The MeerKAT Absorption Line Survey (MALS) data release 3: Cold atomic gas associated with the Milky Way^★

¹ Inter-University Centre for Astronomy and Astrophysics, Post Bag 4, Ganeshkhind, Pune 411 007, India
² Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
³ Ioffe Institute, 26 Politeknicheskaya st., St. Petersburg 194021, Russia
⁴ Observatoire de Paris, Collège de France, PSL University, Sorbonne University, CNRS, LUX, Paris, France
⁵ Université Lyon, ENS de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon UMR5574, 69230 Saint-Genis-Laval, France
⁶ French-Chilean Laboratory for Astronomy, IRL 3386, CNRS and Universidad de Chile, Santiago, Chile
⁷ National Radio Astronomy Observatory, PO Box O, Socorro, NM 87801, USA
⁸ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
⁹ ThoughtWorks Technologies India Private Limited, Yerawada, Pune 411 006, India
¹⁰ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
¹¹ Department of Physics and Electronics, Rhodes University, PO Box 94 Makhanda 6140, South Africa
¹² School of Mathematics, Statistics & Computer Science, University of KwaZulu-Natal, Westville Campus, Durban 4041, South Africa
¹³ Astrophysics Research Centre, University of KwaZulu-Natal, Durban 4041, South Africa
¹⁴ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, Sweden
¹⁵ Institut d’Astrophysique de Paris, UMR 7095, CNRS-SU, 98 bis bd Arago, 75014 Paris, France

^★★ Corresponding author: ngupta@iucaa.in

Received: 29 September 2024
Accepted: 9 March 2025

Abstract

Aims. We present results of a blind search for Galactic H I 21-cm absorption lines toward 19 130 radio sources brighter than 1 mJy at 1.4 GHz, using 390 pointings of the MeerKAT Absorption Line Survey (MALS), each pointing centered on a source brighter than 200 mJy. The spectral resolution, the median spatial resolution, and the median 3σ optical depth sensitivity (τ_3σ) are 5.5 km s⁻¹, ~ 9″, and 0.381, respectively. We used the spectra of the central sources and the other off-axis radio sources within the telescope pointings to constrain the properties of H I gas in the local interstellar medium (LISM) of the Galaxy.

Methods. Through an automated procedure, we detected 3640 H I absorption features over ~800 deg². This represents the largest Galactic H I absorption line catalog to date. We used H I 21-cm emission line measurements from HI4PI, an all sky single-dish survey, and far-infrared maps from COBE/DIRBE and IRAS/ISSA in addition to the Gaussian decomposition of the HI4PI into cold (CNM), lukewarm (LNM), and warm (WNM) neutral medium phases for our analyses.

Results. We find a strong linear correlation with a coefficient of 0.84 between the H I 21-cm emission line column densities (N_HI) and the visual extinction (A_V) measured toward the pointing center, along with the confinement of the absorption features to a narrow range in radial velocities (−25< v_LSR[km s⁻¹]<+25). This implies that the detected absorption lines form a homogeneous sample of H I clouds in the LISM. For central sight lines (median τ_3σ=0.008), the detection rate is 82±5%. All the central MALS sight lines with H I absorption have N_HI(CNM) + N_HI(LNM) ≥ N_HI(WNM). The H I 21-cm absorption optical depth is linearly correlated to N_HI and A_V, with a correlation coefficient in excess of 0.8 up to N_HI ≃ 2 · 10²¹ cm⁻² or, equivalently, A_V ≃ 1 mag. Above this threshold, A_V traces the total hydrogen content, and consequently, A_V and the single-dish N_HI scale, differently. The slopes of N_HI distributions of central sight lines with H I 21-cm absorption detections and non-detection differ at >2σ. A similar difference is observed for H₂ detections and non-detections in damped Lyman-alpha systems at z≳1.8, implying that turbulence-driven WNM-to-CNM conversion is the common governing factor for the presence of H I 21-cm and H₂ absorption. Through a comparison of central and off-axis absorption features, we find the optical depth variations (Δτ) to be higher for pointings centered on regions with a higher N_HI and CNM fraction. However, no such dependence is observed for the covering fraction of the absorbing structures over 0.1–10 pc. The slope (2.327 ± 0.153) of root mean square (rms) fluctuations in optical depth variations in the quiescent gas associated with LISM is shallower than the earlier measurements in the disk. The densities (20–30 cm⁻³) inferred from |Δτ| at the median separation (1.5 pc) of the sample are typical of the CNM values. The negligible (median ~0 km s⁻¹) velocity shifts between central and off-axis absorbers are in line with the hypothesis that the CNM/LNM clouds freeze out of the extended WNM phase.