X-raying galaxies: A Chandra legacy

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 active galactic nuclear feedbackthe 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 kiloparsec. 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 circumgalactic medium, hence the evolution of galaxies.feedback | simulation | x-ray source | hot gas X -ray observations are playing an increasingly important role in the study of galaxies. With its arcsecond spatial resolution, Chandra in particular has made a significant impact on our understanding of discrete x-ray source populations, which mostly represent various stellar end products [e.g., low-and high-mass x-ray binaries (LMXBs and HMXBs) and supernova remnants (SNRs)] as well as active galactic nuclei (AGN). The resolution also allows for a clean excision of such sources from the data so low-surface brightness emission (e.g., from diffuse hot gas) can be mapped out. An underappreciated aspect of Chandra is its spectroscopic capability in the study of diffuse hot gas when the low-and high-energy grating instruments are used. Although the sensitivities of the instruments are quite limited, the existing observations of a dozen or so bright objects (AGN and LMXBs) have yielded data of high enough quality for unprecedented x-ray absorption line spectroscopic measurements of the global hot gas in and around our galaxy. Useful constraints have also been obtained on the overall content of hot gas around galaxies within certain impact distances of the sight lines toward the AGN. These measurements, compared with physical models and simulations of the hot gas, are shedding important insights on its relationship to the feedback from stars and AGN. These aspects of Chandra's legacy are reviewed in the following. Discrete SourcesThe overall x-ray lu...

Spectroscopic evidence of charge exchange X‐ray emission from galaxies

“…In a joint analysis, even spectral lines that are not individually detected can be useful, as demonstrated in existing X-ray absorption line spectroscopy (e.g., Yao & Wang 2005;Wang et al 2010 and references therein). Such an analysis will become possible with the upcoming CX spectral model (see the presentation by Smith et al in this workshop).…”

An X‐ray view of the hot circum‐galactic medium

“…We emphasize that reliable observations of many CGM phases are still very limited, so the numbers in Table are just the order‐of‐magnitude estimate with very different samples (many are just case studies) and often highly uncertain assumptions. The basic conclusion is that a significant fraction of SNe energy is not detected around galaxies, which is often regarded as the “missing galactic feedback” problem (e.g., Wang ). Such a problem is also revealed by the lack of heavy elements from feedback in the CGM (e.g., Wang ).…”

“…The basic conclusion is that a significant fraction of SNe energy is not detected around galaxies, which is often regarded as the “missing galactic feedback” problem (e.g., Wang ). Such a problem is also revealed by the lack of heavy elements from feedback in the CGM (e.g., Wang ).…”

Chandra’s first decade of discovery

“…The search for this gas through absorption-line spectroscopy and by other means was the main topic discussed by invited speaker Daniel Wang (45). Wang also pointed out that the current failure to locate this gas component may be because it is in a hotter galactic halo or has been somehow pushed away.…”