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 á ñ = -M 20.03, and á ñ =-L 3.1 10 2 10 keV 42 erg s −1 , respectively. Five objects have detectable broad Hα emission in their optical spectra, indicating black hole masses of ( -) ☉ M 1.1 10.4 10 6 . 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-rayselected 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.

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