SCUBA observations of the host galaxies of four dark gamma-ray bursts

(This article is based on the work published in Barnard et al., 2003, MNRAS 338, 1-6. Please see this paper for citations of work referred to below.)

Gamma-ray bursts (GRBs) are the focus of much current astronomy research and yet many questions remain unanswered about these hugely energetic phenomena. Whilst it is now clear that the GRB population is extragalactic, the actual cause and progenitors of these massive explosions are not yet certain. Most models currently involve the endpoints in the evolution of massive stars, and so the host galaxies of GRBs are expected to have high star-formation rates. These ideas are supported by studies of the relative rates of star formation and GRB explosions in the Universe as a whole, which suggest a strong correlation.

Radiation from the site of GRBs comes in three major phases: the initial explosion, during which the GRB may be the most luminous object in the Universe; followed by the afterglow as an ultrarelativistic shock wave passes through the surrounding ISM; followed eventually by emission from the host galaxy of the GRB progenitor. The recent increase in GRB research was sparked by the detection of the first afterglow (GRB 970508), essentially because the afterglow is relatively long-lasting and passes through a range of wavelengths, which allows far better positional accuracy than the glimpses of brief high-energy initial explosion radiation.

Afterglows from GRBs have now been observed at nearly all wavelengths, and the information garnered by following the evolution of a GRB's afterglow has led to the general acceptance of the synchrotron fireball model. Details such as polarisation and collimation of afterglows are now the focus of research.

However, there is a large fraction of GRBs for which, despite rapid and deep searches, optical afterglows were not found. This fraction is around 50 per cent and so these 'dark' GRBs may represent an important population. It is not clear however whether these GRBs are intrinsically different objects to those with optically bright afterglows, or whether their optical faintness depends on their surroundings. Several authors have suggested that the apparent optical faintness of these GRBs may be due to obscuring dust along the line of sight. Since the energy of the shock wave is expected to clear out dust in the GRB's vicinity, this explanation would require the GRBs to be located in generally dusty galaxies. This, in combination with the expected high star-formation rates of GRB host galaxies, makes them a target for SCUBA observations.

We observed a small sample of optically dark GRBs (with good positions from radio and/or X-ray afterglows) with SCUBA in photometry mode. Four host galaxies were observed. Of these, only one appears to represent a possible detection, and this has been confirmed by combining our result with that of another group (see Berger et al., astro-ph/0210645, and the March 2002 article). Overall, we find that the 'dark' GRB host galaxy population is no more likely to be dusty than the general galaxy population. This is illustrated in figure 1, where the results of our sample are compared with two galaxy evolution models (marked BSIK and BJSLKI) which represent the SCUBA galaxy population. In this figure, the fraction of hosts with 850µm flux density greater than the value S is plotted. The smooth curves correspond to the two models, whilst the solid stepped line shows the cumulative results from our sample (and the dotted lines represent 1 sigma errors).

This result indicates that to characterise the optically dark GRB population as residing in dusty host galaxies is incorrect; in fact a variety of explanations probably need to be applied to the whole population. This is further reinforced by the findings of other researchers that some GRB hosts are bright in the submillimetre even when the optical afterglow was observed. Hence the optical afterglow of a GRB is not a reliable guide to the submillimetre luminosity of the host galaxy. We are currently completing a wider survey of all GRBs, to further understand the links between the GRB host galaxy population and the SCUBA galaxy population.

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