--- title: MeerKAT Absorption Line Survey (MALS) url: https://www.emergentmind.com/topics/meerkat-absorption-line-survey-mals-c88cc013-e842-4068-93a1-f8b82ac42eea type: topic --- # MeerKAT Absorption Line Survey (MALS) MeerKAT Absorption Line Survey (MALS) is a MeerKAT continuum and spectral-line survey primarily designed to carry out a search for intervening and associated neutral hydrogen (H I) 21-cm and hydroxyl radical (OH) 18-cm absorption lines at \(00.6\), while UHF-band pointings should be centered on bright sources at \(z>1.4\) [1708.07371]. Later survey descriptions state that pointings are centered on bright radio AGN, specifically sources brighter than 200 mJy at 1 GHz, and note that MeerKAT’s field of view is 88 arcmin FWHM at 1 GHz, which makes deep commensal continuum science unavoidable rather than incidental [2412.12470]. Two optical spectroscopic campaigns supplied the source-selection backbone. “MALS-NOT: Identifying Radio-Bright Quasars for the MeerKAT Absorption Line Survey” selected southern radio sources using a single flux-density limit \(F_{1.4\,{\rm GHz}}>200\) mJy together with WISE color cuts \[ W_1-W_2 < 1.3\,(W_2-W_3)-3.04, \qquad W_1-W_2 > 0.6, \] and spectroscopically observed 99 candidates with the Nordic Optical Telescope. It identified 72 quasars, of which 64 have \(z>0.6\) and 48 have \(z>1.4\), directly serving the L-band and UHF-band pointing requirements [1802.00482]. The later SALT–NOT campaign extended that effort to a larger, dust-unbiased, MIR-selected sample of powerful radio-bright AGN. It produced a sample containing 250 AGN with emission lines, 26 with no emission lines, and 27 without optical counterparts, with a median redshift of 1.8. The paper also summarized the full MALS central-AGN pool as about 650 AGN, giving the practical target base from which the survey pointings were assembled [2107.09705]. An important consequence is that MALS background sources are not merely bright; they are characterized by spectroscopic redshifts, optical classes, and MIR properties that are directly relevant for interpreting both associated and intervening absorption. ## 3. Data processing and public releases The computational scale of MALS led to the development of ARTIP, the Automated Radio Telescope Imaging Pipeline, written in Python 3.6 and built extensively on CASA. ARTIP was designed to enable the geographically distributed MALS team to process large volumes of radio interferometric data collaboratively, and early MALS work demonstrated its use on both MeerKAT and uGMRT data for blind H I and OH absorption searches [2007.04347]. The pipeline structure separates calibration, spectral-line cube production, continuum imaging, and diagnostics, and later MALS releases continued to use ARTIP for survey data products [2308.12347]. Three public releases define the present survey state. | Release | Content | Key figures | |---|---|---| | DR1 [2308.12347] | Stokes \(I\) image catalogues at 1006 and 1381 MHz from 391 L-band pointings | 495,325 sources over 2289 deg\(^2\) at 1006 MHz; 240,321 over 1132 deg\(^2\) at 1381 MHz; median rms 25 and 22 \(\mu\)Jy beam\(^{-1}\) | | DR2 [2408.16619] | Wideband L-band continuum catalogues from the same 391 pointings | 4344 deg\(^2\), depth 10 \(\mu\)Jy beam\(^{-1}\), 971,980 sources | | DR3 [2504.00097] | Blind Galactic H I 21-cm absorption catalogue | 19130 radio sight lines, 3640 H I absorption features | DR1 established the first large MALS continuum reference. It released Stokes \(I\) images and catalogues from 391 MeerKAT L-band pointings at \(\delta\lesssim+20^\circ\), observed between 2020 April 1 and 2021 January 18, with median spatial resolution \(12''\) at 1006 MHz and \(8''\) at 1381 MHz. The abstract states that the flux-density scale ratio and astrometric accuracy deduced from multiply observed sources are less than 1% with 8% scatter and \(1''\), respectively, and that external comparisons place the catalog accuracy better than 6% with 15% scatter in flux density and better than \(0.8''\) in astrometry [2308.12347]. DR2 converted the same 391 L-band pointings into the first full wideband continuum release. It used multi-term multi-frequency synthesis over a usable bandwidth of 802.5 MHz and delivered wideband source catalogues, Gaussian-component catalogues, primary-beam-corrected Stokes \(I\) images, and spectral-index images. The catalogue spans five orders of magnitude in flux density and is described as providing a robust view of the extragalactic radio population down to 200 \(\mu\)Jy. DR2 also quantified completeness, false-detection behavior, and a declination-dependent source-density systematic that had to be modeled explicitly in cosmological applications [2408.16619]. DR3 turned the same survey geometry into a Galactic absorption survey. Using 390 pointings and 19130 unique sight lines, it detected 3640 Galactic H I absorption features, which the paper describes as the largest Galactic H I absorption line catalog to date. The central sight lines reach much better optical-depth sensitivity than the full source ensemble, so DR3 also formalized the distinction between high-quality central spectra and the much larger but shallower off-axis sample [2504.00097]. ## 4. Intervening and associated absorption science Early MALS science verification already showed the survey’s wide-band absorption capability. The first MeerKAT UHF spectrum, obtained toward PKS1830−211, 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, the OH satellite lines. The paper decomposed the OH lines into thermal and stimulated contributions, identified the 1612 and 1720 MHz lines as conjugate, and inferred a total OH 1720 MHz emission-line luminosity of 6100 \(L_\odot\), described there as the most luminous known 1720 MHz maser line [2101.00188]. MALS science-verification work also demonstrated