New light on gamma-ray burst host galaxies withHerschel

Abstract

Until recently, dust emission has been detected in very few host galaxies of gamma-ray bursts (GRBHs). With Herschel, we have now observed 17 GRBHs up to redshift z ∼ 3 and detected seven of them at infrared (IR) wavelengths. This relatively high detection rate (41%) may be due to the composition of our sample which at a median redshift of 1.1 is dominated by the hosts of dark GRBs. Although the numbers are small, statistics suggest that dark GRBs are more likely to be detected in the IR than their optically-bright counterparts. Combining our IR data with optical, near-infrared, and radio data from our own datasets and from the literature, we have constructed spectral energy distributions (SEDs) which span up to 6 orders of magnitude in wavelength. By fitting the SEDs, we have obtained stellar masses, dust masses, star-formation rate (SFR), and extinctions for our sample galaxies. We find that GRBHs are galaxies that tend to have a high specfic SFR (sSFR), and like other star-forming galaxies, their ratios of dust-to-stellar mass are well correlated with sSFR. Dust masses of GRBHs relative to stellar mass and SFR fall within the range of other star-forming galaxies in the local universe, and of sub-millimeter galaxies (SMGs) and luminous IR galaxies for redshift z > ∼ 1. We incorporate our Herschel sample into a larger compilation of GRBHs, after checking for consistency in mass and SFR estimations. This combined sample is compared to SFR-weighted median stellar masses of the widest, deepest galaxy survey to date in order to establish whether or not GRBs can be used as an unbiased tracer of cosmic comoving SFR density (SFRD) in the universe. In contrast with previous results, this comparison shows that GRBHs are medium-sized galaxies with relatively high sSFRs, as might be expected for galaxies selected on the basis of SFR because of the explosive GRB event. Stellar masses and sSFRs of GRBHs as a function of redshift are similar to what is expected for star-forming galaxy populations at similar redshifts. We conclude that there is no strong evidence that GRBs are biased tracers of SFRD; thus they should be able to reliably probe the SFRD to early epochs.

GRB hosts through cosmic time

“…16 The figure and results remain conceptually unchanged if we use the host's 12 + log(O/H) and SFR to derive a stellar mass via Mannucci et al (2010) to compare it with the SFR-weighted stellar-mass function. 17 Similar values are inferred through the analysis of Perley et al (2013b), Hunt et al (2014) and Sobral et al (2014), which use deep photometric surveys (Kajisawa et al 2009;Ilbert et al 2013) to calculate a galaxy stellar mass of 10 9.7−10.2 M at z ∼ 0.6 above or below which half of all star formation occurs. This implies a metallicity between Z = 0.9 Z and Z = 1.4 Z in our oxygen-abundance scale (Mannucci et al 2010).…”

“…The relative sparsity of high A V events at low redshift is however not easily explained by selection effects. At the average mass of the SF-weighted galaxy population of M ∼ 10 10 M at z ∼ 0.6 Hunt et al 2014), the average reddening for galaxies is E B−V ∼ 0.35 mag (Zahid et al 2014, and references therein). For GRBs to trace star formation representatively, half of the sample would be expected above this value and half below, while we observe (82 ± 8)% of all hosts below E B−V = 0.35 mag at z < 1.…”

“…Because of these inherent uncertainties from sample selection and small number statistics, the question of how directly GRBs trace star formation, and how representatively they select star-forming galaxies remains a matter of debate (e.g., Michałowski et al 2012;Vergani et al 2015;Schulze et al 2015): a metal-dependence on the GRB selection is shown by e.g., Graham & Fruchter (2013) or Perley et al (2013b), while Hunt et al (2014) advocate no strong evidence for GRBs to provide a biased census of star formation.…”

Are long gamma-ray bursts biased tracers of star formation? Clues from the host galaxies of theSwift/BAT6 complete sample of bright LGRBs

“…Studies in the past have reached contradictory conclusions regarding the LGRB efficiency, largely because of the heterogeneous nature of investigated samples (e.g. Le Floc'h et al 2003Floc'h et al , 2006Fruchter et al 2006;Savaglio et al 2009;Levesque et al 2010a;Svensson et al 2010;Mannucci et al 2011;Graham & Fruchter 2013;Perley et al 2013;Hunt et al 2014). However, the large number of LGRBs detected by the Swift satellite (Gehrels et al 2004) accumulated in the past ten years and carefully chosen selection criteria have recently resulted in several unbiased LGRB samples, highly complete in redshift: the GROND , BAT6 , TOUGH , and SHOALS (Perley et al 2016a) samples.…”

EXPLORING DAMPED Lyα SYSTEM HOST GALAXIES USING GAMMA-RAY BURSTS

“…MAGPHYS models templates to the data and returns an SED with fitted parameters which include SFR, stellar mass (M * ), dust mass (M dust ), and A V . This particular package is well suited for > z 1 galaxies and takes into account bursty star formation which is appropriate for GRB host galaxies as suggested by Hunt et al (2014). MAGPHYS uses a continuous model of star formation with superimposed random bursts that happen at equal probability at all times up to the age of the galaxy.…”