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East-Asian ALMA Science Workshop 2019
February 19(Wed)-21(Fri), 2020
ASIAA, Taipei, Taiwan

Oral Presentation

Star-formation rates of two GRB host galaxies at z~2 and a [C II] deficit observed with ALMA

Author(s): Tetsuya Hashimoto (NTHU), Bunyo Hatsukade (Univ. of Tokyo), Tomotsugu Goto (NTHU), Seong Jin Kim (NTHU), Kouji Ohta (Kyoto Univ.), Tohru Nagao (Ehime Univ.), Albert K. H. Kong (NTHU), Kouichiro Nakanishi (NAOJ), and Jirong Mao (Yunnan Obs.)

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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