High-Resolution Images of Diffuse Neutral Clouds in the Milky Way. I. Observations, Imaging, and Basic Cloud Properties

TL;DR

This study provides high-resolution images and basic properties of diffuse neutral hydrogen clouds in the inner Milky Way, revealing their irregular shapes, mass range, and thermal phases, contributing to understanding the ISM's structure.

Contribution

First high-resolution imaging of diffuse neutral HI clouds in the inner Galaxy, detailing their properties and thermal phases, expanding knowledge of the interstellar medium.

Findings

Cloud diameters range from 10 to 100 pc

HI column densities range from 0.8 to 2.9 x 10^{20} cm^{-2}

Some clouds show evidence of two thermal phases

Abstract

A set of diffuse interstellar clouds in the inner Galaxy within a few hundred pc of the Galactic plane has been observed at an angular resolution of ~1 arcmin combining data from the NRAO Green Bank Telescope and the Very Large Array. At the distance of the clouds the linear resolution ranges from ~1.9 pc to ~2.8 pc. These clouds have been selected to be somewhat out of the Galactic plane and are thus not confused with unrelated emission, but in other respects they are a Galactic population. They are located near the tangent points in the inner Galaxy, and thus at a quantifiable distance: $2.3 \leq R \leq 6.0$ kpc from the Galactic Center, and $-1000 \leq z \leq +610$ pc from the Galactic plane. These are the first images of the diffuse neutral HI clouds that may constitute a considerable fraction of the ISM. Peak HI column densities range from $N_{HI} = 0.8-2.9 \times 10^{20}$ cm$^{-2}$. Cloud diameters vary between about 10 and 100 pc, and their HI mass spans the range from less than a hundred to a few thousands Msun. The clouds show no morphological consistency of any kind except that their shapes are highly irregular. One cloud may lie within the hot wind from the nucleus of the Galaxy, and some clouds show evidence of two distinct thermal phases as would be expected from equilibrium models of the interstellar medium.