2017ApJ...837...48C

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_r>=-20.03, <M_*>=4.6×10⁹ M_☉, and <L_2–10keV>=3.1×10⁴² erg s^–1, respectively. Five objects have detectable broad Hα emission in their optical spectra, indicating black hole masses of (1.1–10.4)×10⁶ 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.