Chandra Survey of Nearby Galaxies: The Catalog

TL;DR

This survey compiles X-ray data from the Chandra archive for 719 nearby galaxies, identifying active nuclei and analyzing their properties, revealing a higher detection rate of weak AGNs especially in star-forming regions.

Contribution

It provides the first large-scale catalog of X-ray detected AGNs in nearby galaxies, demonstrating the effectiveness of X-ray surveys in identifying weak and obscured AGNs.

Findings

Approximately 60% of elliptical and early-type spirals host X-ray AGNs.

X-ray surveys detect more weak AGNs than optical methods, especially in star-forming regions.

Stacked analysis shows faint X-ray emission possibly from X-ray binaries in non-detected galaxies.

Abstract

We searched in the public archive of the Chandra X-ray Observatory as of March 2016 and assembled a sample of 719 galaxies within 50 Mpc with ACIS observations available. By cross-correlation with the optical or near-infrared nuclei of these galaxies, 314 of them are identified to have an X-ray active galactic nucleus (AGN). The majority of them are low-luminosity AGNs and are unlikely X-ray binaries based upon their spatial distribution and luminosity functions. The AGN fraction is around 60% for elliptical galaxies and early-type spirals, but drops to roughly 20% for Sc and later types, consistent with previous findings in the optical. However, the X-ray survey is more powerful in finding weak AGNs, especially from regions with active star formation that may mask the optical AGN signature. For example, 31% of the H II nuclei are found to harbor an X-ray AGN. For most objects, a single power-law model subject to interstellar absorption is adequate to fit the spectrum, and the typical photon index is found to be around 1.8. For galaxies with a non-detection, their stacked Chandra image shows an X-ray excess with a luminosity of a few times 10^37 erg/s on average around the nuclear region, possibly composed of faint X-ray binaries. This paper reports on the technique and results of the survey; in-depth analysis and discussion of the results will be reported in forthcoming papers.