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

• Published in: The Astrophysical journal 2017-03, Vol.837 (1), p.48
• Main Authors: Chen (陳建廷), C.-T. J., Brandt, W. N., Reines, A. E., Lansbury, G., Stern, D., Alexander, D. M., Bauer, F., Moro, A. Del, Gandhi, P., Harrison, F. A., Hickox, R. C., Koss, M. J., Lanz, L., Luo, B., Mullaney, J. R., Ricci, C., Trump, J. R.
• Format: Article
• Language: English
• Subjects: Active galactic nuclei; Astrophysics; Black holes; Deposition; Emission lines; Emission spectra; Galaxies; galaxies: active; galaxies: dwarf; H alpha line; Infrared astronomy; Luminosity; Object recognition; Polls & surveys; Red shift; Spectroscopic telescopes; Spectroscopy; Stars & galaxies; Stellar mass; X ray absorption; X-ray emissions; X-rays: galaxies
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/aa5d5b

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 , to be at , and to have optical r-band magnitudes at least 0.5 mag fainter than an galaxy at its redshift. The median values of absolute magnitude, stellar mass, and 2-10 X-ray luminosity of our sample are , , and erg s−1, respectively. Five objects have detectable broad H emission in their optical spectra, indicating black hole masses of . We find that 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 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 ( 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 ( ), thus providing a critical step in understanding AGN demographics in low-mass galaxies.