The MeerKAT Absorption Line Survey (MALS) data release 3: Cold atomic gas associated with the Milky Way
Journal article, 2025
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 (∆τ) 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 |∆τ| 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.
techniques: interferometric
Galaxy: halo
radio lines: ISM
ISM: structure
ISM: clouds
dust, extinction
Author
N. Gupta
Inter-University Centre for Astronomy and Astrophysics
J. Kerp
University of Bonn
S. Balashev
Russian Academy of Sciences
A. P.M. Morelli
University of Bonn
F. Combes
Paris Observatory
J. K. Krogager
University of Chile (UCH)
École Normale Supérieure de Lyon
E. Momjian
National Radio Astronomy Observatory Socorro
D. Borgaonkar
Inter-University Centre for Astronomy and Astrophysics
Partha P. Deka
Inter-University Centre for Astronomy and Astrophysics
K.L. Emig
National Radio Astronomy Observatory
J. Jose
ThoughtWorks Technologies India
G. I.G. Józsa
Max Planck Society
Rhodes University
H. R. Klockner
Max Planck Society
K. Moodley
University of KwaZulu-Natal
Sebastien Muller
Chalmers, Space, Earth and Environment, Onsala Space Observatory
P. Noterdaeme
Centre national de la recherche scientifique (CNRS)
P. Petitjean
Centre national de la recherche scientifique (CNRS)
Jonah D. Wagenveld
Max Planck Society
Astronomy and Astrophysics
0004-6361 (ISSN) 1432-0746 (eISSN)
Vol. 698 A120Subject Categories (SSIF 2025)
Atom and Molecular Physics and Optics
Astronomy, Astrophysics, and Cosmology
Infrastructure
Onsala Space Observatory
DOI
10.1051/0004-6361/202452407