-5 show the frequency range valid for 21-cm line search and actually
contributing to the sensitivity function ($g(z)$), respectively. The candidate
detections are marked using $\star$.

For spectral line analysis, RR and LL spectra in the heliocentric frame were
extracted at the location of radio continuum peaks from the spectral line
cubes. The spectra show systematic oscillations or ripples due to residual
bandpass calibration, and numerous positive/ negative spikes (for an example
see Fig. 5). The ripple is not identical in the two parallel hands and also
varies from one target source to other. We removed its effect by iteratively
fitting the underlying structure using Savitsky-Golay filtering with window
length = 24 and polynomial order = 3. In each iteration, the pixels deviating
beyond the threshold were flagged and excluded from the subsequent iterations.
The continuum was interpolated across the masked pixels and the process was
repeated until no deviant pixels were found.

The above determined continuum fit was subtracted from the RR and LL spectra,
and an error spectrum was generated by calculating a rolling standard
deviation ($\sigma_{\rm rolling}$; window size=48 channels). For Band-3, an
additional step was to merge the spectra from adjacent spectral windows and
unmask the spikes to obtain the final RR and LL spectra covering the entire
300-500 MHz. These were then averaged with appropriate statistical weights to
obtain the final Stokes-$I$ spectra. The resultant Stokes-$I$ spectra have
flat baselines but numerous positive and negative spikes (for example see Fig.
6).

The Band-2 spectrum of J1312$-$2026 is presented in Fig. 7. These data were
severely affected by the radio frequency interference (RFI). The broad-band
RFI mostly affected the shorter baselines ($<$4 k$\lambda$) which were
completely flagged. There was also narrow-band impulse-like bursts of RFI,
which affected all the baselines. Overall $\sim$55% of the data was flagged
due to antenna/baseline-based problems and RFI. The spectral rms in the
Stokes-$I$ spectrum presented here is 8.5 mJy/beam, which for the unsmoothed
spectrum presented here corresponds to a 1$\sigma$ optical depth sensitivity
of 0.003. There are several statistically significant narrow-band features
with widths of 1-2 spectral channels detected in the spectrum. But all of
these are coincident with the spikes present in the RFI-spectrum, and are most
likely due to the low-level narrow-band RFI which could not be detected in the
individual baselines.

In general, no true emission is expected in our spectra and only a tiny
fraction of negative spikes are expected to represent true absorption. The
majority of these spikes are RFI artefacts. The biggest issue in spectral line
search at low frequencies is to distinguish between true absorption and RFI
artefacts. The rest of this section is concerned with absorption line search
in Band-3 spectra.

Figure 7:  H i 21-cm absorption spectrum towards the highest redshift quasar
M1312-2026 in our sample. The vertical dashed line marks the redshifted H i
21-cm absorption frequency corresponding to $z_{\rm q}$. The filled histogram
in the background is the ratio of the extent of data flagged due to frequency-
dependent and frequency-independent flags.

The worst RFI in Band-3 spectra (e.g., at 360-380 MHz) was flagged by applying
an initial mask prior to any calibration (see shaded regions in Fig. 6).
Further, to identify artefacts due to weaker but persistent RFI, we generated
median Stokes-$I$ spectra for each observing run and the full survey. The
median spectrum from the survey and 08SEP run in which M0422-2034 was observed
are shown in Fig. 6. The pixels deviating by more than 5$\sigma$ in the median
spectra were taken to represent RFI artefacts. We rejected corresponding
pixels in the individual source spectrum. In Fig. 6, such pixels for
M0422-2034 are plotted in red.

After this we created a list of 550 absorption line candidates using a
detection algorithm which requires: (i) flux density at a pixel $j$, F($j$)
$<-5\times\sigma_{rolling}$($j$) and (ii) Heliocentric frequency at $j$,
$\nu$($j$) $\geq\nu_{\rm 21cm}$/(1 + ($z_{em}$ \+ 0.01) ), where $\nu_{\rm
21cm}$ is the rest-frame 21-cm line frequency. The factor of 0.01 in the
denominator, which approximately corresponds to a outflowing velocity of
$\sim$3000 km s-1, allows for the possibility of detecting redshifted
absorption associated with AGN (see Fig. 21 of Gupta et al., 2006).

Next, we created a false-detection spectrum by identifying pixels based on
following two characteristics. First, we identified all positive spikes with
F($j$) $>5\times\sigma_{rolling}$($j$). These are unphysical and hence false
detections because H i emission lines are too weak to be detectable at $z>2$
in our spectra. Second, we identified all negative spikes i.e., F($j$)
$<-5\times\sigma_{rolling}$ but only at $\nu$($j$) $<\nu_{\rm 21cm}$/(1 +
($z_{em}$ \+ 0.01)). These are unphysical because absorbing gas must be in the
front of radio source. In Fig. 6, we mark three candidates using $\star$. Two
of these are clearly false detections whereas third one at 395.5 MHz
(approximately +800km s-1 with respect to $z_{em}$) could be a true absorption
associated with the AGN. The cumulative distributions of all the false
absorption (528) and emission (1359) detections from the survey are shown in
Fig. 8. These represent frequency ranges that may be affected by sporadic RFI.
The majority of these are at the edges of frequency ranges masked in Fig. 10.
An updated RFI mask would get rid of them. This will certainly be the
preferred strategy for defining frequency ranges to be used for continuum
imaging. Here, we rejected all the absorption candidates that are within one
frequency channel of any of these false detections. This step reduced the
number of absorption candidates by a factor of $\sim$10.

Figure 8:  Distribution of false absorption (blue) and emission (red)
detections for the survey. The majority of these are the edges of frequency
regions masked in Fig. 10. The locations of absorption candidates (see column
12 of Table LABEL:tab:wisesamp) are marked by $\star$.

We visually examined RR and LL spectra of the remaining 48 absorption
candidates for consistency. Specifically, we imposed the criteria that
integrated optical depths estimated using the RR and LL spectra match within
3$\sigma$ and within errors the absorption profiles appear similar.

