This article describes a series of scientific endeavors undertaken that led to the compilation and characterization of nearly half a million radio sources observed as part of the MeerKAT Absorption Line Survey (MALS, P.I.- Prof. Neeraj Gupta). Remarkably, many of these sources are detected for the first time, thanks to the excellent surface-brightness sensitivity of MeerKAT. The sources from this catalog will be subsequently searched for signatures of cold gas: fuel reservoir for both star-formation and AGN-activity driving galaxy evolution. MALS aims at tracing the evolution of cold gas in galaxies in the past 10 Gyrs, the cosmic epoch marked by steep evolution in both star-formation and AGN-activity. This publication marks the initial release in a sequence of forthcoming data releases, including both continuum and spectral-line data from MALS. For the latest project status and the opportunity to explore the sources firsthand, please visit https://mals.iucaa.in/.

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Background and Motivation

The observable universe hosts countless galaxies, each with a mix of baryonic elements (stars, planets, gas, and dust) and non-baryonic components like dark matter. Most galaxies have supermassive black holes (SMBH) at their cores, attracting matter that spirals into them through an accretion disc. This process heats the matter, causing it to radiate across the electromagnetic (EM) spectrum, resulting in an ‘active’ SMBH and forming an Active Galactic Nuclei (AGN). See Fig. 1 for a schematic picture of an AGN with its various components. AGNs often outshine their host galaxies in energy output and emit jets of highly-energetic ionized particles that penetrate the interstellar medium (ISM) among the stars, energizing the ISM and influencing host-galaxy properties.

Figure 1: An artistic impression of the structure of an AGN (by Astrophysicist Salvatore Orlando). Besides the SMBH and the accretion disk, a typical AGN is surrounded by the following structures (moving radially outwards from the centre): (1) Broad Line Region (BLR; 0.1 - 1 pc): consists of dense (~ 10¹⁰ cm^-3 ) gas clouds at very high velocities (> 1000 km/s) which produces the permitted broad emission lines in the optical spectrum; (2) Obscuring torus (1 - 10 pc): a donut shaped structure containing gas and dust that blocks the direct view of the SMBH when viewed edge-on and reprocesses UV / optical light from the AGN and re-emits in infrared; (3) Narrow Line Region (NLR; up-to ~100 pc): low density (~ 10³ cm^-3) gas clouds at low velocities (<1000 km/s) giving rise to narrow forbidden lines in the optical spectrum. Depending on the viewing angle, an AGN may appear different and are classified into Type 1 (central region not obscured), Type 2 (central region obscured) and Blazars (seen very close to the jet axis).

The gas in the ISM is composed of diffuse and dense components of ionized, neutral and molecular phases. Hydrogen is the most ubiquitous element in the ISM by mass (~70%), and found in both neutral and ionized phases. Particularly interesting are the cold neutral (HI; T~100 K) and molecular (H₂; T < 100 K) phases, directly responsible for the onset of both AGN activity and star-formation, the primary processes steering galaxy evolution. The interplay between energetic output from AGN and cold gas in the host galaxies is central to understanding the fueling of massive black holes and the evolution of galaxies hosting these.

Astronomers use absorption line spectroscopy (Morganti & Oosterloo, 2018), a powerful technique, to detect cold gas by observing intensity drops at specific wavelengths in the spectrum of a bright background source. These ‘absorption lines’ act as fingerprints, revealing the elemental composition of the gas cloud.

In studying extragalactic objects, a key parameter is their redshift (z), indicating distance from the observer. Due to the finite speed of light, light from a distant object, received now, was emitted in an earlier epoch, defining redshift as a measure of lookback time from the present epoch (z = 0). The cosmological expansion drives objects apart, causing a Doppler shift in the wavelength of spectral lines from the original wavelength emitted by elements in those objects. Measuring this shift gives a direct estimate of the redshift.

