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

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.