Table 3: High probability 21-cm absorption candidates

Source name | $z_{em}$ | $z_{abs}$(21-cm) | $\int\tau$dv(21-cm) | $\Delta$V90
---|---|---|---|---
|  |  | (km s-1) | (km s-1)
(1) | (2) | (3) | (4) | (5)
M0513$+$0100 | 2.673 | 1.9526 | 3.59$\pm$0.67 | 107
M0618$-$3158 | 2.134 | 1.9642 | 0.49$\pm$0.07 | 15
M1244$-$0446 | 3.114 | 2.3871 | 1.61$\pm$0.27 | 70
M1312$-$2026 | 5.064 | 3.0324 | 0.34$\pm$0.08 | 20
M1540$-$1453 | 2.098 | 2.1139 | 9.14$\pm$0.28 | 144

Note. — Column 1: source name. Column 2: emission redshift. Columns 3-5: H i
21-cm absorption redshift, integrated 21-cm optical depth limit and velocity
width of absorption profile based on the survey spectra presented in Fig. 9,
respectively. The spectra have been normalized using the corresponding peak
flux densities.

After all the statistical filtering described above, we were left with a total
of 15 candidates towards the sight lines which are identified in column 12 of
Table LABEL:tab:wisesamp. We extracted Stokes-$I$ spectra of gain calibrators
corresponding to these. For the following candidates: M0212-3822 ($z_{\rm
abs}$=2.1666), M0250-2627 ($z_{\rm abs}$=2.1665), M0422-2034 ($z_{\rm
abs}$=2.5924), M0513+0100 ($z_{\rm abs}$=2.1612, 2.3183, 2.6434), M0515-0120
($z_{\rm abs}$=2.1753), M0702-3302 ($z_{\rm abs}$=2.2769), M1448-1122 ($z_{\rm
abs}$=2.2973) and M2017-2933 ($z_{\rm abs}$=2.0733), we find an ‘absorption’
feature at the same redshifted frequency in the gain calibrator spectrum. The
angular separation between a target source and its gain calibrator is
typically 15 degree. Thus, it is unrealistic that at $z>2$ a true absorption
is present in both of them. Therefore we rejected these 10 candidates.

Figure 9: High probability absorption candidates. The survey and reobservation
spectra are shown as dotted (red) and solid (blue) lines, respectively.

Finally, we have 5 high probability candidates. These are listed in Table
LABEL:tab:abscand. We also estimated integrated optical depths ($\int\tau$dv)
and velocity width ($\Delta$V90) corresponding to the 5% and 95% percentiles
of the apparent optical depth distribution. These are very similar to the
values observed for 21-cm absorption lines detected in various surveys (see
e.g., Gupta et al., 2009; Dutta et al., 2017c).

We reobserved these high probability candidates with uGMRT (see Section 3.1
and Table LABEL:tab:obslog) using a bandwidth of 6.25 MHz centered at H i
21-cm line frequency corresponding to $z_{abs}$(21-cm) given in column 3 of
Table LABEL:tab:abscand. These observations were carried out at night to
reduce the effect of RFI. For better RFI mitigation, the frequency setup was
chosen to provide a spectral resolution of $\sim$1 km s-1. Recall, the survey
observations had a spectral resolution of $\sim$18 km s-1. In Fig. 9, we
present profiles from the survey and reobservation spectra. Clearly, only
M1540-1453 is confirmed. The remaining 4 candidates are due to RFI.

To summarize, based purely on the uGMRT survey spectra and blind 21-cm line
search, we identified 5 absorption features (4 intervening and 1 associated
systems). The followup observations confirmed only one of these i.e.,
absorption associated with the radio source M1540-1453 at $z_{em}$ = 2.098.
The distribution of 5$\sigma$ 21-cm optical depth limits at 420 MHz estimated
assuming $\Delta v$=25 km s-1 is shown in Fig. 4. The median 0.535 km s-1 is
well below the sensitivity (1.1 km s-1) required to detect CNM (i.e T$\sim$100
K) in DLAs (i.e log $N$(H i)$\geq$ 20.3).

##  4 Associated H i 21-cm absorption detection towards M1540-1453

Figure 10: Associated H i 21-cm absorption detection towards M1540-1453. The
zero of the velocity scale is defined with respect to the peak of the
absorption i.e., $z_{\rm abs}$= 2.1139. The solid horizontal line corresponds
to $\Delta V_{90}$.

The H i 21-cm absorption spectrum of M1540-1453 based on the followup
observation is presented in Fig. 10. It is normalized using the corresponding
peak continuum flux density of 652 mJy beam-1. The absorption is spread over
$\sim$ 300 km s-1. We measure a total optical depth, $\int\tau$dv =
11.30$\pm$0.07 km s-1 and about 90% of this is confined to $\Delta V_{90}$ =
167 km s-1. This translates to a H i column density of $(2.06\pm 0.01)\times
10^{21}({T_{S}\over 100})({1\over f_{c}})$ cm-2. Here, $T_{\rm s}$ is the spin
temperature in Kelvin and $f_{c}$ is covering factor of absorbing gas. We have
assumed $T_{\rm s}$ = 100 K for the CNM (Heiles & Troland, 2003) and $f_{c}$ =
1. The inferred $N$(H i) will be higher if the gas is warmer and/or partially
covers the absorbing gas.

We obtained an optical spectrum of M1540-1453 using the Robert Stobie
Spectrograph (RSS; Burgh et al., 2003; Kobulnicky et al., 2003) on SALT as
part of our optical survey summarized in Section 2. The spectrum has a typical
signal-to-noise ratio (SNR) of 7 per pixel. It shows C iv and [C iii] emission
lines. The spectral SNR close to the [C iii] emission line is poor due to
residuals from the subtraction of skylines. Hence, we focus on the C iv
emission line. The peak of C iv emission corresponds to $z_{\rm em}$$\simeq$
2.113 which is consistent with the 21-cm absorption peak (Fig. 10). The
emission line is superimposed with absorption lines possibly also of C iv but
at a redshift slightly lower than the 21-cm absorption.

