Unveiling a population of galaxies harboring low-mass black holes with X-rays

We report the discovery of three low-mass black hole (BH) candidates residing in the centers of low-mass galaxies at z < 0.3 in the Chandra Deep Field-South Survey. These BHs are initially identified as candidate active galactic nuclei based on their X-ray emission in deep Chandra observations. Multi-wavelength observations are used to strengthen our claim that such emission is powered by an accreting supermassive BH. While the X-ray luminosities are low at L_X ∼ 10⁴⁰ erg s^-1 (and variable in one case), we argue that they are unlikely to be attributed to star formation based on Hα or UV fluxes. Optical spectroscopy from Keck and the VLT allows us to (1) measure accurate redshifts, (2) confirm their low stellar host mass, (3) investigate the source(s) of photo-ionization, and (4) estimate extinction. With stellar masses of M _* < 3 × 10⁹ M _⊙ determined from Hubble Space Telescope/Advanced Camera for Surveys imaging, the host galaxies are among the lowest mass systems known to host actively accreting BHs. We estimate BH masses M _BH ∼ 2 × 10⁵ M _⊙ based on scaling relations between BH mass and host properties for more luminous systems. In one case, a broad component of the Hα emission-line profile is detected, thus providing a virial mass estimate. BHs in such low-mass galaxies are of considerable interest as the low-redshift analogs to the seeds of the most massive BHs at high redshift which have remained largely elusive to date. Our study highlights the power of deep X-ray surveys to uncover such low-mass systems.