--- title: MeerKAT Absorption Line Survey (MALS) url: https://www.emergentmind.com/topics/meerkat-absorption-line-survey-mals type: topic --- # MeerKAT Absorption Line Survey (MALS) 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 \(0 10^{19}\,{\rm cm}^{-2}\), 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\(^{-1}\) [1708.07371]. The frequency coverage was selected to make the redshift path scientifically distinctive. For H I 21-cm, L-band corresponds to \(00.6\) for L-band and \(z>1.4\) 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 \(\sim 2000\) sources brighter than 30 mJy at \(\sim 1\) GHz in the survey area [1708.07371]. The central-source strategy required precursor optical spectroscopy. In “MALS-NOT: Identifying Radio-Bright Quasars for the MeerKAT Absorption Line Survey” [1802.00482], candidates were selected with \(F_{1.4\,\mathrm{GHz}} > 200~\mathrm{mJy}\) together with the WISE cuts \[ W_1 - W_2 < 1.3 \times (W_2 - W_3) - 3.04,\qquad W_1 - W_2 > 0.6. \] Of 99 observed candidates, 72 were securely identified as quasars, with 64 at \(z>0.6\) and 48 at \(z>1.4\). 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 \(z_{\rm med}=1.8\) [2107.09705]. 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 [2007.04347]. The first public continuum release, “The MeerKAT Absorption Line Survey (MALS) data release I: Stokes I image catalogs at 1-1.4 GHz” [2308.12347], 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 \(>5\) over 2289 deg\(^2\) at 1006 MHz, and 240,321 sources over 1132 deg\(^2\) at 1380.9 MHz. The median synthesized beam size is \(12^{\prime\prime}\) at 1006 MHz and \(8^{\prime\prime}\) at 1380.9 MHz, and the median rms noise away from the pointing center is 25 and 22 \(\mu\)Jy beam\(^{-1}\), respectively. Internal repeatability implies a flux-density scale ratio less than 1% with 8% scatter and astrometric precision of \(1^{\prime\prime}\); comparison with NVSS and FIRST gives flux-density accuracy better than 6% with 15% scatter and astrometric accuracy better than \(0\farcs8\) [2308.12347]. The second public release, “The MeerKAT Absorption Line Survey Data Release 2: Wideband continuum catalogues and a measurement of the cosmic radio dipole” [2408.16619], uses all 391 L-band pointings to build wideband continuum catalogues from multi-term multi-frequency synthesis with \(nterms=2\). The full wideband catalogue covers 4344 deg\(^2\), reaches a depth of about \(10~\mu\mathrm{Jy\,beam^{-1}}\), and contains 971,980 sources, of which 58,122 are flagged as false detections. The typical restoring beam is \(8.9''\times6.6''\), and the released wideband images are retained at full size, \(3.3^\circ\) 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 [2408.16619]. Across the releases, MALS continuum processing exploits the standard radio spectral convention \[ S_\nu \propto \nu^\alpha. \] 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 [2308.12347; 2412.12470]. ## 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” [2007.04347], 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 \(z=0.19\) and the OH absorber at \(z=0.89\). The same paper used a blind uGMRT pilot survey of 72 bright radio sources to derive upper limits on absorber incidence, reporting \(n_{21}(z\sim0.18)<0.14\) and \(n_{\rm OH}(z\sim0.40)<0.12\), and constraining the cold-gas covering factor of galaxies at \(50\,\mathrm{kpc}<\rho<150\,\mathrm{kpc}\) 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 \(\Delta z\sim10^{3-4}\), would provide a statistical census [2007.04347]. The first UHF-band verification spectrum, “PKS1830-211: OH and HI at z=0.89 and the first MeerKAT UHF spectrum” [2101.00188], 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\(^{-1}\) channel. The spectrum detected the known H I 21-cm and OH 18-cm main lines at \(z=0.89\), 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 \(6100\,L_\odot\), described as the most luminous known 1720 MHz maser line [2101.00188]. 