Our SALT spectrum does not cover ${\rm Ly}\alpha$ absorption for M1540-1453.
But it covers the rest wavelength range of 1436 to 2414Å. Although the region
is affected by skylines, in principle, we have access to the Fe ii lines
associated with the 21-cm absorption. We detect an absorption feature exactly
at the redshifted wavelength corresponding to Fe ii$\lambda$2383 line. The
redshift and rest equivalent width of the absorption are $z_{\rm abs}$=
2.11404 and $W_{\rm r}$ = $1.05\pm 0.25$Å, respectively. We also find
absorption dips at the expected positions of the Fe ii$\lambda$2344, Fe
ii$\lambda$2374 and Si ii$\lambda$1526 lines. These coincidences are
interesting because metal absorption line ratios can be a reasonable indicator
of H i column density (Rao et al., 2006; Gupta et al., 2012; Dutta et al.,
2017b). A better quality optical spectrum covering Ly$\alpha$ and these metal
lines is needed to extract physical conditions prevailing in the absorbing
gas.

We follow the method described in Srianand et al. (2008) to constrain the
visual extinction, $A_{V}$. Using our flux calibrated SALT spectrum along with
the Small Magellanic Cloud (SMC) type extinction curve and the average QSO
spectral energy distribution (SED) given in Selsing et al. (2016), we measure,
$A_{V}=0.13\pm 0.01$. The moderate extinction observed towards M1540-1453 is
consistent with the idea that cold atomic gas is accompanied by dust (see Fig.
9 of Dutta et al., 2017b).

To date only three associated H i 21-cm absorbers are known222We note that H i
21-cm absorber ($z_{abs}$ = 1.9436) has been detected towards the QSO PKS
1157+014 at $z_{em}=$ 1.978 (Wolfe et al., 1981) which is slightly below the
redshift cut-off used here. It is suggested that in this case the absorption
originates from a galaxy bound to the cluster containing the QSO. at $z>2$.
These are: $z_{\rm abs}$= 3.3968 towards B2 0902+345 (Uson et al., 1991;
Briggs et al., 1993), $z_{\rm abs}$= 2.6365 towards MG J0414+0534 (Moore et
al., 1999) and $z_{\rm abs}$= 3.52965 towards 8C 0604+728 (Aditya et al.,
2021). Thus, M1540-1453 absorber reported here is only the fourth detection at
$z>2$. The inferred column densities for reasonably assumed values of spin
temperature and covering factor ($T_{\rm s}$ = 100 K; $f_{c}$ =1) imply $N$(H
i)$>>2\times 10^{20}$ cm-2 which is the formal DLA cut-off. So, in all these
cases if the optical and radio sightlines coincide then one expects to see a
DLA at the 21-cm absorption redshift. We investigate this for B2 0902+345 and
MG J0414+0534, the two sources for which optical spectra are available in
literature.

B2 0902+345 is associated with a radio galaxy that exhibits ${\rm Ly}\alpha$
emission extended up to 50 kpc. The associated radio continuum emission
($\alpha$ = -0.94) has a highly distorted radio morphology over 6′′ ($\sim$45
kpc at $z_{abs}$) and rotation measure in excess of 1000 rad m-2 (Carilli et
al., 1994). However, no signatures of ${\rm Ly}\alpha$ absorption associated
with the 21-cm absorber are seen. In fact, the 21-cm absorption is found to be
redshifted with respect to the ${\rm Ly}\alpha$ emission (shift $\sim$ +300 km
s-1; see Adams et al., 2009, for details).

MG J0414+0534 is a highly reddened gravitationally lensed quasar ($A_{V}\sim$5
Lawrence et al., 1995b). The weakness of ${\rm Ly}\alpha$ emission prevents us
from searching for a DLA. However 4 associated strong Fe ii absorption
components are detected in the redshift range 2.6317-2.6447 (Lawrence et al.,
1995a). These are within the range over which CO emission is detected but do
not exactly coincide with the 21-cm absorption redshift, which itself is
shifted by $\sim$200 km s-1 with respect to the peak of the CO emission line.
The 21-cm absorption in this case may actually be towards a steep-spectrum
radio jet component not spatially coinciding with the AGN (Moore et al.,
1999). The same scenario may also apply to B2 0902+345.

In comparison, the radio emission associated with M1540-1453 is compact in
VLASS. It has a deconvolved size of $1.8^{\prime\prime}\times
0.7^{\prime\prime}$ with a position angle of $155^{\circ}$ which corresponds
to $\sim$15 pc at $z_{abs}$. The CNM clouds responsible for H i 21-cm
absorption are generally larger than this (Gupta et al., 2018b). Thus, it is
most likely the case that $f_{c}=1$ for M1540-1453. Further, the radio
continuum peak coincides well with the PS1/MIR counterparts. All these explain
the coincidence between 21-cm absorption and metal absorption lines observed
towards this source. This is also consistent with the profile of 21-cm
absorption which is only slightly asymmetric at the base and does not show
signatures of outflows i.e., blue-shifted absorption (Vermeulen et al., 2003;
Gupta et al., 2006). In low-$z$ samples, such absorption likely originates
from the circumnuclear disk or gas clouds associated with the host galaxy
(e.g., Geréb et al., 2015; Srianand et al., 2015). In Section 6, we note that
the quasars in our sample are generally hosted in gas and dust poor galaxies.
The CO emission line and millimetre continuum observations of M1540-1453 will
reveal the nature of its host galaxy ISM and shed light on the origin of gas
detected in 21-cm absorption.

##  5 Intervening absorption statistics

In this section, we constrain the occurrence of intervening H i 21-cm
absorbers at $z>2$ using a blind spectroscopic search. We also use ${\rm
Ly}\alpha$ and metal absorption lines detected in our SALT spectra to
interpret these statistics.

###  5.1 Blind H i 21-cm absorption line search

To estimate the incidence of intervening absorbers, we first determine the
sensitivity function, $g({\cal{T}},z)$, as a function of integrated optical
depth (${\cal{T}}$) and redshift ($z$). For this we follow the formalism
provided in Gupta et al. (2021). The two crucial inputs required to determine
$g({\cal{T}},z)$ are spectral weight ($W$) and completeness fraction ($C$).
The former accounts for the possibility that some of the targets in the sample
may not have spectroscopic redshifts. Since, all the targets in our sample
have spectroscopic redshifts, we assign $W$ = 1.

