MeerKAT Absorption Line Survey (MALS)

• MALS is a deep radio survey using MeerKAT’s L- and UHF-band receivers to detect blind H I and OH absorption, targeting redshifts 0 to 2.
• It employs high spectral resolution with 32,768 channels and advanced pipelines like ARTIP to achieve sensitivity goals and robust calibration.
• The survey’s multi-purpose data releases support studies of radio continuum sources, Galactic interstellar medium, and cosmological anisotropies.

Searching arXiv for recent MALS papers to ground the article. The MeerKAT Absorption Line Survey (MALS) is a MeerKAT Large Survey Project conceived as a deep, dust-unbiased radio survey of cold gas, designed primarily to search for blind H I 21-cm and OH 18-cm absorption over $0$\Omega_{\rm HI}$, so that the central problem is not only how much neutral gas exists, but how much of it is in the cold phase traced by radio absorption. In practice, MALS has developed into both a spectral-line survey and a commensal continuum survey, with public data releases that support absorber searches, radio source population studies, Galactic interstellar-medium work, and cosmological number-count analyses (Gupta et al., 2017).

1. Survey conception and target architecture

MALS was designed to use MeerKAT’s L- and UHF-band receivers to carry out a sensitive search for intervening H I 21-cm and OH 18-cm absorption. In the survey design, the sensitivity goal is expressed as $N({\rm HI}) > 10^{19}\,{\rm cm}^{-2}$, with a target $5\sigma$ integrated optical-depth sensitivity at the pointing center of

$\int \tau\,dv = 0.045~{\rm km\,s^{-1}}.$

The planned survey configuration comprises 740 L-band pointings and 370 UHF-band pointings, with 56 minutes per L-band pointing and 121 minutes per UHF-band pointing, for a total observing time of about 1655 hr. The spectral setup uses 32,768 channels and 4 s correlator dumps, giving a spectral resolution of about 5 km s$^{-1}$ (Gupta et al., 2017).

The frequency coverage was selected to make the redshift path scientifically distinctive. For H I 21-cm, L-band corresponds to $0$\Omega_{\rm HI}$0 for L-band and $\Omega_{\rm HI}$1 for UHF-band. Within the design study, this geometry was expected to yield roughly 100 intervening 21-cm absorbers toward the central bright sources alone, about 200 intervening 21-cm absorbers overall, and roughly 500 associated 21-cm absorbers among the $\Omega_{\rm HI}$2 sources brighter than 30 mJy at $\Omega_{\rm HI}$3 GHz in the survey area (Gupta et al., 2017).

The central-source strategy required precursor optical spectroscopy. In “MALS-NOT: Identifying Radio-Bright Quasars for the MeerKAT Absorption Line Survey” (Krogager et al., 2018), candidates were selected with $\Omega_{\rm HI}$4 together with the WISE cuts

$\Omega_{\rm HI}$5

Of 99 observed candidates, 72 were securely identified as quasars, with 64 at $\Omega_{\rm HI}$6 and 48 at $\Omega_{\rm HI}$7. The later SALT–NOT campaign expanded this into a dust-unbiased, WISE mid-infrared color-selected sample of 303 AGN, comprising 250 sources with emission lines, 26 emission-line-less sources, and 27 dark fields, with median redshift $\Omega_{\rm HI}$8 (Gupta et al., 2021). These preparatory surveys established the spectroscopic backbone for MALS absorption sightlines while also quantifying the mix of broad-line AGN, narrow-line AGN, BL Lac candidates, and optically faint targets.

2. Observing mode, processing pipeline, and continuum data releases

MALS observations use the MeerKAT-64 array and the 32K correlator mode. In the L-band implementation described for the early data products, the total bandwidth is 856 MHz centered at (1283.9869) MHz, split into 32,768 channels with 26.123 kHz spacing; after flagging radio-frequency interference and band edges, the usable bandwidth is 802.5 MHz. Each L-band target was typically observed for 56 minutes total, divided into three scans at different hour angles. Processing is performed with the Automated Radio Telescope Imaging Pipeline (ARTIP), a CASA-based framework written in Python 3.6 with modular stages for calibration, cube production, continuum imaging, and diagnostics (Gupta et al., 2020).

