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

doi:10.3847/1538-4357/aa5d5b

paper

Hard X-ray selected AGNs in low-mass galaxies from the NuSTAR serendipitous survey

arXiv:1701.08768 · doi:10.3847/1538-4357/aa5d5b

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 $⟨ M_{r} ⟩ = - 20.03$ , $⟨ M_{⋆} ⟩ = 4.6 \times 1 0^{9} M_{⊙}$ , and $⟨ L_{2 - 10 keV} ⟩ = 3.1 \times 1 0^{42}$ erg s $^{- 1}$ , respectively. Five objects have detectable broad H $α$ emission in their optical spectra, indicating black-hole masses of $(1.1 - 10.4) \times 1 0^{6} M_{⊙}$ . We find that $3 0_{- 10}^{+ 17} %$ 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.

arXiv commentAccepted for publication in ApJ. 27 pages, 6 tables, 10 figures, including one online figure set available at https://ctjchen.github.io/ctc_nustardwarf_figset.pdf (v2: updated acknowledgement.)

Hard X-ray selected AGNs in low-mass galaxies from the NuSTAR serendipitous survey

Abstract

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