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” [2207.01807] reported a new absorber at \(z_{\rm abs}=1.172635\) with \(\int \tau\,dv = 0.161 \pm 0.030\ \mathrm{km\ s^{-1}}\) 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” [2211.09355] 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 [2207.01807; 2211.09355]. At higher redshift, “MALS discovery of a rare HI 21-cm absorber at \(z\sim1.35\): origin of the absorbing gas in powerful AGN” [2311.00336] reported the first associated H I 21-cm absorber discovered by MALS, toward the quasar J2339-5523 at \(z_{\rm em}=1.3531\). The profile is broad, with \(\Delta V_{\rm null}\approx 397\ {\rm km\,s^{-1}}\), and redshifted by roughly \(200\ {\rm km\,s^{-1}}\). 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 [2311.00336]. A broader associated-absorption analysis appears in “HI 21-cm absorption in low- and high-excitation radio-loud AGNs at \(z<0.5\) from MALS” [2604.19896]. Cross-matching the MALS 1006 MHz catalogue with SDSS DR18 yielded 1908 radio sources at \(z<0.5\); 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 \[ {\rm EI} = \log({\rm [O\,III]}/\mathrm{H}\beta) - \frac{1}{3}\left[\log({\rm [N\,II]}/\mathrm{H}\alpha)+\log({\rm [S\,II]}/\mathrm{H}\alpha)+\log({\rm [O\,I]}/\mathrm{H}\alpha)\right]. \] A radio-bright subsample of 99 AGNs with \(S_{1.4\,\mathrm{GHz}}>4\) mJy produced five new associated H I 21-cm detections at \(0.298\), confirmed a flattening of spectral indices with decreasing flux density, and identified 140 ultra steep-spectrum sources with \(\alpha<-1.3\) as prospective high-\(z\) radio galaxies. The same release identified 1308 variable and 122 transient radio sources, comprising primarily AGN, using the 26-year baseline between NVSS and MALS [2308.12347]. This commensal continuum capability underpins specialized discovery programs. “Fermi Unassociated Sources in the MeerKAT Absorption Line Survey” [2412.12470] uses MALS DR1 as a radio-image catalogue to search for steep-spectrum counterparts to Fermi unassociated \(\gamma\)-ray sources. In that work, MALS DR1 is described as having median rms noise of 22 to 25 \(\mu\)Jy and 735,649 sources, of which 657,318 are compact and 551,069 compact sources have in-band spectral indices. The pulsar-oriented search applies the criterion \[ \alpha < -1.4,\qquad S_\nu \propto \nu^{+\alpha}, \] 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 [2412.12470]. 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” [2506.20211] 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 \(0.041.5\). Mid-infrared and environmental analysis suggests that many hosts are radiatively efficient, star-forming systems, while 9 of the 36 candidates are located near galaxy-cluster centers. The paper argues that hybrid morphology likely arises from a combination of orientation and dense environments, but it also stresses that these remain candidates identified by visual inspection and require higher-resolution follow-up for confirmation [2506.20211]. ## 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” [2504.00097], 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 \(48\farcm5\) of the pointing centers, the authors conducted a blind, automated search with Gaussian-component model selection based on the Bayesian 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 \(\mathrm{S/N}>6\) cut. The paper describes this as the largest Galactic H I absorption-line catalog to date [2504.00097]. 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 \[ -25 < v_{\rm LSR} < +25\ \mathrm{km\,s^{-1}}, \] and a strong gas-to-dust relation toward the pointing centers, \[ N_{\rm HI} = (2.21 \pm 0.20)\times 10^{21}\,A_V\ {\rm cm^{-2}\,mag^{-1}}, \] with correlation coefficient 0.84. The paper reports that \(\int \tau\,dv\) is linearly correlated with \(N_{\rm HI}\) and \(A_V\) up to \(A_V\sim1\) mag, and interprets the contrasting \(N_{\rm HI}\) distribution slopes for detections and non-detections as evidence for turbulence-driven conversion from warm neutral medium to cold neutral medium. A central-versus-off-axis comparison over projected separations of \(\sim 0.1\)–10 pc yields tiny median velocity shifts and an rms optical-depth fluctuation law \[ \Delta\tau_{\rm rms}(x) = \Delta\tau_{\rm rms}^o\,x^{(\alpha-2)/2}, \] with \(\alpha = 2.327 \pm 0.153\), corresponding to inferred densities \(n_{\rm HI}\sim 20\)–30 cm\(^{-3}\), typical of diffuse CNM [2504.00097]. 