A Wide and Deep Exploration of Radio Galaxies with Subaru HSC (WERGS). I. The Optical Counterparts of FIRST Radio Sources

TL;DR

This study cross-matched FIRST radio sources with Subaru HSC data, revealing over 3600 optical counterparts, including many faint and high-redshift radio galaxies and quasars, significantly expanding the known population of radio AGNs.

Contribution

It provides the first large-scale optical identification of FIRST radio sources with deep Subaru HSC data, uncovering a substantial population of faint and high-redshift radio AGNs.

Findings

Over 3600 optical counterparts identified within 1" matching radius.

Faint radio galaxies show a flatter source count slope, indicating less massive or distant sources.

High radio-loudness sources at z > 1 are included in the optically faint sample.

Abstract

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.