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 _ { \star } galaxy at its redshift .
The median values of absolute magnitude , stellar mass and 2–10 X-ray luminosity of our sample are \langle M _ { r } \rangle = -20.03 , \langle M _ { \star } \rangle = 4.6 \times 10 ^ { 9 } M _ { \sun } , and \langle L _ { 2 - 10 \mathrm { keV } } \rangle = 3.1 \times 10 ^ { 42 } erg s ^ { -1 } , respectively .
Five objects have detectable broad H \alpha emission in their optical spectra , indicating black-hole masses of ( 1.1 - 10.4 ) \times 10 ^ { 6 } M _ { \sun } .
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 can not 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 .