Abstract
Abstract We present a study of 21 dark gamma-ray burst (GRB) host galaxies, predominantly using X-ray afterglows obtained with the Chandra X-Ray Observatory (CXO) to precisely locate the burst in deep Hubble Space Telescope (HST) imaging of the burst region. The host galaxies are well-detected in F160W in all but one case and in F606W imaging in 60 per cent of cases. We measure magnitudes and perform a morphological analysis of each galaxy. The asymmetry, concentration, and ellipticity of the dark burst hosts are compared against the host galaxies of optically bright GRBs. In agreement with other studies, we find that dark GRB hosts are redder and more luminous than the bulk of the GRB host population. The distribution of projected spatial offsets for dark GRBs from their host galaxy centroids is comparable to that of optically bright bursts. The dark GRB hosts are physically larger, more massive and redder, but are morphologically similar to the hosts of bright GRBs in terms of concentration and asymmetry. Our analysis constrains the fraction of high redshift (z > 5) GRBs in the sample to 14 per cent, implying an upper limit for the whole long-GRB population of ≤4.4 per cent. If dust is the primary cause of afterglow darkening amongst dark GRBs, the measured extinction may require a clumpy dust component in order to explain the observed offset and ellipticity distributions.
Highlights
Long-duration Gamma-ray bursts (GRBs)1 are the most luminous events in the Universe (e.g. Racusin et al 2008), arising from the violent explosions of massive stars (e.g. Hjorth et al 2003)
We present a study of 21 dark gamma-ray burst (GRB) host galaxies, predominantly using X-ray afterglows obtained with the Chandra X-Ray Observatory (CXO) to precisely locate the burst in deep Hubble Space Telescope (HST) imaging of the burst region
We present a study of 21 dark GRBs and their host galaxies, observed with the Chandra X-ray Observatory (CXO) and Hubble Space Telescope (HST)
Summary
Long-duration Gamma-ray bursts (GRBs) are the most luminous events in the Universe (e.g. Racusin et al 2008), arising from the violent explosions of massive stars (e.g. Hjorth et al 2003). The first example where this was found to be the case was GRB 970828 (Groot et al 1998), which showed no afterglow down to an r-band limit of AB mag 23 within 12 h post-burst. Such events have subsequently become known as dark bursts (Fynbo et al 2001). A commonly used formal definition for dark GRBs is an X-ray to optical spectral slope βOX of less than 0.5 (Jakobsson et al 2004), effectively the limit allowed by standard synchrotron afterglow theory. It should be noted that GRB emission can deviate from this simple synchrotron model, with plateaus,
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