2020ApJ...901....3I

Utilizing the latest and the most sensitive radio and optical catalogs, we completed a new blazar candidate catalog, Blazar Radio and Optical Survey (BROS), which includes 88,211 sources located at decl. δ > -40° and outside the galactic plane (| b| > 10^○). We list compact flat-spectrum radio sources of α > -0.6 (α is defined as F_ν ∝ ν^α) from 0.15 GHz TGSS to 1.4 GHz NVSS catalogs. We further identify optical counterparts of the selected sources by cross-matching with Pan-STARRS1 photometric data. Color-color and color-magnitude plots for the selected BROS sources clearly show two distinct populations, a "quasar-like" population consisting of both flat-spectrum radio quasars and BL Lac type objects. On the other hand, an "elliptical-like" population of mostly BL Lac type objects is buried in the elliptical galaxy. The latter population is also reported in previous catalogs, but the BROS catalog provided a new larger sample of this population, due to the lower radio flux threshold of our selection. Model calculations show that the "elliptical-like" population consists of elliptical galaxies located at redshift z <= 0.5, which is also supported by the logN-logS distribution of the power-law index of 1.49 ± 0.05. This BROS catalog is useful for identifying the electromagnetic counterparts of ultra-high-energy cosmic rays and PeV neutrinos recently detected by IceCube, as well as nearby BL Lac objects detectable by future high-sensitivity TeV telescopes, such as the Cherenkov Telescope Array.