Context. Accretion of gas from the intergalactic medium is required to
fuel star formation in galaxies. We have recently suggested that this
process can be studied using host galaxies of gamma-ray bursts (GRBs).
Aims: Our aim is to test this possibility by studying in detail
the properties of gas in the closest galaxy hosting a GRB (980425). Methods: We obtained the first ever far-infrared (FIR) line
observations of a GRB host, namely Herschel/PACS resolved [C II] 158
μm and [O I] 63 μm spectroscopy, and an APEX/SHeFI CO(2-1) line
detection and ALMA CO(1-0) observations of the GRB 980425 host.
Results: The GRB 980425 host has elevated [C II]/FIR and [O I]/FIR
ratios and higher values of star formation rates (SFR) derived from line
([C II], [O I], Hα) than from continuum (UV, IR, radio)
indicators. [C II] emission exhibits a normal morphology, peaking at the
galaxy centre, whereas [O I] is concentrated close to the GRB position
and the nearby Wolf-Rayet region. The high [O I] flux indicates that
there is high radiation field and high gas density at these positions,
as derived from modelling of photo-dissociation regions. The [C II]/CO
luminosity ratio of the GRB 980425 host is close to the highest values
found for local star-forming galaxies. Indeed, its CO-derived molecular
gas mass is low given its SFR and metallicity, but the [C II]-derived
molecular gas mass is close to the expected value. Conclusions:
The [O I] and H I concentrations and the high radiation field and
density close to the GRB position are consistent with the hypothesis of
a very recent (at most a few tens of Myr ago) inflow of atomic gas
triggering star formation. In this scenario dust has not had time to
build up (explaining high line-to-continuum ratios). Such a recent
enhancement of star formation activity would indeed manifest itself in
high SFRline/SFRcontinuum 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). Within a sample of 32 other GRB hosts, 20 exhibit
SFRline/SFRcontinuum> 1 with a mean ratio of
1.74 ± 0.32. This is consistent with a very recent enhancement of
star formation that is common among GRB hosts, so galaxies that have
recently experienced inflow of gas may preferentially host stars
exploding as GRBs. Therefore GRBs may be used to select a unique sample
of galaxies that is suitable for the investigation of recent gas
accretion.
Herschel is an ESA space observatory with science instruments provided
by European-led Principal Investigator consortia and with important
participation from NASA.