Narrow-line HI and cold structures in the ISM

TL;DR

This study confirms the existence of narrow-line HI emission clouds in the ISM using GASS data, compares their properties with other cold ISM structures, and suggests they are intermediate phases in the evolution of cold neutral medium to star-forming regions.

Contribution

It provides the first confirmation of NHIE clouds with GASS data and compares their properties with HISA, Planck cold clumps, and IRDCs, proposing an evolutionary link.

Findings

NHIE clouds are similar to HISA but warmer and more extended.

NHIE and HISA are likely the same type of clouds observed under different conditions.

Both may represent an intermediate phase in ISM evolution.

Abstract

Context. In the HI line profiles in the Leiden-Argentina-Bonn (LAB) all-sky database, we have found a population of very cold HI clouds. So far, the role of these clouds in the interstellar medium (ISM) has remained unclear. Aims. In this paper, we attempt to confirm the existence of the narrow-line HI emission (NHIE) clouds by using the data from the Parkes Galactic all-sky survey (GASS) and try to find their place among other coldest constituents of the ISM. Methods. We repeat the search of NHIE with the GASS data and derive or compile some preliminary estimates for the distribution, temperatures, distances, linear sizes, column and number densities, masses, and the composition of NHIE clouds, and compare these data with corresponding estimates for HI self-absorption (HISA) features, the Planck cold clumps (CC), and infrared dark clouds (IRDC). Results. We demonstrate that from LAB and GASS we can separate comparable NHIE complexes, and the properties of the obtained NHIE clouds are very similar to those of HISA features, but both of these types of clouds are somewhat warmer and more extended and have lower densities than the cores in the Planck CC and IRDC. Conclusions. We conclude that NHIE may be the same type of clouds as HISA, but in different observing conditions, in the same way as the Planck CC and IRDC are most likely similar ISM structures in different observing conditions and probably in slightly different evolutionary stages. Both NHIE and HISA may be an intermediate phase between the diffuse cold neutral medium and star-forming molecular clumps represented by the Planck CC and IRDC.