Chandra Survey of Nearby Galaxies: A Significant Population of Candidate Central Black Holes in Late-type Galaxies

Abstract: Based on the Chandra data archive as of March 2016, we have identified 314 candidate active galactic nuclei in 719 galaxies located closer than 50 Mpc, among them late-type (Hubble types Sc and later) galaxies that previously had been classified from optical observations as containing star-forming (H II) nuclei. These late-type galaxies comprise a valuable subsample to search for low-mass (<~ 10^6 solar masses) central black holes. For the sample as a whole, the overall dependence of the fraction of active nuclei on galaxy type and nuclear spectral classification is consistent with previous results based on optical surveys. We detect 51 X-ray cores among the 163 H II nuclei and estimate that, very conservatively, ~74% of them with luminosities above 10^38 erg/s are not contaminated by X-ray binaries; the fraction increases to ~92% for X-ray cores with a luminosity of 10^39 erg/s or higher. This allows us to estimate a black hole occupation fraction of >~ 21% in these late-type, many bulgeless, galaxies.