Galactic Small Scale Structure Revealed by the GALFA-HI Survey

TL;DR

The GALFA-HI survey reveals numerous small, cold, and faint HI clouds in the Milky Way, providing new insights into small-scale Galactic structure and the interstellar medium.

Contribution

This study presents the first large-scale detection and characterization of compact HI clouds in the Milky Way using high-resolution GALFA-HI survey data.

Findings

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

Clouds are cold, faint, and compact, with median size 5 arcmin.

Most clouds are related to the Galactic disk within a few kpc.

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 an angular resolution of 3.5 arcmin. The unprecedented resolution and sensitivity of the GALFA-HI survey have resulted in the detection of many 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$^2$ searched so far, we have detected 96 such compact clouds. The detected clouds are cold with kinetic temperature less than 300 K. Moreover, they are quite compact and faint, with median values of 5 arcmin in angular size, 0.75 K in peak brightness temperature, and $5\times10^{18}$ cm$^{-2}$ in HI column density. From the modeling of spatial and velocity distributions of the whole compact cloud population, we find that the bulk of clouds are related to the Galactic disk, and are within a few kpc distance. We present properties of the compact clouds sample and discuss various possible scenarios for the origin of this clouds population and its role in the Galactic interstellar medium studies.