A Wide and Deep Exploration of Radio Galaxies with Subaru HSC (WERGS).
  I. The Optical Counterparts of FIRST Radio Sources
  We report the result of optical identifications of FIRST radio sources with
the Hyper Suprime-Cam Subaru Strategic Program survey (HSC-SSP). The positional
cross-match within 1" between the FIRST and HSC-SSP catalogs (i ~< 26) produced
more than 3600 optical counterparts in the 156 deg^2 of the HSC-SSP field. The
matched counterparts account for more than 50% of the FIRST sources in the
search field, which substantially exceed previously reported fractions of SDSS
counterparts (i ~< 22) of ~30%. Among the matched sample, 9% are optically
unresolved sources such as radio-loud quasars. The optically faint (i > 21)
radio galaxies (RGs) show that the fitting linear function of the 1.4 GHz
source counts has a slope that is flatter than that of the bright RGs, while
optically faint radio quasars show a slope steeper than that of bright radio
quasars. The optically faint RGs show a flat slope in the i-band number counts
down to 24 mag, implying either less massive or distant radio-active galactic
nuclei (AGNs) beyond 24 mag. The photometric redshift and the comparison of
colors with the galaxy models show that most of the matched RGs are distributed
at redshifts from 0 to 1.5. The optically faint sample includes the high
radio-loudness sources that are not seen in the optically bright sample. Such
sources are located at redshift z > 1. This study gives ~1500 radio AGNs lying
at the optically faint end and high-redshift regime not probed by previous
searches.
