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

C. T.J. Chen, W. N. Brandt, A. E. Reines, G. Lansbury, D. Stern, D. M. Alexander, 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

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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 Hα 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.

Original languageEnglish (US)
Article number48
JournalAstrophysical Journal
Issue number1
StatePublished - Mar 1 2017

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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