GRB 980425 host: [CII], [OI] and CO lines reveal recent enhancement of
  star formation due to atomic gas inflow
  We have recently suggested that gas accretion can be studied using host
galaxies of gamma-ray bursts (GRBs). We obtained the first ever far-infrared
(FIR) line observations of a GRB host, namely Herschel/PACS resolved [CII] 158
um and [OI] 63 um spectroscopy, as well as APEX CO(2-1) and ALMA CO(1-0)
observations of the GRB 980425 host. It has elevated [CII]/FIR and [OI]/FIR
ratios and higher values of star formation rate (SFR) derived from line ([CII],
[OI], Ha) than from continuum (UV, IR, radio) indicators. [CII] emission
exhibits a normal morphology, peaking at the galaxy center, whereas [OI] is
concentrated close to the GRB position and the nearby Wolf-Rayet region. The
high [OI] flux indicates high radiation field and gas density. The [CII]/CO
luminosity ratio of the GRB 980425 host is close to the highest values found
for local star-forming galaxies. Its CO-derived molecular gas mass is low given
its SFR and metallicity, but the [CII]-derived molecular gas mass is close to
the expected value. The [OI] and HI concentrations, and the high radiation
field and density are consistent with the hypothesis of a very recent (at most
a few tens of Myr ago) inflow of atomic gas triggering star formation. Dust has
not had time to build up (explaining high line-to-continuum ratios). Such a
recent enhancement of star-formation would indeed manifest itself in high
SFR_line/SFR_continuum ratios, because the line indicators are sensitive only
to recent (<10 Myr) activity, whereas the continuum indicators measure the SFR
averaged over much longer periods (~100 Myr). Other GRB hosts exhibit a mean
SFR_line/SFR_continuum of 1.74+-0.32. This is consistent with a very recent
enhancement of star formation being common among GRB hosts, so galaxies which
have recently experienced inflow of gas may preferentially host stars exploding
as GRBs. Hence GRB hosts may be used to investigate recent gas accretion.
