Issue |
A&A
Volume 698, June 2025
|
|
---|---|---|
Article Number | A120 | |
Number of page(s) | 29 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361/202452407 | |
Published online | 06 June 2025 |
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
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 (τ3σ) 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 (NHI) and the visual extinction (AV) measured toward the pointing center, along with the confinement of the absorption features to a narrow range in radial velocities (−25< vLSR[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 τ3σ=0.008), the detection rate is 82±5%. All the central MALS sight lines with H I absorption have NHI(CNM) + NHI(LNM) ≥ NHI(WNM). The H I 21-cm absorption optical depth is linearly correlated to NHI and AV, with a correlation coefficient in excess of 0.8 up to NHI ≃ 2 · 1021 cm−2 or, equivalently, AV ≃ 1 mag. Above this threshold, AV traces the total hydrogen content, and consequently, AV and the single-dish NHI scale, differently. The slopes of NHI 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 NHI 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.
Key words: techniques: interferometric / ISM: clouds / dust, extinction / ISM: structure / Galaxy: halo / radio lines: ISM
The MALS images and spectra are publicly available at https://mals.iucaa.in
© The Authors 2025
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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