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” [2206.09940] 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 \(\sim 10\) kpc. After modelling and subtracting the local H I emission, the absorption measurement gives \[ \int \tau(v)\,dv = 0.26 \pm 0.03 \ \mathrm{km\,s^{-1}}. \] A symmetric tilted-ring interpretation implies \(T_s \approx 530\) K, but the velocity mismatch between the model and the absorption peak, \(\Delta v \approx 38\ \mathrm{km\,s^{-1}}\), together with the Na I/Ca II behaviour, is used to argue that the absorbing gas may instead be extra-planar [2206.09940]. A related nearby-gas application appears in “Origin of gas in the Magellanic Bridge: MeerKAT detection of HI 21-cm absorption” [2509.21970]. 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\(^{-1}\) continuum source. Combined with earlier ATCA detections, the new sightline extends cold-gas probing across 4–6 kpc in the Magellanic Bridge. The absorption centroid is \(224.59 \pm 0.97\ \mathrm{km\ s^{-1}}\), the bridge H I column density is \((1.18 \pm 0.07)\times 10^{20}\ \mathrm{cm^{-2}}\), and the inferred gas-to-dust ratio, \[ N_{\mathrm{H\,I}}/E(B-V) = (1.29 \pm 0.04\ (\mathrm{stat}) \pm 0.17\ (\mathrm{sys})) \times 10^{22}\ \mathrm{cm^{-2}\,mag^{-1}}, \] 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 [2509.21970]. ## 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” [2302.10696]. 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\(^2\). The analysis emphasizes that dipole measurements are limited not only by shot noise but by inhomogeneous completeness, false detections near bright sources, primary-beam attenuation, flux-scale offsets, and source splitting. A practical homogenization parameter, \(\sigma_{20}\), is introduced as the rms value at 20% coverage of the rms map. The study concludes that number counts can be corrected down to 100–200 \(\mu\)Jy, and that 100 MALS pointings should be sufficient for a \(3\sigma\) radio dipole detection if the catalogues are homogenized properly [2302.10696]. The wider DR2 analysis turned that feasibility study into a measurement. The dipole interpretation uses the kinematic relation \[ \mathcal D=[2+x(1-\alpha)]\beta \] for source counts, with \(x\simeq0.69\) for the MALS flux-density distribution and \(\langle\alpha\rangle\approx-0.75\), implying an expected CMB-based amplitude \(\mathcal D\approx0.40\times10^{-2}\). The decisive observational issue is a declination-dependent source-density systematic, which the DR2 paper links to array projection effects and models through the major axis of the restoring beam, \(\theta_{B,\mathrm{maj}}\). Once this linear correction is included, the MALS dipole estimate stabilizes. With a flux-density cut of 400 \(\mu\)Jy, the final result is \[ \mathcal D = 0.67^{+0.31}_{-0.29}\times10^{-2}, \] with a direction around \((\mathrm{R.A.},\mathrm{Dec.})\approx(176^\circ,20^\circ)\), consistent with the CMB dipole within the uncertainties. The paper explicitly contrasts this with many recent centimetre-wavelength catalogues that yield significantly larger amplitudes [2408.16619]. 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 [2207.01807; 2211.09355]. The HyMoRS study explicitly states that higher-resolution follow-up is still necessary to confirm candidate morphologies [2506.20211]. The low-\(z\) AGN absorption study finds no convincing evidence for strong redshift or radio-power dependence within the present sample and sensitivity limits [2604.19896]. 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 [1708.07371; 2408.16619]. Source: https://www.emergentmind.com/topics/meerkat-absorption-line-survey-mals