Figure 11:  Completeness corrected total redshift paths ($\Delta
z({\cal{T}})\equiv g({\cal{T}})$) for the 21-cm line search. The horizontal
dashed line represents total redshift path without completeness correction.
The vertical dashed lines correspond to integrated optical depth, ${\cal{T}}$
= 1.1 km s-1. This corresponds to a 5$\sigma$ detection limit of $N$(H i) =
$2\times 10^{20}$ cm-2 for $T_{s}$ = 100 K.

Figure 12:  The sensitivity function, $g(z)$, for H i 21-cm absorbers with
integrated 21-cm optical depth ${\cal{T}}\geq$ 1.1 km s-1. The abrupt dips are
caused by the spectral channels removed from the data due to RFI.
Approximately 30% of the redshift path is lost due to RFI.

The completeness fraction accounts for the detectability of absorption line
features of different line shapes. To determine this we consider the
absorption profiles of all the intervening absorbers detected from our surveys
in last 15 years. We inject 200 single Gaussian components with widths
consistent with the distribution of $\Delta V_{90}$ of these absorbers (see
Fig. 8 of Gupta et al., 2021) at each pixel and apply the detection algorithm
described in Section 3.3 to compute $C({\cal{T}}_{j},z_{k})$ as

$C({\cal{T}}_{j},z_{k})=\frac{1}{N_{inj}}\sum_{i=1}^{N_{inj}}F({\cal{T}}_{j},z_{k},\Delta
V_{i}),$ (2)

where $N_{inj}$ is the number of injected systems and $F=1$ if the injected
system is detected and 0 if not. The total completeness corrected redshift
path of the survey, $g({\cal{T}}_{j}$), considering all sight lines is plotted
in Fig. 11. The redshift path starts falling off rapidly below ${\cal{T}}$ =
1.5 km s-1.

It is of particular interest to consider the detectability of H i 21-cm
absorption in DLAs, i.e., ${\cal{T}}$ = 1.1 km s-1(refer to vertical dashed
line in Fig. 11). The sensitivity function providing the number of spectra in
which it is possible to detect CNM in DLAs is shown in Fig. 12. The total
redshift and comoving path length are $\Delta z=$ 38.3 and $\Delta X=$ 130.1,
respectively. Then, the incidence or number of 21-cm absorbers per unit
redshift and comoving path length are $n_{21}<$ 0.048 and $\ell_{21}<$ 0.014,
respectively. These 1$\sigma$ upper limits are based on small number Poisson
statistics (Gehrels, 1986).

The formalism to search for H i 21-cm absorption line presented above can also
be applied to OH main lines. For stronger OH main line at 1667.359 MHz,
${\cal{T}}$ = 1.1 km s-1 and excitation temperature of 3.5 K will correspond
to $N$(OH) = 8.6$\times 10^{14}$ cm-2. The total redshift and comoving path
length are $\Delta z=$ 44.9 and $\Delta X<$ 167.7, respectively. The number of
OH absorbers per unit redshift and comoving path length are $n_{\rm OH}$ =
0.041 and $\ell_{\rm OH}<$ 0.011, respectively.

Besides H i 21-cm absorption, CNM at high-$z$ may also be searched using
absorption lines of H2 and C i (e.g., Srianand et al., 2012; Noterdaeme et
al., 2018). Recently, Krogager & Noterdaeme (2020) using a canonical two phase
model of atomic gas and the observed statistics of H2 and C i absorbers at
high-$z$ estimated the comoving path length of CNM, $n_{\rm CNM}$ = 0.012. The
upper limit of $n_{21}<$ 0.048 obtained through our blind survey is consistent
with this. This result is the first study comparing the CNM cross-section of
galaxies at $z>2$ estimated using radio and optical/ultraviolet absorption
lines. The detectability of H2 and C i absorption at optical/ultraviolet and H
i 21-cm absorption at radio wavelength are affected by different systematic
effects. Indeed, there does not exist a one-to-one correspondence between the
presence of H2 and H i 21-cm absorption, and the difference may be due to
small sizes of H2 bearing clouds (see Srianand et al., 2012, for a
discussion). Much larger radio surveys are needed to disentangle these
possibilities.

###  5.2 Relationship with ${\rm Ly}\alpha$ and metal lines

Figure 13: Voigt profile fits to the 6 DLAs detected in our new radio loud
quasar sample. The profiles given in solid and long-dashed lines correspond to
the best fit and 1$\sigma$ range around it respectively. The absorption
redshift and the best fit $N$(H i) obtained are also provided in each panel.
Table 4: Properties of the DLAs derived from our observations

QSO | $z_{\rm abs}$ | log $N$(H i) | W(Si ii) | Z(P08) | Z | $\int\tau_{21}dv$ | $T_{s}$
---|---|---|---|---|---|---|---
|  |  | (Å) |  |  | (km s-1) | (K)
(1) | (2) | (3) | (4) | (5) | (6) | (7) | (8)
M0416-3339 | 2.8561 | 20.50$\pm$0.10 | 0.12$\pm$0.03 |  $-$2.2 | $\leq-2.3^{a}$ | RFI | ….
M0610-2301 | 2.3987 | 20.85$\pm$0.10 | 0.66$\pm$0.02 |  $-$1.2 | $\leq-1.1^{b}$ | $\leq 0.64$ |  $\geq$603
M1013-2546 | 2.6834 | 20.35$\pm$0.15 | $\leq 0.90$ | …. | …. | $\leq 0.72$ |  $\geq$169
M1351-0129 | 2.7719 | 20.50$\pm$0.10 | 0.19$\pm$0.03 | $-1.9$ | $\leq-1.2^{c}$ | RFI | ….
M1619-0939 | 2.7934 | 20.55$\pm$0.10 | 1.03$\pm$0.05 | $-0.9$ | $\leq-1.9^{c}$ | RFI | ….
M2154-3826 | 2.7613 | 21.00$\pm$0.10 | 0.53$\pm$0.01 | $-1.3$ | $-1.60\pm 0.11^{d}$ | …. | ….

