Cosmic star formation history revealed by AKARI and Hyper Suprime Cam

Tomotsugu Goto

Revealing cosmic star formation history is one of the major goals of observational astronomy. However, the extragalactic background suggests half the energy generated by stars reprocessed into the infrared (IR) by dust. Especially, at z∼1.3, 90% of star formation is obscured by dust. To fully understand the cosmic star formation history, it is critical to investigate infrared emission. The first Japanese infrared space telescope, AKARI, performed deep mid-infrared observation using its continuous 9-band filters in the NEP field (5.4 deg2), using ∼ 10% of the entire pointed observations available throughout satellite’s lifetime. AKARI’s mid-IR data are truly unique in that Spitzer lacks filters between 8 and 24μm. Similarly WISE also has a wide gap between 4 and 12μm filters. No other telescope can provide continuous 9-band photometry in mid-IR wavelength (2-24μm) in foreseeable future. However previously, we only had shallow optical imaging (with CFHT, r ∼25.9mag) in a small area of 1.0 deg2. As a result, there remained ~11,000 AKARI’s infrared sources undetected in optical. Redshift and IR luminosity of these sources are unknown. They may carry a significant amount of cosmic star-formation rate density (CSFRD). For example, if they all lie at 1< z <2, the CSFRD will be twice as high at the epoch. With the advent of Hyper Suprime Cam, for the first time, we were able to obtain deep enough optical images of the entire AKARI NEP field in 5 broad bands (g~27.5mag). These provided photometric redshift, and thereby IR luminosity for the previously undetected 11,000 faint AKARI IR sources, over the entire 5.4deg2. Combined with AKARI’s unique mid-IR AGN/SF diagnosis, and accurate mid-IR luminosity measurement, we performed a complete census of dust-obscured cosmic star-formation/AGN accretion history in the entire AKARI NEP field.