The strength of absorption lines is directly proportional to the column density of the absorbing cloud, indicating the atom count per unit surface area. Neutral hydrogen with high column densities (N_HI ~ 2e+20 cm^-2) appears as damped Ly-alpha absorbers (DLA) at a rest wavelength of 1216 Angstrom in the far-UV part of the EM spectrum. Practical constraints limit effective observation of DLAs to z>2 when Ly-alpha redshifts to optical wavelengths. At lower redshifts (z<0.2), 21-cm emission observations offer reasonable constraints on the neutral hydrogen fraction. However, for the intermediate range (0.2<z<2), constraints on cold gas fraction are currently lacking, impacted by mostly observational limitations. Moreover, due to wavelength dependent extinction, optical surveys tend to miss the densest systems, causing a bias in statistics at all redshifts.

To address limitations in the intermediate redshift range, 21-cm absorption from cold neutral HI and 18-cm absorption from molecular hydrogen can serve as tracers to study cold gas in the radio regime. However, technical constraints, like narrow bandwidth receivers and challenges due to radio frequency interference, historically restricted the search for 21-cm absorption to a limited redshift range, resulting in fewer than 100 detections (Gupta et al., 2021). Modern telescopes, such as MeerKAT and ASKAP with wide instantaneous bandwidth receivers, will revolutionize the study of cold gas properties and its fraction in galaxies using the 21-cm and 18-cm absorption lines, propelling scientific explorations.

Figure 2: Both star-formation and AGN activity are fed by cold gas, funneling down from a common, potentially hot gas reservoir. In turn, both the processes inject the ISM with significant energy, thereby creating a self-regulatory process known as feedback, an essential component of all galaxy evolution models. The figure is adapted from Harrison, C., 2017.

The MeerKAT Absorption Line Survey (MALS):

The ongoing MeerKAT Absorption Line Survey (MALS) is a large-scale project utilizing the highly sensitive MeerKAT telescope operating at cm wavelengths. Comprising 64 antennas of 13.5m diameter each, MeerKAT will be utilized for ~1700 hours, observing nearly 500 pointings in both L- (856-1711 MHz) and UHF- (580-1015 MHz) observation bands at declinations between -90 degrees and +20 degrees. MALS aims to seamlessly explore HI 21-cm and OH 18-cm absorption lines in the redshift ranges 0<z<1.4 and 0<z<1.8, respectively. This project is expected to offer unprecedented insights into the evolution of cold gas fraction in galaxies within the poorly constrained redshift range of 0<z<2.

Figure 3: Sky distribution of the 391 MALS pointings observed in L-band shown in Mollweide projection in equatorial coordinates (J2000). The dotted line marks the Galactic plane. See Deka et al., 2023 for more details.

While MALS is primarily a spectral line survey, it also serves as a highly sensitive and competitive continuum survey. Presently, all L-band pointings have been observed, and UHF-band observations are in progress. Fig. 3 displays the distribution of the 391 observed L-band pointings. Data from these observations undergo processing at IUCAA to generate both continuum and cube images. The catalog of sources detected in the continuum images has been compiled and released to the community, and I delve into the technical and scientific aspects of generating these catalogs below.

I. Technical aspects:

For processing, the 856 MHz wide L-band was split into 15 spectral windows (SPW0, …., SPW14), each of width 60 MHz. The SPW2 (central frequency ~ 1006 MHz) and SPW9 (central frequency ~ 1380 MHz) were considered for the first data release (DR1). This is due to the relatively RFI-free coverage of SPW2, and SPW9 being centered at ~1.4 GHz facilitates comparison of astrometry and flux density scale of MALS with existing surveys in the literature, e.g. the NRAO VLA Sky Survey (NVSS; Condon et al., 1998) and the Faint Images of the Radio Sky at Twenty cm (FIRST; Becker et al., 1995). Moreover, flux density measurements at two frequencies allow measurements of spectral indices of the detected sources, a quantity which provides important information on both the size and nature of the radio emission.

Data from these two SPWs were processed using the VROOM cluster at IUCAA. An indigenously developed pipeline named ARTIP (Automated Radio Telescope Imaging Pipeline) manages the automated processing of these data and produces both continuum and cube images (see Gupta et al. 2021, for details).

The properties of the radio sources detected in the continuum images of SPW2 and SPW9 were cataloged using PyBDSF (Python Blob Detector and Source Finder; Mohan & Rafferty, 2015) software. A few important notes for the users regarding the accuracy of these catalogs are summarized below. Please refer to Deka et al., 2023 for their detailed discussion.

