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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. and Brandt, W. N. and Reines, A. E. and Lansbury, G. and Stern, D. and Alexander, D. M. and Bauer, F. and Moro, A. Del and Gandhi, P. and Harrison, F. A. and Hickox, R. C. and Koss, M. J. and Lanz, L. and Luo, B. and Mullaney, J. R. and Ricci, C. and Trump, J. R. (2017) 'Hard X-ray-selected AGNs in low-mass galaxies from the NuSTAR serendipitous survey.', Astrophysical journal., 837 (1). p. 48.


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.

Item Type:Article
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Publisher statement:© 2017. The American Astronomical Society. All rights reserved.
Date accepted:27 January 2017
Date deposited:03 July 2017
Date of first online publication:01 March 2017
Date first made open access:03 July 2017

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