Sources of X-rays from galaxies

TL;DR

Galactic X-ray emission arises from diverse high-energy processes, including accretion-powered sources and diffuse emission, providing insights into stellar evolution, feedback mechanisms, and galaxy evolution, though many aspects remain uncertain.

Contribution

This paper reviews the sources and characteristics of X-ray emission in galaxies, highlighting recent detection capabilities and the complexities in understanding diffuse X-ray processes.

Findings

Bright X-ray sources are mainly accretion-powered objects.

Fainter sources correlate with star formation and stellar mass.

Diffuse X-ray emission mechanisms are still uncertain.

Abstract

Galactic X-ray emission is a manifestation of various high-energy phenomena and processes. The brightest X-ray sources are typically accretion-powered objects: active galactic nuclei and low- or high-mass X-ray binaries. Such objects with X-ray luminosities of > 10^{37} ergs/s can now be detected individually in nearby galaxies. The contributions from fainter discrete sources (including cataclysmic variables, active binaries, young stellar objects, and supernova remnants) are well correlated with the star formation rate or stellar mass of galaxies. The study of discrete X-ray sources is essential to our understanding of stellar evolution, dynamics, and end-products as well as accretion physics. With the subtraction of the discrete source contributions, one can further map out truly diffuse X-ray emission, which can be used to trace the feedback from active galactic nuclei, as well as from stars, both young and old, in the form of stellar winds and supernovae. The X-ray emission efficiency, however, is only about 1% of the energy input rate of the stellar feedback alone. The bulk of the feedback energy is most likely gone with outflows into large-scale galactic halos. Much is yet to be investigated to comprehend the role of such outflows in regulating the ecosystem, hence the evolution of galaxies. Even the mechanism of the diffuse X-ray emission remains quite uncertain. A substantial fraction of the emission cannot arise directly from optically-thin thermal plasma, as commonly assumed, and most likely originates in its charge exchange with neutral gas. These uncertainties underscore our poor understanding of the feedback and its interplay with the galaxy evolution.