Note. — Column 1: source name. Columns 2 and 3: DLA redshift and H i column
density. Column 4: Si ii equivalent width. Column 5: metallicity inferred
using W(Si ii) and correlation from Prochaska et al. (2008a). Column 6:
metallicity measured using weak Si ii or S i lines. Column 7: 5$\sigma$ 21-cm
optical depth limit, considering $\Delta v$ = 25 km s-1.

$a$: Based on Si ii$\lambda 1301$ line; $b$: Based on S ii$\lambda$1250 line;
$c$: Based on Si ii$\lambda$1808 line; $d$: Using the curve of growth
analysis.

Our SALT spectra allow us to search for Fe ii$\lambda\lambda\lambda$2343,
2374, 2383 for $z_{\rm abs}$$\leq$2.15 and DLAs at $z>2.65$. For this search,
we complement our SALT-NOT survey spectra with more sensitive long-slit SALT
observations of 25 quasars $\mathrm{z_{em}>2.7}$ obtained to search for
extended ${\rm Ly}\alpha$ emission halos associated with powerful radio loud
quasars (see Shukla et al., 2021, for an example). In total, we identify 7
DLAs and 1 strong Fe ii absorber in our sample. Implications of lack of 21-cm
absorption in these high H i column density absorbers are discussed below. In
the redshift range 2.15 $\leq$ $z_{\rm abs}$$\leq$ 2.65 we also identify 21 C
iv absorbers for which neither Fe ii nor DLA can be searched in our spectrum.
The redshifted H i 21-cm line frequencies corresponding to these C iv
absorbers are unaffected by RFI and no H i absorption is detected. Note that C
iv can trace a wide range of ionization stages and so is not a good indicator
of the presence of a DLA or 21-cm absorption. We will use this only as an
indicator of the possible presence of multi-phase gas along the sight line.

First, we focus on the subset of 23 quasars that have sufficient SNR in
optical continuum and, hence, suitable to search for ${\rm Ly}\alpha$
absorption. The absorption profiles of 6 DLAs detected from this subsample are
shown in Fig. 13. The measured absorption redshifts are consistent with 5 of
these being intervening systems and remaining one ($z_{\rm abs}$= 2.7613
towards M2154-3826) being a PDLA.

We also detect strong ${\rm Ly}\alpha$ absorption at the systemic redshift of
M0507$-$3624\. While we see the evidence of the damping wings and a wide range
of absorption from singly ionised species, we also see non-zero flux in the
core of the ${\rm Ly}\alpha$ absorption line. It is possible that this system
is similar to the associated H2 bearing DLAs studied by Noterdaeme et al.
(2019) where the presence of flux in the absorption core is related to the
partial coverage. Based on damping wings we estimate log $N$(H i)$\sim$20.2.
However, with the present spectra we are unable to confirm the origin of
residual flux in the core. Higher spectral resolution dataset covering both
${\rm Ly}\alpha$ and Ly$\beta$ absorption is needed to get an accurate
estimate of the $N$(H i) for this absorber. For the purpose of this paper, we
consider this as a candidate PDLA.

For a total redshift path of 9.3, the detection of 5 intervening DLAs
correspond to a number of DLAs per unit redshift, $n_{DLA}$ = 0.54 $\pm$ 0.24.
This is slightly higher but due to large uncertainties consistent with the
measurement of 0.24$\pm$0.02 based on SDSS DLAs by Noterdaeme et al. (2012),
and $0.26^{+0.06}_{-0.05}$ towards radio-loud quasars based on the combined
CORALS and UCSD samples by Jorgenson et al. (2006). Since, the quasars in our
sample are fainter than previous surveys (see Fig. 1), it is also possible
that there is indeed a dependence between $n_{\rm DLA}$ and the faintness of
quasars as noted by Ellison et al. (2001) for the CORALS sample.

As discussed in next section, in comparison to associated H i 21-cm absorption
detection rates in low-$z$ AGNs, the detection of just 2 PDLAs and one
associated H i 21-cm absorber (M1540-1453) from our sample may seem surprising
low. But in fact the detection of 3 PDLAs from our sample is in fact a factor
of 3 larger than what we expect from the statistics of PDLAs observed at
$z\sim 3$ in SDSS (Prochaska et al., 2008a). Interestingly, from the
statistics of damped H2 absorption lines, Noterdaeme et al. (2019) have
suggested that the PDLA fraction in Prochaska et al. (2008b) may have been
underestimated. A complete search of ${\rm Ly}\alpha$ absorption towards all
the targets in our sample will confirm the above mentioned excesses of DLAs
and PDLAs. Specifically, from the observations of remaining $2<z<2.7$ sources
($\Delta z$ = 20.4), we expect to detect another $\sim$10 DLAs.

Using SALT 1D spectra we measure redshift, $N$(H i) and rest equivalent widths
of metal absorption lines corresponding to these DLAs. These measurements are
provided in Table. LABEL:tab:dla. The quoted error in $N$(H i) also include
continuum placement uncertainties. The single component Voigt profile fits to
the DLAs are shown in Fig 13. The rest equivalent width of Si ii$\lambda$1526Å
lines are provided in column 4 of Table. LABEL:tab:dla. The metallicities
inferred using the W(Si ii) and metallicity correlation (see equation 1 of
Prochaska et al., 2008a) are given in column 5. We also estimated metallicity
using weak transitions of Si ii or S ii. These are provided in column 6. For
$z_{\rm abs}$= 2.7613 PDLA towards M2154$-$3826 we detect several weak
transitions of Fe ii, Ni ii and Si ii. We used single component curve of
growth to measure metallicities in this case. Overall, the metallicities of
these DLAs are typically less than a tenth of the solar metallicity. However,
given the poor spectral resolution of our data these estimates may suffer from
hidden saturation effects.