sensitivity to time-variable cold gas structure in known high-redshift absorbers. Toward the \(\gamma\)-ray blazar PKS 2355−106, MALS found the emergence of a new H I 21-cm absorption component at \[ z_{\rm abs}=1.172635, \] in addition to the previously known component at \(z_{\rm abs}=1.173019\). The paper interpreted the new component as evidence for a large optical-depth gradient over a length scale of \(\sim 0.35\) pc, while the older component remained nearly uniform over the same scale, directly linking MALS sensitivity to sub-parsec absorber structure and variability systematics in radio-optical line comparisons [2207.01807]. A different MALS case study, PKS1413+135 at \(z=0.24671\), combined strong H I absorption, tentative OH 1667 MHz absorption, and clear OH 1720 MHz emission. The source had already been known as a variable molecular absorber, and the MALS observation extended that temporal baseline: the paper states that over 30 years the H I line varied by 20 per cent in depth, while the OH 1720 MHz depth varied by a factor 4, with line centroids and widths also changing. The preferred interpretation was changing sightlines through a clumpy absorber caused by superluminal motions of radio knots [2211.09355]. MALS has also begun to populate the rare class of high-redshift associated absorbers. “MALS discovery of a rare H I 21-cm absorber at \(z\sim1.35\)” reported associated 21-cm absorption toward the quasar J2339−5523 at \(z_{\rm em}=1.3531\), with a profile width of about 400 km s\(^{-1}\) and a peak redshifted by about 200 km s\(^{-1}\) relative to the quasar. The notable result was that HST/COS and Magellan/MIKE spectra showed no corresponding optical or FUV absorption features, leading the authors to argue that the H I seen by MALS lies along radio sight lines not sampled by the optical AGN continuum, probably in the inner circumnuclear disk and/or a cocoon created by jet–ISM interaction [2311.00336]. MALS has also been used to extend associated-absorption work to lower-power radio AGN at \(z<0.5\). A study based on MALS DR1 continuum catalogues and MALS L-band spectra classified 426 radio AGN into 327 low-excitation radio galaxies and 99 high-excitation radio galaxies, searched 99 radio-bright systems for associated H I absorption, and reported five new detections at \(0.292\sigma\), and the paper explicitly compares this to the difference between H\(_2\) detections and non-detections in damped Ly\(\alpha\) systems at \(z>1.8\). The proposed interpretation is that turbulence-driven WNM-to-CNM conversion is the common governing factor for the presence of both H I 21-cm and H\(_2\) absorption [2504.00097]. Central versus off-axis comparisons also showed that the slope of rms fluctuations in optical-depth variations in quiescent LISM gas is shallower than earlier measurements in the Galactic disk, with inferred densities of 20–30 cm\(^{-3}\), typical of CNM gas [2504.00097]. MALS-relevant nearby-environment science has also extended beyond the Milky Way proper. A later paper used MALS data to detect H I 21-cm absorption from Magellanic Bridge gas toward J033242.97−724904.5 with a peak signal-to-noise ratio of 10. Together with earlier ATCA detections, the new line allowed the authors to compare cold atomic gas over 4–6 kpc. The gas-to-dust ratio along the new sight line was measured as \[ N_{\rm HI}/E(B-V) = (1.29\pm0.04_{\rm stat}\pm0.17_{\rm sys})\times10^{22}\ {\rm cm^{-2}\,mag^{-1}}, \] which they argued is more consistent with Large Magellanic Cloud than Small Magellanic Cloud gas, supporting a direct LMC–SMC collision scenario for the Bridge [2509.21970]. This nearby application shows that the MALS observing strategy is sensitive not only to cosmological absorbers but also to weak H I absorption in the Local Group environment. ## 6. Continuum legacy and commensal science Although conceived as an H I/OH absorption survey, MALS has developed into a substantial continuum-survey resource. DR1 already showed that 391 L-band pointings yielded hundreds of thousands of radio sources, in-band spectral indices, 140 ultra steep-spectrum sources as prospective high-\(z\) radio galaxies, 1308 long-term variable sources, and 122 transients [2308.12347]. The later Fermi-unassociated-source study made this broader significance explicit by using MALS DR1 as the starting point for image-based searches for steep-spectrum radio counterparts to \(\gamma\)-ray sources. Within Fermi error ellipses, the authors identified nine pulsar candidates toward six Fermi sources and 41 steep-spectrum radio galaxy candidates, and argued that MALS can detect 80 percent of the known pulsar population, largely because its in-band spectral indices suppress false positives from non-contemporaneous multi-survey flux comparisons [2412.12470]. DR2 demonstrated that MALS continuum products are also precise enough for cosmological number-count work. Using the 971,980-source wideband catalogue, the DR2 paper modeled completeness, false detections, and a declination-dependent source-density systematic, then measured the cosmic radio dipole. Its preferred result at a 400 \(\mu\)Jy flux-density cut was \[ \mathcal{D}=0.67_{-0.29}^{+0.31}\times10^{-2}, \] with direction \[ {\rm R.A.}=176_{-30}^{+30}\degree,\qquad {\rm Dec.}=20_{-40}^{+32}\degree, \] which the authors described as consistent with the cosmic microwave background dipole in both direction and amplitude [2408.16619]. The same paper effectively established MALS as a bridge survey between shallow all-sky cm-wave catalogues and very small ultra-deep fields. Morphological radio-source studies have likewise benefited from the MALS continuum footprint. A 2025 study reported 36 new hybrid morphology radio sources from MALS radio maps, described as the largest collection of such sources in the southern sky to date. The sample spans \(0.04