The first public continuum release, “The MeerKAT Absorption Line Survey (MALS) data release I: Stokes I image catalogs at 1-1.4 GHz” (Deka et al., 2023), presents catalogues from 391 L-band pointings observed between 2020, April, 01 and 2021, January, 18. The release focuses on SPW2 at 1006.0 MHz and SPW9 at 1380.9 MHz. It contains 495,325 radio sources detected at signal-to-noise ratio $\Omega_{\rm HI}$9 over 2289 deg$N({\rm HI}) > 10^{19}\,{\rm cm}^{-2}$0 at 1006 MHz, and 240,321 sources over 1132 deg$N({\rm HI}) > 10^{19}\,{\rm cm}^{-2}$1 at 1380.9 MHz. The median synthesized beam size is $N({\rm HI}) > 10^{19}\,{\rm cm}^{-2}$2 at 1006 MHz and $N({\rm HI}) > 10^{19}\,{\rm cm}^{-2}$3 at 1380.9 MHz, and the median rms noise away from the pointing center is 25 and 22 $N({\rm HI}) > 10^{19}\,{\rm cm}^{-2}$4Jy beam$N({\rm HI}) > 10^{19}\,{\rm cm}^{-2}$5, respectively. Internal repeatability implies a flux-density scale ratio less than 1% with 8% scatter and astrometric precision of $N({\rm HI}) > 10^{19}\,{\rm cm}^{-2}$6; comparison with NVSS and FIRST gives flux-density accuracy better than 6% with 15% scatter and astrometric accuracy better than $N({\rm HI}) > 10^{19}\,{\rm cm}^{-2}$7 (Deka et al., 2023).

The second public release, “The MeerKAT Absorption Line Survey Data Release 2: Wideband continuum catalogues and a measurement of the cosmic radio dipole” (Wagenveld et al., 2024), uses all 391 L-band pointings to build wideband continuum catalogues from multi-term multi-frequency synthesis with $N({\rm HI}) > 10^{19}\,{\rm cm}^{-2}$8. The full wideband catalogue covers 4344 deg$N({\rm HI}) > 10^{19}\,{\rm cm}^{-2}$9, reaches a depth of about $5\sigma$0, and contains 971,980 sources, of which 58,122 are flagged as false detections. The typical restoring beam is $5\sigma$1, and the released wideband images are retained at full size, $5\sigma$2 on a side, with no primary-beam cutoff in the imaging products. The release also formalizes empirical flux-density uncertainty and bias relations and recommends the use of Isl_Total_flux for most scientific work (Wagenveld et al., 2024).

Across the releases, MALS continuum processing exploits the standard radio spectral convention

$5\sigma$3

This convention underlies both the source-population products and the in-band spectral-index measurements that later became central to image-based pulsar searches, radio-galaxy selection, and dipole modelling (Deka et al., 2023, Himes et al., 2024).

3. Absorption-line program and the cold-gas physics of AGN sightlines

The first science-verification paper, “Blind HI and OH absorption line search: first results with MALS and uGMRT processed using ARTIP” (Gupta et al., 2020), established the operational logic of the survey. A 40-minute MeerKAT observation of PKS1830-211 produced the most sensitive spectrum of that source to date, detecting the known H I absorber at $5\sigma$4 and the OH absorber at $5\sigma$5. The same paper used a blind uGMRT pilot survey of 72 bright radio sources to derive upper limits on absorber incidence, reporting $5\sigma$6 and $5\sigma$7, and constraining the cold-gas covering factor of galaxies at $5\sigma$8 to be less than 0.022. The argument advanced there was that the pilot was limited primarily by path length, whereas the full MALS survey, with expected $5\sigma$9, would provide a statistical census (Gupta et al., 2020).

The first UHF-band verification spectrum, “PKS1830-211: OH and HI at z=0.89 and the first MeerKAT UHF spectrum” (Combes et al., 2021), demonstrated the diagnostic power of simultaneous H I and OH coverage. With only 90 minutes on source, the continuum signal-to-noise ratio reached about 4000 per 6 km s$\int \tau\,dv = 0.045~{\rm km\,s^{-1}}.$0 channel. The spectrum detected the known H I 21-cm and OH 18-cm main lines at $\int \tau\,dv = 0.045~{\rm km\,s^{-1}}.$1, and for the first time at that redshift revealed the OH satellite lines. The OH analysis decomposed the line set into thermal and stimulated contributions, identified conjugate 1612/1720 MHz behaviour, and measured an isotropic 1720 MHz emission-line luminosity of about $\int \tau\,dv = 0.045~{\rm km\,s^{-1}}.$2, described as the most luminous known 1720 MHz maser line (Combes et al., 2021).

Subsequent single-sightline studies used MALS to probe absorber structure and variability. “Emergence of a new HI 21-cm absorption component at z~1.1726 towards the gamma-ray blazar PKS~2355-106” (Srianand et al., 2022) reported a new absorber at $\int \tau\,dv = 0.045~{\rm km\,s^{-1}}.$3 with $\int \tau\,dv = 0.045~{\rm km\,s^{-1}}.$4 and interpreted its appearance, relative to earlier GMRT spectra, as evidence for a large optical-depth gradient over a transverse scale of roughly 0.35 pc. “PKS1413+135: OH and HI at z = 0.247 with MeerKAT” (Combes et al., 2022) found high-significance H I absorption, clear OH 1720 MHz maser emission, tentative OH 1667 MHz absorption, and line variability over decades: the H I line depth changed by about 20%, while the OH-1720 MHz depth changed by a factor 4. The narrow widths of the cm-wave lines were used to argue that the absorber arises in an outer gas ring rather than the nuclear region (Srianand et al., 2022, Combes et al., 2022).