Unfortunately strong persistent RFI at 360-380 MHz prevents H i 21-cm line
search at $z$ = 2.73-2.94 (see Fig. 12). Thus, we could observe 21-cm line
only for the $z_{\rm abs}$= 2.3987 DLA towards M0610-2301 and $z_{\rm abs}$=
2.6834 towards M1013-2546. We do not detect 21-cm absorption from these
systems. The 5$\sigma$ integrated optical depths are provided in column 7 of
Table LABEL:tab:dla. M0610-2301 is a compact radio source in the modest
quality VLASS quick look image. The deconvolved source size is
$1.3^{\prime\prime}\times 0.1^{\prime\prime}$ with a position angle of
$180^{\circ}$ (i.e., size $<$ 11 pc at $z_{\rm abs}$). M1013-2546 also appears
to be a core-dominated source. Thus, we assume complete coverage i.e.,
$f_{c}=1$. This together with the observed $N$(H i) translates to a lower
limit on the spin temperature, $T_{S}\geq 603$ K for M0610$-$2301 and $\geq$
169K for M1013$-$2546\. These limiting values of spin temperatures are higher
than the measured median $N$(H i) weighted $T_{S}$ of 70 K for the cold
neutral medium (CNM) in our galaxy (see Heiles & Troland, 2003). We note that
H i 21-cm observations of 23 DLAs from radio-selected samples of CORALS, UCSD
and Ellison et al. (2008) are available in the literature (Srianand et al.,
2012; Kanekar et al., 2014). The overall 21-cm absorption detection rate of
3/25 (12${}^{+11}_{-7}$%) is consistent with the measurements from optically
selected samples and the conclusion that DLAs at $z>2$ are predominantly warm
and tend to show high spin temperatures (see also Petitjean et al., 2000).
Much larger radio-selected surveys ($\Delta{\rm X}\gtrsim 10^{4}$) are needed
to uncover the population of dusty DLAs.

Our SALT spectra also allow us to detect Fe ii$\lambda$2383 line for $z_{\rm
abs}$$\leq 2.15$. We detect a strong Fe ii absorber at $z_{\rm abs}$= 2.1404
towards M0652-3230. This system also shows absorption lines from other singly
ionized species (i.e Si ii and Al ii). All these suggest high $N$(H i) (Dutta
et al., 2017b). But as can be seen from Fig. 3, the radio emission is
dominated by the double lobe structure. The separation between the two lobes
is $\sim 27^{\prime\prime}$ (250 kpc at $z_{\rm abs}$). Therefore, the radio
and optical sight lines are well separated. This explains the non-detection of
21-cm absorption at the redshift of Fe ii absorbers.

##  6 Associated absorption statistics

Figure 14:  Distributions of quasar redshift (filled histogram for all targets
and hatched for those unaffected by RFI) and 5$\sigma$ 21-cm optical depth
limits for associated H i absorption. The vertical dashed lines mark the
median for each distribution. The dotted line corresponds to $\tau_{21}$dv =
1.1 km s-1.

We searched for H i 21-cm absorption within 3000 km s-1 to the quasar emission
line redshift. In 28/88 cases (32%), the redshifted frequency is affected by
RFI. For remaining 60 sources, the distributions of redshift and 5$\sigma$
21-cm optical depth limit for a width of 25 km s-1 are shown in Fig. 14. The
median redshift of this subsample which includes M1312-2026 ($z=5.064$) is
2.288. For sources with resolved morphology (Fig. 3), we have searched for
absorption towards multiple components but consider only the optical depth
limit for the strongest component for statistics.

The only 21-cm absorption detection from the survey is described in Section 4.
For non-detections, the optical depth limits span a range of 0.103 - 3.6 km
s-1. The detection rate of $1.6^{+3.8}_{-1.4}$% without any sensitivity cut is
among the lowest for a 21-cm absorption line survey. If we adopt the detection
rate based on low-$z$ surveys, we would expect to detect approximately 20 H i
21-cm absorbers from our sample. For example, Maccagni et al. (2017) reported
a 27%$\pm$5% detection rate from the observations of 248 AGNs at
$0.02<z<0.25$. The 3$\sigma$ peak optical depth (resolution $\sim$ 16 km s-1)
limits are typically between 0.005 - 0.2 (median $\sim$ 0.04). For 25 km s-1
width adopted by us, the range corresponds to 5$\sigma$ integrated optical
depth, ${\cal{T}}$, of 0.2 - 7.1 km s-1(median $\sim$ 1.41). In our survey, we
achieve ${\cal{T}}$ better than 1.1 km s-1 in 90% of the cases. Thus, stark
contrast in detection rates is not due to differences in the sensitivity.

At low-$z$, the vast majority of targets have WISE color $W1-W2$, which is an
indicator of AGN accretion rate, to be less than 0.6. Among these the higher
fraction of detections are associated with larger $W2-W3$ colors, which is an
indicator of star formation rate (Glowacki et al., 2017; Chandola et al.,
2020). The compact radio sources still embedded within such gas and dust rich
environments are well placed to exhibit 21-cm detections. The powerful quasars
identified by our MIR-wedge (see equation 1) do not overlap with the above-
mentioned low-$z$ AGNs in color space defined by $W1-W2$ and $W2-W3$. Thus, we
hypothesize that the low non-detection rate in our sample is a consequence of
the gas poor environment of host galaxies. Since the detectability of H i
21-cm absorption also depends on the thermal state of the gas and the
morphology of radio emission, we validate this hypothesis through ${\rm
Ly}\alpha$ and metal lines covered in our SALT-NOT spectra.

First, we consider following four sources333We exclude M1312-2026 at $z=5.064$
as all the lines except ${\rm Ly}\alpha$ are redshifted into IR and not
covered in our SALT spectrum. at 2.7$<z<$3.5 : M0507$-$3624, M1013$-$2546,
M1351$-$1019 and M2047$-$1841\. For these we have access to both ${\rm
Ly}\alpha$ and metal absorption lines and the 21-cm frequency is not affected
by RFI. In the cases of M1013$-$2546 and M2047$-$1841, we do not detect a PDLA
or any associated C iv absorption system. This suggests the absence of neutral
and metal enriched ionized gas along the sight lines. In the case of
M0507$-$3624, we identified a potential PDLA. For log $N$(H i)$\sim$20.2
inferred from the damping wing of ${\rm Ly}\alpha$ absorption and H i 21-cm
non-detection, we place a lower limit of 216 K for the spin temperature
($f_{c}=1$). In the case of M1351$-$1019 while ${\rm Ly}\alpha$ absorption is
not prominent we do detect associated C iv absorption. In this case, we also
detect extended ${\rm Ly}\alpha$ emission. All this suggests that these two
sources are associated with gas rich environment. Therefore, the lack of H i
21-cm absorption here may just be due to lower CNM filling factor.

