Are Gamma-Ray Bursts good Star Formation Indicators?

1. Are Gamma-Ray Bursts good Star Formation Indicators? Nial Tanvir University of Hertfordshire Abstract Since long-duration gamma-ray bursts (GRBs) are associated with the deaths of massive stars, the are also indicators of star formation activity. Advantages they have over conventional methods are that they are extremely bright, and hence visible to high redshifts, and detectable in gamma-rays through very high columns of gas and dust. If they are unbiased tracers of star formation activity, then the GRB host galaxies should contain all populations of star forming galaxies, in proportion to their contribution to the global star formation rate. In particular, if a high proportion of star formation occurs in obscured mode, then a similar proportion of GRBs and their hosts should also show signs of dusty environments. We describe progress towards testing this prediction. Effect of metallicity? There is already some evidence that GRBs show some preference for low metallicity environments. For example, Ramirez-Ruiz et al. (2002 ApJ 565 L9) found that GRBs located in the outer parts of their host galaxies, presumably with lower metallicity, are somewhat brighter then those closer to the centres. Fynbo et al. (2003 A&A 406 L63) pointed out that Ly-α is seen strongly in emission (where it falls in the optical window) in five out of six GRB hosts. This is a substantially greater than the proportion of Ly-break galaxies showing such strong emission lines, and is again suggestive of a low metallicity, low dust environment. Introduction This conference attests to the importance placed on studies of star formation in our understanding of the evolution of baryonic matter in the universe. Although significant progress has been made in recent years, there remains debate about the fraction of star-formation occurring in obscured mode, which is hard to study directly, and uncertainty about the star formation occurring at very high redshifts (z>5). In the latter case, because galaxies (and quasars) are faint and few at high-z, traditional methods become ever more difficult to apply. Long-duration GRBs are known to accompany the deaths of massive stars (eg. Hjorth et al. 2003 Nature 423 847), and since such stars have very short lifetimes, are also indicators of recent star formation. We may therefore hope that simply counting the GRB rate as a function of redshift will tell us the star formation history of the universe. As star formation indicators GRBs have a number of potentially important advantages over other approaches: Explaining the discrepancy The question therefore arises as to whether GRBs avoid ULIRG-like hosts, or whether the amount of star formation in such galaxies has been substantially overestimated. In fact, the discrepancy may be worse since the 3 hosts which are confidently (>3σ) detected at 850μm are not representative of typical submm selected galaxies: they are faint and blue, with little evidence of optical extinction, and lie at z<1.5. It is possible that current samples are deficient in GRBs from very dusty hosts, simply because their optical light is attenuated so that no afterglow is seen. However, Barnard et al. (2003 MNRAS 338 1) showed that a sample of 4 hosts of “dark” bursts with good X-ray positions did not show strongly enhanced submm emission either. It is also possible that a bias towards finding lower redshift GRB afterglows means that we are not yet seeing the peak of the global star formation. A recent relevant development has been the discovery of GRB 030115, which shows evidence of strong reddening of its afterglow light, and also resides in a ERO-like host galaxy (fig 2). GRB 030115 may therefore represent an intermediate case between the generally low extinction afterglows which dominate the observed samples, and rarer highly extinguished bursts. Testing the method Current samples of GRBs are too small and incomplete, especially in terms of redshifts, to make firm statements about global star formation. However, if GRBs are unbiased tracers of star formation then samples of their hosts should contain populations of galaxies in proportion to the contribution of each population to the overall star formation rate (eg. Trentham et al. 2002 MNRAS). Generally the GRB hosts are small, blue (in optical/nIR colours) galaxies in the redshift range 0.5<z<2.5 (eg. Le Floc’h et al. 2003 A&A 400 499). Most would be too faint to be included in samples of Ly-break galaxies, or, for that matter, in samples of submm selected galaxies as we show below. To attempt to quantify the relationship between GRB rate and star-formation rate more precisely we have observed a sample of GRB hosts using SCUBA on the JCMT. Blind submm surveys and the integrated IR background suggest that a majority of the star formation in the early universe took place in dust enshrouded systems. In such galaxies the UV flux from OB stars is largely reprocessed by the dust to far IR wavelengths. Thus submm measurements provide an independent estimate of star formation rate in the host galaxies. Combining our observations of GRB hosts with those of Berger et al. (2003 ApJ 588 99) we construct a sample of 21 galaxies. Although comparatively small, this is sufficient to test the hypothesis that GRB rate and star formation rate are directly proportional. The results are illustrated in figure 1 (from Tanvir et al. 2004 MNRAS 352 1073). • They should be detectable in the nIR to very high redshift, z~20, if they existed then (eg. Lamb and Reichart 2000 ApJ 536 1). • The gamma-ray flash from GRBs should itself be detectable through significant columns of gas and dust, and also destroy dust for tens of parsecs (eg. Waxman and Draine 2000 ApJ 537 796). • There is no need to detect individual host galaxies, in any passband, since redshifts can be obtained from the afterglows. • Conversely, potential drawbacks with GRBs are: • There could be a metallicity or other environmental dependence of GRB rate or luminosity which is hard to account for. Indeed there is some evidence of such a metallicity dependence (see below). • Some GRBs may be so heavily enshrouded in dust that we only detect their gamma-rays, making redshift estimates more uncertain. • GRBs are short lived, so rapid, pre-planned observations are required if we are to obtain redshifts for statistical samples. • GRBs are rare, building up sufficient numbers is a slow process, consuming considerable telescope time. • GRBs can only be used as global indicators and aren’t useful for studying the star formation rate in individual galaxies. Fig 2. GRB 030115 shows the most reddened afterglow found to-date, and resides in an ERO-like host. This does indicated that at least some GRBs are found in highly dusty environments, albeit that a majority are not (Levan et al. in prep.) Conclusions and Future Work GRBs are potentially powerful tracers of the global star formation history of the universe back to very early times. However, with evidence that GRBs may favour low metallicity environments, and disfavour intensively star-forming ULIRGs, their use in practice may be hard. A number of new developments promise to speed up these investigations in the near future: • The SWIFT satellite will be launched late 2004 and will provide many more rapid GRB alerts. With good localisations, and rapid optical monitoring (on-board and ground-based), the proportion of bursts which remain optically dark is likely to fall considerably. Importantly the Swift X-ray telescope (XRT) is expected to given positions good to a few arcsecs, so GRBs which are optically dark but visible in X-rays can be followed up with SCUBA to search for dusty hosts. • The UK now has access to a broader range of powerful telescopes for GRB followup, including the 2m robotic Liverpool Telescope and the Faulkes Telescopes. These are crucial for obtaining large, and statistically well defined samples of redshifts (time already awarded to teams led by O’Brien et al., Mundell et al. and Tanvir et al.). Fig 1. The histogram shows the distribution of submm luminosities of GRB hosts. Of 21 in the sample, only 3 were significantly detected (middle bin). The points (with error bars from counting statistics) show the predicted distribution based on the models of Blain et al. (1999 MNRAS 302 632). Of these disadvantages, only the first may turn out to be a show stopper. To investigate possible external influences on GRB rate/luminosity, we can either look directly at the properties of GRBs and their galactic environments, or assume that no such influences exist, and compare the star formation inferred from GRBs with other measures. Both these approaches are discussed below. The figure shows that to-date the sample of GRB hosts is deficient in very bright submm galaxies.