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Register a new userKeck Observations of 160 Gamma-Ray Burst Host Galaxies
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Daniel A. Perley
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We present a preliminary data release from our multi-year campaign at Keck Observatory to study the host galaxies of a large sample of Swift-era gamma-ray bursts via multi-color ground-based optical imaging and spectroscopy. With over 160 targets observed to date (and almost 100 host detections, most of which have not previously been reported in the literature) our effort represents the broadest GRB host survey to date. While targeting was heterogeneous, our observations span the known diversity of GRBs including short bursts, long bursts, spectrally soft GRBs (XRFs), ultra-energetic GRBs, X-ray faint GRBs, dark GRBs, SN-GRBs, and other sub-classes. We also present a preview of our database (currently available online via a convenient web interface) including a catalog of multi-color photometry, redshifts and line IDs. Final photometry and reduced imaging and spectra will be available in the near future.
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Due to their extreme luminosities, gamma-ray bursts (GRBs) can be detected in hostile regions of galaxies, nearby and at very high redshift, making them important cosmological probes. The investigation of galaxies hosting long-duration GRBs (whose progenitor is a massive star) demonstrated their connection to star formation. Still, the link to the total galaxy population is controversial, mainly because of the small-number statistics: ~ 1,100 are the GRBs detected so far, ~ 280 those with measured redshift, and ~ 70 the hosts studied in detail. These are typically low-redshift (z < 1.5), low luminosity, metal poor, and star-forming galaxes. On the other hand, at 1.5< z <4, massive, metal rich and dusty, interacting galaxies are not uncommon. The most distant population (z > 4) is poorly explored, but the deep limits reached point towards very small and star-forming objects, similar to the low-z population. This `back to the future behavior is a natural consequence of the connection of long GRBs to star formation in young regions of the universe.
GRB-selected galaxies are broadly known to be faint, blue, young, star-forming dwarf galaxies. This insight, however, is based in part on heterogeneous samples of optically selected, lower-redshift galaxies. To study the statistical properties of GRB-selected galaxies we here introduce The Optically Unbiased GRB Host (TOUGH) complete sample of 69 X-ray selected Swift GRB host galaxies spanning the redshift range 0.03-6.30 and summarise the first results of a large observational survey of these galaxies.
We use galaxy catalogues constructed by combining high-resolution N-body simulations with semi-analytic models of galaxy formation to study the properties of Long Gamma-Ray Burst (LGRB) host galaxies. We assume that LGRBs originate from the death of massive young stars and analyse how results are affected by different metallicity constraints on the progenitor stars. As expected, the host sample with no metallicity restriction on the progenitor stars provides a perfect tracer of the cosmic star formation history. When LGRBs are required to be generated by low-metallicity stars, they trace a decreasing fraction of the cosmic star formation rate at lower redshift, as a consequence of the global increase in metallicity. We study the properties of host galaxies up to high redshift (~9), finding that they typically have low-metallicity (Z<0.5 Z_sun) and that they are small (M<10^9 M_sun), bluer and younger than the average galaxy population, in agreement with observational data. They are also less clustered than typical L_* galaxies in the Universe, and their descendents are massive, red and reside in groups of galaxies with halo mass between 10^{13} M_sun to 10^{14} M_sun.
We obtained CO(2-1) observations of seven GRB hosts with the APEX and IRAM 30m telescopes. We analysed these data together with all other hosts with previous CO observations. We obtained detections for 3 GRB hosts (980425, 080207, and 111005A) and upper limits for the remaining 4 (031203, 060505, 060814, and 100316D). In our entire sample of 12 CO-observed GRB hosts, 3 are clearly deficient in molecular gas, even taking into account their metallicity (980425, 060814, and 080517). Four others are close to the best-fit line for other star-forming galaxies on the SFR-MH2 plot (051022, 060505, 080207, and 100316D). One host is clearly molecule rich (111005A). Finally, the data for 4 GRB hosts are not deep enough to judge whether they are molecule deficient (000418, 030329, 031203, and 090423). The median value of the molecular gas depletion time, MH2/SFR, of GRB hosts is ~0.3 dex below that of other star-forming galaxies, but this result has low statistical significance. A Kolmogorov-Smirnov test performed on MH2/SFR shows an only ~2sigma difference between GRB hosts and other galaxies. This difference can partly be explained by metallicity effects, since the significance decreases to ~1sigma for MH2/SFR versus~metallicity. We found that any molecular gas deficiency of GRB hosts has low statistical significance and that it can be attributed to their lower metallicities; and thus the sample of GRB hosts has molecular properties that are consistent with those of other galaxies, and they can be treated as representative star-forming galaxies. Given the concentration of atomic gas recently found close to GRB and supernova sites, indicating recent gas inflow, our results about the weak molecular deficiency imply that such an inflow does not enhance the SFRs significantly, or that atomic gas converts efficiently into the molecular phase, which fuels star formation.
Long gamma-ray bursts (GRBs), among the most energetic events in the Universe, are explosions of massive and short-lived stars, so they pinpoint locations of recent star formation. However, several GRB host galaxies have recently been found to be deficient in molecular gas (H2), believed to be the fuel of star formation. Moreover, optical spectroscopy of GRB afterglows implies that the molecular phase constitutes only a small fraction of the gas along the GRB line-of-sight. Here we report the first ever 21 cm line observations of GRB host galaxies, using the Australia Telescope Compact Array, implying high levels of atomic hydrogen (HI), which suggests that the connection between atomic gas and star formation is stronger than previously thought, with star formation being potentially directly fuelled by atomic gas (or with very efficient HI-to-H2 conversion and rapid exhaustion of molecular gas), as has been theoretically shown to be possible. This can happen in low metallicity gas near the onset of star formation, because cooling of gas (necessary for star formation) is faster than the HI-to-H2 conversion. Indeed, large atomic gas reservoirs, together with low molecular gas masses, stellar and dust masses are consistent with GRB hosts being preferentially galaxies which have very recently started a star formation episode after accreting metal-poor gas from the intergalactic medium. This provides a natural route for forming GRBs in low-metallicity environments. The gas inflow scenario is also consistent with the existence of the companion HI object with no optical counterpart ~19 kpc from the GRB 060505 host, and with the fact that the HI centroids of the GRB 980425 and 060505 hosts do not coincide with optical centres of these galaxies, but are located close to the GRB positions.
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