In general, 22 i.e., $\sim$23% quasars in our sample show associated C iv
absorption within 3000 km s-1 to the $z_{\rm em}$. In 11 of these cases 21-cm
absorption could not be searched due to strong RFI. Among remaining 10, we
also detect H i ${\rm Ly}\alpha$ absorption in 4 cases but none are DLAs. In
the remaining 6 cases, the ${\rm Ly}\alpha$ absorption is not covered in the
spectra but we do not detect corresponding absorption due to any low-
ionization species such as Fe ii (in 4 cases) and Si ii (in 6 cases). Since C
iv may come from a wide range of ionization stages, the absence of strong
${\rm Ly}\alpha$ and low-ionization absorption lines indicate the lack of
sufficient high column density neutral gas along the line of sight.

From the above we conclude that the high fraction of quasars in our sample are
indeed residing in gas and dust poor environments. An interesting counterpoint
to the lack of H i 21-cm absorption in our sample is provided by the 100%
detection rate of molecular gas through CO emission in a sample of eight
hyper-luminous WISE/SDSS (WISSH) quasars at $z\sim 2-4$ (Bischetti et al.,
2021). We note that a majority (6/8) of these would be selected by our MIR-
wedge (Equation 1). But WISSH quasars are much brighter (1.5 Vega magnitude
compared to our sample) in the $W4$ band (22$\mu$m) of WISE. The deliberate
selection of WISSH quasars as most luminous infrared sources ensures that they
are being observed through dust clouds (Weedman et al., 2012), and perhaps
represent an early phase in the evolution of quasar. Approximately only
$\sim$10% of quasars in our sample have $W4$ magnitudes comparable to above-
mentioned WISSH quasars with CO detections, and only in 3 cases ${\cal{T}}<$
1.1 km s-1 i.e., the sensitivity to detect CNM in $N$(H i)$>10^{20}$ cm-2 is
achieved. Although this may seem to conflict with our MIR-selected sample
strategy but luminous quasars only spend a small fraction of their total
lifetime ($\sim 10^{7}$ yr; Martini & Weinberg, 2001) in the dust-obscured
phase. Therefore, the the representation of dust-obscured quasars in our
unbiased sample will also be proportionately small. Considering only the AGNs
with sensitivity to detect CNM in $N$(H i)$>10^{20}$ cm-2 gas, we estimate the
CNM covering factor in the unobscured phase of quasars with radio luminosity
log $L_{\rm 1.4\,GHz}\simeq 27-29.3$ W Hz-1 to be 0.02.

Although, the most straightforward explanation for low detection rate in our
sample is gas and dust poor environment at host galaxy scales but the
distribution of cold gas at nuclear scales may also be influenced by the
additional factors such as high intrinsic radio or ultraviolet luminosities of
AGN (Curran & Whiting, 2010; Aditya et al., 2016; Grasha et al., 2019) and the
distribution of gas at nuclear scales in the context of unification model may
also contribute (Gupta & Saikia, 2006). Unfortunately, none of the CO-detected
quasars from Bischetti et al. (2021) are bright enough at radio wavelengths to
search for H i 21-cm absorption. Interestingly, their available SDSS spectra
do not show the presence of high-column density neutral gas i.e., a PDLA along
the quasar sight line (although ionized gas outflows are present). Thus,
although the molecular gas is distributed in rotating disks (extent 1.7 - 10
kpc; Bischetti et al., 2021), it is oriented such that cold gas cross-section
towards the quasar sight line is minimal. An extended bright radio source
embedded in such an environment may have still shown H i 21-cm absorption
(e.g., refer to the cases of B2 0902+345 and MG J0414+0534 in Section 4).

Finally, we note the non-detection towards the highest redshift quasar
($z_{\rm em}$= 5.062) in our sample: M1312-2026 (see also Carilli et al.,
2007, for 21-cm non-detections towards two $z\sim$5 AGNs). This brightest
radio loud quasar at $z>5$, has a radio-loudness parameter of R =
$f_{\nu,5GHz}$/$f_{\nu,4400\AA}$ = $1.4\times 10^{4}$. This R value is an
order-of-magnitude greater than that of any other $z>5$ AGN known-to-date
(Momjian et al., 2018; Saxena et al., 2018). The host galaxies of quasars at
such high redshifts can be associated with large amounts of dust and molecular
gas ($>10^{10}\,{\rm M_{\odot}}$), and high inferred star formation rates
($>100\,{\rm M_{\odot}\,yr^{-1}}$) (Venemans et al., 2017; Decarli et al.,
2018; Feruglio et al., 2018). Our H i 21-cm non-detection corresponds to a
5$\sigma$ upper limit of $N$(H i)$<4\times 10^{19}$ cm-2= 100 K; $f_{c}$ = 1
assumed) but can miss narrow absorption components due to the RFI. The current
SALT spectrum also covers only ${\rm Ly}\alpha$ emission. Further
investigation on the nature of this very intriguing non-detection require IR
spectra and sub-arcsecond scale radio imaging which are in progress.

##  7 Summary and outlook

This paper described a spectroscopically blind search for H i 21-cm absorption
lines in the wide band uGMRT spectra of 88 AGNs at $2<z<5.1$. We also applied
the same formalism to constrain the occurrence of intervening OH 18-cm main
lines. We show that compared to previous radio-selected samples of quasars to
search for DLAs, our sample for the uGMRT survey has targeted fainter objects
(median $i$ = 19.5 mag; see Fig. 2) and is a close representation of the
underlying luminosity function of quasars. Thus, our dust-unbiased sample of
AGNs with median radio spectral index, $\alpha^{1.4}_{0.4}$ = $-0.38$,
redshift, $z$ = 2.5 and spectral luminosity, log $L_{\rm 1.4GHz}$ = $27-29.3$
W Hz-1 is ideally suited to determine the occurrence of cold atomic gas
($T\sim$100 K) towards powerful quasars at $z>2$.

