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 ( \lesssim 10 ^ { 6 } M _ { \sun } ) 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 , \sim 74 % of them with luminosities above 10 ^ { 38 } erg s ^ { -1 } are not contaminated by X-ray binaries ; the fraction increases to \sim 92 % for X-ray cores with a luminosity of 10 ^ { 39 } erg s ^ { -1 } or higher .
This allows us to estimate a black hole occupation fraction of \gtrsim 21 % in these late-type , many bulgeless , galaxies .