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

Artikel i vetenskaplig tidskrift, 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 (∆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.