The Carina Nebula and Gum 31 molecular complex: II. The distribution of the atomic gas revealed in unprecedented detail

TL;DR

This study provides high-resolution HI 21cm line observations of the Carina Nebula and Gum 31, revealing complex filamentary structures, bubbles from star clusters, and insights into atomic gas distribution, temperature, and phase transition regions.

Contribution

It offers the first detailed high-resolution HI map of the region, identifying structures and cold gas properties, and links atomic to molecular gas transition areas with dust and molecular features.

Findings

Identification of filamentary structures and bubbles in the HI map.

Detection of cold atomic gas with spin temperatures around 100 K.

Atomic mass of Gum 31 constitutes about 35% of total gas mass.

Abstract

We report high spatial resolution observations of the HI 21cm line in the Carina Nebula and the Gum 31 region obtained with the Australia Telescope Compact Array. The observations covered $\sim$ 12 deg$^2$ centred on $l= 287.5\deg,b = -1\deg$, achieving an angular resolution of $\sim $ 35 arcseconds. The HI map revealed complex filamentary structures across a wide range of velocities. Several "bubbles" are clearly identified in the Carina Nebula Complex, produced by the impact of the massive star clusters located in this region. An HI absorption profile obtained towards the strong extragalactic radio source PMN J1032--5917 showed the distribution of the cold component of the atomic gas along the Galactic disk, with the Sagittarius-Carina and Perseus spiral arms clearly distinguishable. Preliminary calculations of the optical depth and spin temperatures of the cold atomic gas show that the HI line is opaque ($\tau \gtrsim$ 2) at several velocities in the Sagittarius-Carina spiral arm. The spin temperature is $\sim100$ K in the regions with the highest optical depth, although this value might be lower for the saturated components. The atomic mass budget of Gum 31 is $\sim35 \%$ of the total gas mass. HI self absorption features have molecular counterparts and good spatial correlation with the regions of cold dust as traced by the infrared maps. We suggest that in Gum 31 regions of cold temperature and high density are where the atomic to molecular gas phase transition is likely to be occurring.