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 2016 March, we have identified 314 candidate active galactic nuclei in 719 galaxies located closer than 50 Mpc, among them late-type galaxies (Hubble types Sc and later) 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 {M}_⊙ ) 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 10³⁸ {erg} {{{s}}}^-1 are not contaminated by X-ray binaries; the fraction increases to ∼92% for X-ray cores with a luminosity of 10³⁹ {erg} {{{s}}}^-1 or higher. This allows us to estimate a black hole occupation fraction of ≳ 21% in these late-type galaxies, many of which are bulgeless.