Dr. Ken Mawatari (ICRR)
"Challenge to the Most Distant Evolved Galaxies: Hint on Star-formation Activity during the First 500 Million Years of the Cosmic History"
The cosmic star-formation rate densities have been measured up to redshift 10. However, this is a limit of observations of direct star-formation activity with current instruments such as the Hubble Space Telescope. Here we bring a novel approach for the cosmic star-formation rate density at an unexplored epoch in the cosmic history. Using very deep imaging data from the Spitzer, Subaru, and VISTA telescopes, we have identified candidate massive galaxies at redshift around 6 which are dominated by old stars. The "passiveness" of the galaxies indicated from their spectral energy distributions suggests that their star-formation should have been finished by redshift 15. From their number density and star-formation history, we obtained the first (indirect) estimate of the star-formation rate density at redshift ~ 18.