New results from Galactic ISM surveys: The frothy ISM

Abstract

The past 10 years have seen a resurgence in studies of the Milky Way's interstellar medium (ISM) driven by new surveys of the Galactic plane at wavelengths from 4 microns to 20 cm. These surveys, which include the H I International Galactic Plane Survey, the Spitzer GLIMPSE and MIPSGAL surveys, the recently commenced Herschel HI-GAL survey, and several surveys of CO J = 1-0, are allowing us to trace the evolution of the ISM on scales of parsecs and sub-parsecs from warm and diffuse through to cold, dusty molecular gas. Together with hydrodynamical simulations of ever-increasing sophistication our understanding of the ISM in the disk of the Milky Way has evolved from the 1970's simplistic models of cold clouds surrounded by warm gas to a complex mixture of phases intertwined and interconnected, whose cooling is driven by colliding flows. Here I focus on the "frothy" ISM, where structure and evolution is impacted by bubbles, shells and supershells. I discuss the role of these objects in cooling, the formation of molecules and even the transfer of matter out of the disk of the Galaxy.

Hi Shells and Supershells in the I-GALFA Hi 21-cm Line Survey: I. Fast-Expanding Hi Shells Associated with Supernova Remnants

“…The interstellar medium (ISM) is pervaded by small and large expanding neutral atomic shells (e.g., Heiles 1979Heiles , 1984McClure-Griffiths et al 2002;Ehlerová & Palouš 2005;McClure-Griffiths 2012). These Hi shells are the interstellar material swept up by supersonic shock waves produced by mechanical energy sources, including Hii regions, stellar winds, supernova (SN) explosions, and infalling high-velocity clouds.…”