X-raying Galaxies: A Chandra Legacy

TL;DR

This paper reviews Chandra's contributions to understanding hot gas in nearby galaxies, highlighting its properties, distribution, and relation to stellar feedback, and compares observations with galaxy evolution simulations.

Contribution

It provides the first detailed characterization of hot gas properties in our Galaxy and other nearby galaxies, linking X-ray observations to galaxy formation theories.

Findings

Hot gas is concentrated around galactic bulges and disks.

The hot gas accounts for a small fraction of feedback energy.

Hot gas properties correlate with star formation rate.

Abstract

This presentation reviews Chandra's major contribution to the understanding of nearby galaxies. After a brief summary on significant advances in characterizing various types of discrete X-ray sources, the presentation focuses on the global hot gas in and around galaxies, especially normal ones like our own. The hot gas is a product of stellar and AGN feedback -- the least understood part in theories of galaxy formation and evolution. Chandra observations have led to the first characterization of the spatial, thermal, chemical, and kinetic properties of the gas in our Galaxy. The gas is concentrated around the Galactic bulge and disk on scales of a few kpc. The column density of chemically-enriched hot gas on larger scales is at least an order magnitude smaller, indicating that it may not account for the bulk of the missing baryon matter predicted for the Galactic halo according to the standard cosmology. Similar results have also been obtained for other nearby galaxies. The X-ray emission from hot gas is well correlated with the star formation rate and stellar mass, indicating that the heating is primarily due to the stellar feedback. However, the observed X-ray luminosity of the gas is typically less than a few percent of the feedback energy. Thus the bulk of the feedback (including injected heavy elements) is likely lost in galaxy-wide outflows. The results are compared with simulations of the feedback to infer its dynamics and interplay with the circum-galactic medium, hence the evolution of galaxies.