At higher redshift, “MALS discovery of a rare HI 21-cm absorber at $\int \tau\,dv = 0.045~{\rm km\,s^{-1}}.$5: origin of the absorbing gas in powerful AGN” (Deka et al., 2023) reported the first associated H I 21-cm absorber discovered by MALS, toward the quasar J2339-5523 at $\int \tau\,dv = 0.045~{\rm km\,s^{-1}}.$6. The profile is broad, with $\int \tau\,dv = 0.045~{\rm km\,s^{-1}}.$7, and redshifted by roughly $\int \tau\,dv = 0.045~{\rm km\,s^{-1}}.$8. Optical and far-UV spectra show no corresponding absorption at the 21-cm redshift, despite a flat-spectrum high-frequency radio core. The paper interprets this mismatch as evidence that no large H I column lies along the optical/UV sightline to the nucleus, and uses a literature comparison to argue that quasars and radio galaxies trace different cold-gas geometries around AGN (Deka et al., 2023).

A broader associated-absorption analysis appears in “HI 21-cm absorption in low- and high-excitation radio-loud AGNs at $\int \tau\,dv = 0.045~{\rm km\,s^{-1}}.$9 from MALS” (Deka et al., 21 Apr 2026). Cross-matching the MALS 1006 MHz catalogue with SDSS DR18 yielded 1908 radio sources at $^{-1}$0; 613 were classified as radio-loud AGNs, and 426 of these were further divided into 327 low-excitation radio galaxies and 99 high-excitation radio galaxies using a weighted multi-criterion framework centered on the excitation index

$^{-1}$1

A radio-bright subsample of 99 AGNs with $^{-1}$2 mJy produced five new associated H I 21-cm detections at $^{-1}$3, with a sensitivity-matched detection rate of $^{-1}$4. The paper concludes that there is no significant evidence for redshift evolution in the detection rate out to $^{-1}$5, and no convincing evidence that incidence depends strongly on radio luminosity across $^{-1}$6–27.0. The detected profiles show mixed redshifted and blueshifted kinematics and, in the LERG subset, asymmetries and velocity offsets exceeding $^{-1}$7, which are interpreted as disturbed cold gas likely linked to jets, lobes, or merger-driven inflow (Deka et al., 21 Apr 2026).

4. Commensal continuum science and radio-source populations

Although MALS was designed for absorption-line work, its continuum output rapidly became a survey product in its own right. The DR1 continuum analysis established that the MALS 1.4 GHz source counts are in agreement with the literature, measured spectral indices for 125,621 sources with $^{-1}$8, confirmed a flattening of spectral indices with decreasing flux density, and identified 140 ultra steep-spectrum sources with $^{-1}$9 as prospective high-$0Deka et al., 2023).

This commensal continuum capability underpins specialized discovery programs. “Fermi Unassociated Sources in the MeerKAT Absorption Line Survey” (Himes et al., 2024) uses MALS DR1 as a radio-image catalogue to search for steep-spectrum counterparts to Fermi unassociated $0

$0

to compact sources inside the 95% Fermi error regions. The selection pipeline reduces 1,090 compact MALS sources in 74 Fermi ellipses to 50 unique compact steep-spectrum sources, which are then classified with VLASS-QL, TGSS ADR1, RACS-low1, WISE, Pan-STARRS1, and DECaLS into RG, T1, and T2 classes. The final result is 4 Tier 1 pulsar candidates, 5 Tier 2 pulsar candidates, and 41 radio galaxy candidates. A sensitivity comparison to the known pulsar population concludes that most older continuum surveys were sensitive to only 7%–21% of known pulsars, whereas MALS is sensitive to 80% of the known pulsar population (Himes et al., 2024).

MALS continuum imaging has also been used for morphological classification of rare radio galaxies. “Hybrid Morphology Radio Sources from the MeerKAT Absorption Line Survey (MALS): Radio, Mid-infrared and Environmental Characteristics” (Manik et al., 25 Jun 2025) reports 36 new HyMoRS candidates, stated to be the largest collection of such sources in the southern sky to date. The search was conducted by visual inspection of MALS radio maps, cross-checked against RACS-mid. The identified sample spans $0

$0

with one side required to satisfy $0Manik et al., 25 Jun 2025).