Through a spectroscopically blind search of absorption lines in all the uGMRT
spectra, we detected one new associated H i absorption which is towards
M1540-1453 at $z_{\rm abs}$= 2.1139. No intervening H i 21-cm absorption line
is detected. Our detection is only the fourth associated H i 21-cm absorber
known at $z>2$. It has a H i column density of $(2.06\pm 0.01)\times
10^{21}({T_{S}\over 100})({1\over f_{c}})$ cm-2. In our SALT spectrum, the
peak of C iv emission and low-ionization metal absorption lines are coincident
with that of the 21-cm absorption. The overall properties of 21-cm absorption
are consistent with it originating from a circumnuclear disk or gas clouds
associated with the host galaxy. The CO emission line observations and optical
spectra covering the ${\rm Ly}\alpha$ absorption (i.e., $\lambda\sim 3785$Å)
will allow us to understand the origin of cold gas detected in 21-cm
absorption.

Our survey is sensitive to detect CNM in DLAs corresponding to a total
redshift and comoving path length of $\Delta z=$ 38.3 and $\Delta X=$ 130.1,
respectively. Using this we constrain the incidence or number of 21-cm
absorbers per unit redshift and comoving path length to be $n_{21}<$ 0.048 and
$\ell_{21}<$ 0.014, respectively. The same formalism applied to OH main line
at 1667.359 MHz corresponds to total redshift and comoving path length of
$\Delta z=$ 44.9 and $\Delta X=$ 167.7, respectively. The number of OH
absorbers per unit redshift and comoving path length are $n_{\rm OH}<$ 0.041
and $\ell_{\rm OH}<$ 0.011, respectively. We note that the number of DLAs per
unit redshift interval i.e., $n_{\rm DLA}$($z$) at $2.3\leq z\leq 2.9$ is in
the range of 0.21 to 0.29 (Noterdaeme et al., 2012). This implies that the
covering factor of CNM gas in DLAs is $\leq$20 percent. These upper limits are
also consistent with $n_{\rm CNM}$ = 0.012 estimated using H2 and C i
absorbers, also tracers of cold gas, at high-$z$ (Krogager & Noterdaeme,
2020). Our result shows that a moderately larger survey such as MALS with
$\Delta$X$\gtrsim 10^{4}$ is important to precisely characterise the CNM
fraction and its redshift evolution at high-$z$.

The low-$z$ AGNs ($z<0.25$) exhibit H i 21-cm absorption detection rates of
$\sim$30% (e.g., Maccagni et al., 2017). Compared to this the low associated H
i 21-cm absorption detection rate ($1.6^{+3.8}_{-1.4}$%) and the CNM filling
factor of 0.2 from our survey is intriguing. We show that this is most likely
due to the fact that the powerful quasars in our sample are residing in gas
and dust poor environments, and that luminous quasars only spend a small
fraction of their total lifetime in dust-obscured phase. We use the spectral
coverage of ${\rm Ly}\alpha$ and various metal absorption lines in our optical
spectra to confirm the absence of high column density atomic gas towards the
quasar sight lines.

From our SALT spectra, we report detections of 5 intervening DLAs and 2 PDLAs
in our sample. The measured number of DLAs per unit redshift, $n_{\rm DLA}$ =
0.54 $\pm$ 0.24 is slightly higher but due to large uncertainties consistent
with the measurement based on SDSS DLAs (Noterdaeme et al., 2012) and the
combined CORALS and UCSD sample of radio-selected quasars (Jorgenson et al.,
2006). Interestingly, the PDLA detection fraction is also a factor of 3
larger. Since the quasars in our sample are fainter than in the previous
surveys, there may indeed be a dependence between $n_{\rm DLA}$ and optical
faintness of quasars (Ellison et al., 2001). These results also underline the
need for larger surveys of dust-unbiased DLAs. Due to limited spectral
coverage, we could search for ${\rm Ly}\alpha$ in only 30% of our SALT-NOT
sample presented here. A complete search of ${\rm Ly}\alpha$ absorption
towards all the targets will allow us to examine the above-mentioned excesses
at a higher significance level.

Eventually, much larger radio-selected surveys ($\Delta$X $\gtrsim 10^{4}$)
such as MALS are needed to uncover the population of dusty DLAs. The key
science objectives are summarized in Gupta et al. (2017), and the survey is
well underway. The first L- and UHF-band spectra based on the science
verification data are presented in Gupta et al. (2021) and Combes et al.
(2021), respectively. Each MALS pointing is centered at a bright ($>200$ mJy
at 1 GHz) radio source. Through wideband spectra of the central bright radio
source and the numerous off-axis radio sources, it will sample the column
density distribution ($N$(H i)$>5\times 10^{19}$ cm-2; $T_{s}$ = 100 K)
relevant to characterize the cross-section of cold atomic gas in and around
normal galaxies and AGNs at $0<z<1.4$. Simultaneously, it will also be
sensitive to detect OH main line absorption at $z<1.9$ in gas with
$N$(OH)$>2.4\times 10^{14}$ cm-2 (excitation temperature = 3.5 K). Since the
formation of OH is tightly coupled to H2, the measurement of OH cross-section
at $z<2$ will be a crucial input through which to understand the redshift
evolution of CNM cross-section (Balashev et al., 2020).

We thank GMRT staff for their support during the observations. GMRT is run by
the National Centre for Radio Astrophysics of the Tata Institute of
Fundamental Research. This work is based on observations made with SALT and
NOT. The uGMRT data were processed using the MALS data processing facility at
IUCAA. The CASA package is developed by an international consortium of
scientists based at the National Radio Astronomical Observatory (NRAO), the
European Southern Observatory (ESO), the National Astronomical Observatory of
Japan (NAOJ), the Academia Sinica Institute of Astronomy and Astrophysics
(ASIAA), the CSIRO division for Astronomy and Space Science (CASS), and the
Netherlands Institute for Radio Astronomy (ASTRON) under the guidance of NRAO.
The National Radio Astronomy Observatory is a facility of the National Science
Foundation operated under cooperative agreement by Associated Universities,
Inc.

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