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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.

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


C.-T.J. Chen (陳建廷)

W.N. Brandt

A.E. Reines

G. Lansbury

D. Stern

F. Bauer

A. Del Moro

P. Gandhi

F.A. Harrison

R.C. Hickox

M.J. Koss

L. Lanz

B. Luo

J.R. Mullaney

C. Ricci

J.R. Trump


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.

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
Public URL


Published Journal Article (1.9 Mb)

Copyright Statement
© 2017. The American Astronomical Society. All rights reserved.

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