5. Milky Way and nearby-system applications

The third public release, “The MeerKAT Absorption Line Survey (MALS) data release 3: Cold atomic gas associated with the Milky Way” (Gupta et al., 31 Mar 2025), extends the survey into Galactic H I absorption. Using 390 usable MeerKAT L-band pointings and spectra toward 19,130 radio sources brighter than 1 mJy at 1.4 GHz within $0Bayesian Information Criterion. The resulting catalogue contains 3,640 unique absorption features, of which 3,158 were visually confirmed and 2,011 satisfy the recommended peak $0.40Gupta et al., 31 Mar 2025).

The Galactic sample is interpreted as a homogeneous population of H I clouds in the local interstellar medium. The evidence includes the confinement of almost all central-sight-line absorbers to

$0.40

and a strong gas-to-dust relation toward the pointing centers,

$0.40

with correlation coefficient 0.84. The paper reports that $0.40cold neutral medium. A central-versus-off-axis comparison over projected separations of $0.40

$0.40

with $0Gupta et al., 31 Mar 2025).

MALS also supports nearby extragalactic gas studies through the combination of absorption and emission in the same field. “Mapping HI 21-cm in the Klemola 31 group at z = 0.029: emission and absorption towards PKS2020-370” (Maina et al., 2022) is a proof-of-concept case in which MeerKAT detected H I emission from four group members and H I absorption against PKS 2020-370 associated with Klemola 31A at impact parameter $0

$0

A symmetric tilted-ring interpretation implies $0Maina et al., 2022).

A related nearby-gas application appears in “Origin of gas in the Magellanic Bridge: MeerKAT detection of HI 21-cm absorption” (Morelli et al., 26 Sep 2025). Using MALS data toward J033242.97-724904.5, the paper reports an H I absorption detection with peak signal-to-noise ratio 10 against a 244.1 mJy beam$0

$0.64

is found to be closer to the LMC value than the SMC value. The authors therefore conclude that the absorbing Magellanic Bridge gas is more likely LMC-origin material, and that the large-scale kinematics favour a direct LMC–SMC collision scenario over a close fly-by (Morelli et al., 26 Sep 2025).

6. Cosmological number counts, systematics, and the scientific scope of the survey

The cosmological use of MALS continuum catalogues emerged first in a methodological study of ten pointings, “The MeerKAT Absorption Line Survey: Homogeneous continuum catalogues towards a measurement of the cosmic radio dipole” (Wagenveld et al., 2023). That paper constructs a combined catalogue containing 16,313 sources; the abstract gives a sky coverage of 37.5 square degrees, while the detailed description gives 35.7 deg$0.64Wagenveld et al., 2023).

The wider DR2 analysis turned that feasibility study into a measurement. The dipole interpretation uses the kinematic relation

$0.64

for source counts, with $0.64projection effects and models through the major axis of the restoring beam, $0.64$\Omega_{\rm HI}$00Jy, the final result is

$\Omega_{\rm HI}$01

with a direction around $\Omega_{\rm HI}$02, consistent with the CMB dipole within the uncertainties. The paper explicitly contrasts this with many recent centimetre-wavelength catalogues that yield significantly larger amplitudes (Wagenveld et al., 2024).

Taken together, the MALS literature defines a survey whose scientific scope is broader than its title might suggest. It remains an absorption survey in design logic, but it has already delivered a continuum survey with nearly a million sources, a Galactic H I absorption catalogue, a framework for cosmic dipole estimation, and targeted source-population studies ranging from pulsar candidates to hybrid-morphology radio galaxies. Several results also delimit the interpretive regime. Absorption variability toward PKS~2355-106 and PKS1413+135 shows that radio and optical sightlines are not always co-spatial and that absorber structure can vary on sub-parsec scales or over decades (Srianand et al., 2022, Combes et al., 2022). The HyMoRS study explicitly states that higher-resolution follow-up is still necessary to confirm candidate morphologies (Manik et al., 25 Jun 2025). The low-$\Omega_{\rm HI}$03 AGN absorption study finds no convincing evidence for strong redshift or radio-power dependence within the present sample and sensitivity limits (Deka et al., 21 Apr 2026). These results do not narrow the significance of MALS; rather, they specify the conditions under which its products should be interpreted.

In that sense, MALS occupies a distinctive place among SKA-pathfinder surveys. It was conceived to measure the redshift evolution of cold atomic and molecular gas through blind, dust-unbiased absorption, but its execution has shown that the same observing strategy can support statistically meaningful continuum catalogues, source-classification workflows, Galactic interstellar-medium analyses, and precision tests of large-scale anisotropy. This suggests that the survey’s long-term impact will depend not only on its absorber counts, but on the way its multi-purpose data products connect cold gas, AGN environments, radio source populations, and cosmological systematics within a single MeerKAT framework (Gupta et al., 2017, Wagenveld et al., 2024).