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

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.

Abstract

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.

Citation

Chen (陳建廷), C., Brandt, W., Reines, A., Lansbury, G., Stern, D., Alexander, D., …Trump, J. (2017). Hard X-Ray-selected AGNs in Low-mass Galaxies from the NuSTAR Serendipitous Survey. Astrophysical Journal, 837(1), Article 48. https://doi.org/10.3847/1538-4357/aa5d5b

• Journal Article Type: Article
• Acceptance Date: Jan 27, 2017
• Online Publication Date: Mar 1, 2017
• Publication Date: Mar 1, 2017
• Deposit Date: Jun 21, 2017
• Publicly Available Date: Jul 3, 2017
• Journal: Astrophysical Journal
• Print ISSN: 0004-637X
• Electronic ISSN: 1538-4357
• Publisher: American Astronomical Society
• Peer Reviewed: Peer Reviewed
• Volume: 837
• Issue: 1
• Article Number: 48
• DOI: https://doi.org/10.3847/1538-4357/aa5d5b
• Public URL: https://durham-repository.worktribe.com/output/1356953