Hard X-Ray-selected AGNs in Low-mass Galaxies from the NuSTAR Serendipitous Survey

Abstract

We present a sample of 10 low-mass active galactic nuclei (AGNs) selected from the 40-month Nuclear Spectroscopic Telescope Array (NuSTAR) serendipitous survey. The sample is selected to have robust NuSTAR detections at 3-24 keV, to be at z < 0.3, and to have optical r-band magnitudes at least 0.5 mag fainter than an L_★ galaxy at its redshift. The median values of absolute magnitude, stellar mass, and 2–10 X-ray luminosity of our sample are 〈 M_★〉 = -20.03, M_r〉 = 4.6 x 10^9 M_⊙, and 〈L_(2-10 keV)〉 = 3.1 x 10^(42) erg s^(−1), respectively. Five objects have detectable broad Hα emission in their optical spectra, indicating black hole masses of (1.1--10.4) x 10^6 M_⊙. We find that 30_(-10)^(+17)% of the galaxies in our sample do not show AGN-like optical narrow emission lines, and one of the 10 galaxies in our sample, J115851+4243.2, shows evidence for heavy X-ray absorption. This result implies that a non-negligible fraction of low-mass galaxies might harbor accreting massive black holes that are missed by optical spectroscopic surveys and < 10 keV X-ray surveys. The mid-IR colors of our sample also indicate that these optically normal low-mass AGNs cannot be efficiently identified with typical AGN selection criteria based on Wide Field Infrared Survey Explorer colors. While the hard (> 10 keV) X-ray-selected low-mass AGN sample size is still limited, our results show that sensitive NuSTAR observations are capable of probing faint hard X-ray emission originating from the nuclei of low-mass galaxies out to moderate redshift (z > 0.3), thus providing a critical step in understanding AGN demographics in low-mass galaxies.