Compact HI clouds from the GALFA-HI survey

TL;DR

The GALFA-HI survey has discovered numerous small, cold, and faint compact HI clouds in the Milky Way, revealing their properties, distribution, and potential origins, which are significant for understanding the Galactic interstellar medium.

Contribution

This paper presents the first large-scale detection and analysis of isolated compact HI clouds using the GALFA-HI survey, highlighting their characteristics and possible Galactic origins.

Findings

Detected 96 compact HI clouds in 4600 deg$^2$ survey area.

Clouds are cold with median T$_{k,max}$ of 300 K.

Most clouds are within a few kpc of the Galactic disk.

Abstract

The Galactic Arecibo L-band Feed Array HI (GALFA-HI) survey is mapping the entire Arecibo sky at 21-cm, over a velocity range of -700 to +700 km/s (LSR), at a velocity resolution of 0.18 km/s and a spatial resolution of 3.5 arcmin. The unprecedented resolution and sensitivity of the GALFA-HI survey have resulted in the detection of numerous isolated, very compact HI clouds at low Galactic velocities, which are distinctly separated from the HI disk emission. In the limited area of ~4600 deg$^2$ surveyed so far, we have detected 96 of such compact clouds. The detected clouds are cold with a median T$_{k,max}$ (the kinetic temperature in the case in which there is no non-thermal broadening) of 300 K. Moreover, these clouds are quite compact and faint, with median values of 5 arcmin in angular size, 0.75 K in peak brightness temperature, and $5 \times 10^{18}$ cm$^{-2}$ in HI column density. Most of the clouds deviate from Galactic rotation at the 20-30 km/s level, and a significant fraction show evidence for a multiphase medium and velocity gradients. No counterparts for these clouds were found in other wavebands. From the modeling of spatial and velocity distributions of the whole compact cloud population, we find that the bulk of the compact clouds are related to the Galactic disk, and their distances are likely to be in the range of 0.1 to a few kpc. We discuss various possible scenarios for the formation and maintenance of this cloud population and its significance for Galactic ISM studies.