1. Sources detected at signal-to-noise (S/N)>8 have a probability of <0.1% to be a false source.
2. The fraction of artifacts increases near the central and edges of the beam, and hence a more stringent reliability threshold (S/N>15) should be used in these regions.
3. The median RA and DEC offsets of MALS sources when compared to their counterparts in FIRST are 0.02 and 0.05 arcsecs, respectively.
4. Comparing flux densities with NVSS, we observed a median offset of 6% between MALS and NVSS sources, with MALS sources systematically brighter.

Catalog summary: The following table summarizes the two catalogs:

II. Scientific aspects:

We not only provided technical details for the two catalogs but also conducted scientific explorations to demonstrate their utility. These encompassed deriving 1.4 GHz radio source counts, studying the correlation between radio spectral index and flux density, identifying ultra-steep-spectrum (USS) sources as potential high-z galaxies, and identifying transient and variable sources from the catalog. Below, I briefly discuss each of these explorations and direct readers to Deka et al., 2023 for a more comprehensive discussion.

a. 1.4 GHz radio source counts:

In general, radio source counts are defined as the number of radio sources detected per unit flux density bin per unit survey area. From the SPW9 catalog at ~1.4 GHz, we derived the source counts for MALS and compared these with other literature counts at the same frequency (Fig. 4). The comparison underscores that MALS source counts align well with the literature, falling within the scatter of various measurements. When comparing MALS with MeerKAT-DEEP2 counts, SPW9 catalog completeness is 100% down to 2 mJy. However, completeness sharply declines below 0.5 mJy, reaching about 50% completeness at 0.1 mJy.

Figure 4: MALS 1.4 GHz radio source counts (red dashed line) in comparison to literature counts at the same frequency. The ‘katbeam’ (Mauch et al., 2020) and ‘plumber’ beam models are two distinct Primary Beam models for MeerKAT and we show source counts corresponding to both the beams. The ‘Raw’ source counts are derived without any area based correction, i.e. the entire survey area was used to normalize the number of sources detected in each flux density bin. This is not practically true because the RMS across the survey area is not constant, and hence at fainter flux densities, a source might not be detectable over the entire area. Taking this into account we derived an area based correction factor, which for a particular flux density calculates the fraction of the total area over which the source will be detected at >5𝜎 threshold. Refer to Deka et al., 2023 for detailed discussion.

b. Variation of spectral index with flux density:

Several studies have reported flattening of spectral index with decreasing flux density e.g. Prandoni et al., 2006, Gasperin et al., 2018, Tiwari et al., 2019 but counter examples have also been reported e.g., Ibar et al., 2009. The large sample of spectral indices in MALS DR1 presents an opportunity for testing this. We created a suitable sample for sources with spectral index measurements by applying specific quality criteria outlined in Deka et al., 2023.

Figure 5: Flattening of spectral indices measured between SPW2 and SPW9 (1.0 – 1.4 GHz) with decreasing flux density in SPW9 (1.4 GHz). Refer to Deka et al., 2023 for detailed discussion.

As shown in Fig. 5, we found a clear flattening of spectral indices with decreasing flux density. In the literature, this is primarily attributed to the increasing fraction of core-dominated FRI (Fanaroff & Riley, 1974) type sources at lower flux densities. However, higher spatial resolution surveys of statistically large samples are required to confirm this hypothesis.

c. Ultra-steep-spectrum (USS) sources:

Here, we focused on a special population of radio sources exhibiting ultra steep spectral indices (USS; spectral index < -1.3) as prospective high-redshift radio galaxies (HzRGs; z>2) (e.g., Miley & De Breuck, 2008, Bornancini et al., 2007, Saxena et al., 2018). For this we crossmatch all the sources detected in SPW2 with the TIFR GMRT Sky Survey Alternative Data Release - I (TGSS-ADR1; Intema et al., 2017) at 147 MHz. Owing to the high-z nature, these sources are believed to be young and compact. Therefore, we discarded any resolved sources from the sample. Further, to avoid false detections, we rejected any candidate HzRG which was within 3 arcmin from the edge of the SPW2 primary beam. This led to a sample of 182 sources with spectral indices <-1.3 and compact morphology on arc-seconds scales. Follow up high spatial resolution radio images along with optical spectroscopy is necessary to confirm the high redshift nature of these galaxies.

