Optical Identification of Infrared Space Observatory Far-Infrared Sources in the Lockman Hole Using a Deep Very Large Array 1.4 GHz Continuum Survey

By exploiting the far-infrared (FIR) and radio correlation, we have performed a likelihood-ratio analysis to identify optical counterparts to the FIR sources that have been found in an area of ~0.9 deg² during the Infrared Space Observatory (ISO) deep FIR survey in the Lockman Hole. New ground-based observations have been conducted to build up the catalogs of radio and optical objects, which include a deep Very Large Array (VLA) observation at 1.4 GHz, optical R- and I-band imaging with the Subaru 8 m and University of Hawaii 2.2 m telescopes, and optical spectroscopy with the Keck II 10 m and WIYN 3.5 m telescopes. This work is based on FIR samples consisting of 116 and 20 sources selected with the criteria of F^C(90 μm)>=43 mJy and F^C(170 μm)>=102 mJy, respectively, where F^C is the bias-corrected flux. Using the likelihood ratio analysis and the associated reliability, 44 FIR sources have been identified with radio sources. Optical confirmation of the 44 FIR/radio associations was then conducted using accurate radio positions. Redshifts have been obtained for 29 out of the 44 identified sources. One hyperluminous infrared galaxy (HyLIRG) with L_FIR>10¹³ L_solar and four ultraluminous infrared galaxies (ULIRGs) with L_FIR=10¹²-10¹³ L_solar are identified in our sample, while the remaining 24 FIR galaxies have L_FIR<10¹² L_solar. The space density of the FIR sources at z=0.3-0.6 is 4.6×10^-5 Mpc^-3, which is 460 times larger than the local value, implying a rapid evolution of the ULIRG population. Most ISO FIR sources have L(1.4 GHz)/L(90 μm) similar to the star-forming galaxies Arp 220 and M82, indicating that star formation is the dominant mechanism for their FIR and radio luminosity.

At least seven of our FIR sources show evidence for the presence of an active galactic nucleus (AGN) in optical emission lines, radio continuum excess, or X-ray activity. Three out of five (60%) of the ULIRGs/HyLIRGs are AGN galaxies, suggesting that the AGN fraction among the ULIRG/HyLIRG population may not change significantly between z~0.5 and the present epoch. Five of the seven AGN galaxies are within the ROSAT X-ray survey field, and two are within the XMM-Newton survey fields. X-ray emission has been detected in only one source, 1EX030, which is optically classified as a quasar. The nondetection in the XMM-Newton 2-10 keV band suggests a very thick absorption column density of 3×10²⁴ cm^-2 or A_V~1200 mag obscuring the central source of the two AGN galaxies. Several sources have an extreme FIR luminosity relative to the optical R band, L(90 μm)/L(R)>500, which is rare even among the local ULIRG population. While source confusion or blending might offer an explanation in some cases, these observations may represent a new population of galaxies with an extreme amount of star formation in an undeveloped stellar system, i.e., formation of bulges or young elliptical galaxies.