An effective selection method for low-mass active black holes and first spectroscopic identification.

We present a newmethod for effectively selecting objects which may be low-mass active black holes (BHs) at galaxy centers using high-cadence optical imaging data, and our first spectroscopic identification of an active 2.7 × 106 M☉ BH at z = 0.164. This active BH was originally selected due to its rapid optical variability, from a few hours to a day, based on Subaru Hyper Suprime-Cam g-band imaging data taken with a 1 hr cadence. Broad and narrow Hα lines and many other emission ones are detected in our optical spectra taken with Subaru FOCAS, and the BH mass is measured via the broad Hα emission line width (1880 kms-1) and luminosity (4.2 × 1040 erg s-1) after careful correction to the atmospheric absorption around 7580-7720 A° . We measure the Eddington ratio and find it to be as low as 0.05, considerably smaller than those in a previous SDSS sample with similar BH mass and redshift, which indicates one of the special potentials of our Subaru C survey. The g - r color and morphology of the extended component indicate that the host galaxy is a star-forming galaxy. We also show the effectiveness of our variability selection for low-mass active BHs. [ABSTRACT FROM AUTHOR]