d. Long term radio variability and transients:

The majority of NVSS observations were carried out between 1993 and 1996 i.e., about 26 years prior to MALS L-band observations presented here. Through comparison of MALS SPW9 (~1.4 GHz) catalog with NVSS, we identify 1308 variables and 122 transient radio sources. We define the former to be detected in both NVSS and MALS, whereas the latter are detected only in one of these.

Figure 6: WISE color-color plot in Vega magnitudes of various classes of sources (top), reproduced from Wright et al., 2010, and variable radio sources from MALS (bottom). The points have been color coded based on spectral index. Refer to Deka et al., 2023 for detailed discussion.

The WISE (Wright et al., 2010) color-color plot (top panel, Fig. 6) serves as a useful tool to gauge the nature of these sources. In the bottom panel of Fig. 6, we illustrate the W1-W2 color vs. W2-W3 color for 763 variable sources with WISE counterparts. Here, W1, W2 and W3 are the sources’ magnitudes measured at 3.4, 4.6 and 12 micrometers respectively. The majority of these variable sources are AGNs, but some could also be stars. Notably, sources with colors overlapping the powerful AGN locus (W1 - W2 > 0.5) exhibit substantially flatter spectral indices (-0.48± 0.04), suggesting a notable fraction could be blazars. Further exploration of the identified variables and transients necessitates insights from multi-epoch optical images and spectra.

Conclusion & Catalog accessibility

In Deka et al., 2023, we present the radio continuum catalogs at 1.0 and 1.4 GHz derived from the L-band observations of the MeerKAT Absorption Line Survey. We detect 495,325 and 240,321 radio sources at 1.0 and 1.4 GHz, respectively. These catalogs are hosted in https://mals.iucaa.in/, through which the users can download the entire catalog or search for the properties of a particular source. The website also holds a cutout fetching facility, enabling users to download 1’ X 1’ cutouts in FITS format centered at the specified position. Additionally, various selection filters can be implemented in the catalog beforehand to select sources of a specific kind.

For any query regarding the catalog, please write to parthad@iucaa.in or ngupta@iucaa.in. If you are interested in joining the collaboration, please contact Prof. Neeraj Gupta at ngupta@iucaa.in.

Acknowledgement

The MALS team is a global collaboration that includes researchers from around the world. Leading this project is N. Gupta from IUCAA, India. The MeerKAT telescope, operated by the South African Radio Astronomy Observatory (SARAO), is a facility of the National Research Foundation (NRF) in South Africa. IUCAA is responsible for hosting and processing the vast raw survey data, totaling 1.6 petabytes, received from SARAO. This processing is facilitated by an automated pipeline built at IUCAA in partnership with Thoughtworks Technologies India Pvt Ltd. The pipeline heavily relies on tools and tasks from the Common Astronomy Software Applications (CASA) software, developed by the National Radio Astronomy Observatory (NRAO) in the USA.

Original paper: The MeerKAT Absorption Line Survey (MALS) data release I: Stokes I image catalogs at 1-1.4 GHz

First Author: P. P. Deka

Co-authors: N. Gupta, P. Jagannathan, S. Sekhar, E. Momjian, S. Bhatnagar, J. Wagenveld, H.-R. Klöckner, J. Jose, S. A. Balashev, F. Combes, M. Hilton, D. Borgaonkar, A. Chatterjee, K. L. Emig, A. N. Gaunekar, G. I. G. Józsa, D. Y. Klutse, K. Knowles, J-.K. Krogager, A. Mohapatra, K. Moodley, Sébastien Muller, P. Noterdaeme, P. Petitjean, P. Salas, S. Sikhosana

First author’s Institution: Inter-University Centre for Astronomy and Astrophysics, Post Bag 4, Ganeshkhind, Pune 411 007, India