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. (2017). Hard X-Ray-selected AGNs in Low-mass Galaxies from the NuSTAR Serendipitous Survey. The Astrophysical Journal 837
Author(s) from Durham
We present a sample of 10 low-mass active galactic nuclei (AGNs) selected from the 40-month 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 ⟨Mr⟩=−20.03, ⟨M⋆⟩=4.6×109M⊙, and ⟨L2−10keV⟩=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)×106M⊙. We find that 30+17−10% of the galaxies in our sample do not show AGN-like optical narrow emission lines, and one of the ten 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 these optically normal low-mass AGNs cannot be efficiently identified with typical AGN selection criteria based on WISE 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.