Oral Presentation
Star-formation rates of two GRB host galaxies at z~2 and a [C II] deficit observed with ALMA
Presenter: Tetsuya Hashimoto (National Tsing Hua University)
Event rate of long Gamma-Ray Bursts (GRBs) is expected to be an useful tracer of the cosmic
star-formation history. For this purpose, it is necessary to understand what kind of star
formations/galaxies are traced by GRBs. Here we report rest-frame far-infrared (FIR)
continuum detections of GRB 070521 and 080207 host galaxies at z~2 with ALMA band 8
and 9. The FIR photometries provide the reliable star-formation rates (SFRs), because FIR
emission is free from dust extinction and possible radio contamination from long-lived
afterglows of GRBs. The spectral energy distribution fittings indicate 49.85
^{+72.33}_{-2.86} and 123.4^{+25.19}_{-21.78} Msun yr^{-1} for the 070521 and 080207
hosts, respectively. The derived SFRs place them on the 'main sequence' of normal
star-forming galaxies at z~2. The derived SFRs are significantly lower than that of radio
observations. It is inferred that the observed radio fluxes in a previous study are
contaminated by the afterglows. ALMA marginally detected [C II]158um emission line
from the GRB 080207 host galaxy with S/N~4. This is the first detection of [C II]158um
of a GRB host at z~2, and the second detection among known GRBs. The luminosity
ratio of [C II]158um to FIR is 7.5x10^{-4}, which is one of the smallest values among
galaxies at z~1-2 with the same FIR luminosity. The '[C II] deficit' could be a new
physical property to characterise GRB hosts at z~1-2. Possible parameters controlling
the deficit include the metallicity, initial mass function, and gas density
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