2019ApJ...885L...3H

The properties of low-mass galaxies hosting central black holes provide clues about the formation and evolution of the progenitors of supermassive black holes. In this Letter, we present HSC-XD 52, a spectroscopically confirmed low-mass active galactic nucleus (AGN) at an intermediate redshift of z ∼ 0.56. We detect this object as a very luminous X-ray source coincident with a galaxy observed by the Hyper Suprime-Cam (HSC) as part of a broader search for low-mass AGN. We constrain its stellar mass through spectral energy distribution modeling to be LMC-like at M_* ≃ 3 x 10⁹M_☉, placing it in the dwarf regime. We estimate a central black hole mass of M_BH ∼ 10⁶M_☉. With an average X-ray luminosity of L_X~3.5×10⁴³ergs^–1, HSC-XD 52 is among the most luminous X-ray selected AGN in dwarf galaxies. The spectroscopic and photometric properties of HSC-XD 52 indicate that it is an intermediate-redshift counterpart to local low-mass AGN.