Host Galaxies

Studies of GRB host galaxies have shown that most are faint blue galaxies [41,52], with R ~ 22-30mag, and R-K ~2.5 mag. GRBs occasionally (but not always) occur within UV-bright parts of their hosts [12], consistent with their association with star forming (SF) regions.

The broad-band spectral energy distribution (SED) of practically all host galaxies are best fit with starburst galaxy templates [16,33,34,72]. Also, sub-mm and radio observations have shown that at least four GRB hosts (corresponding to 20% of the observed hosts), are intensively star forming galaxies [3,9,23]. A recent survey [90] has led to the conclusion that the optical/infrared properties of the submillimeter-brightest GRB hosts are not typical of the galaxy population selected in sub-mm surveys. Also, there is evidence that Lya emission is common for GRB host galaxies, while it is not for other starbursts at similar redshift [27].

The study of the SF rate in normal galaxies is a controversial topic. Though several different tracers for SF have been proposed and used, there still remain questions and problems. Light dimming by dust biases SF rate measurements, both locally [8, 89] and at high redshift [55, 88], and is the cause of uncertain cross-calibrations between different indicators. GRBs, on the other hand, provide some important advantages (though measurement of the SF in a GRB host galaxy then suffers from the same biases), namely that (i) they can be seen to high red-shifts, (ii) finding the hosts by y-ray detection is unaffected by gas/dust absorption, (iii) they can be detected in hosts which are themselves too faint to appear in any flux-limited galaxy sample.

Morphologically, GRB hosts exhibit a surprisingly broad diversity of galaxy types. Based on Hubble Space Telescope imaging, two thirds of host galaxies are found to be situated in a region of the concentration-asymmetry diagram occupied by spirals or peculiar/merging galaxies while the other are more akin to elliptical galaxies [17]. More interestingly, GRB hosts at z > 1 are different from the general field population at that redshift in terms of light concentration, yet have sizes similar to the general z > 1 population. This is the opposite of the effect at z < 1, where GRB hosts are smaller than average. This implies that GRB hosts trace the starburst population at high redshift, as similarly concentrated galaxies at z > 2 are undergoing a disproportionate amount of SF for their luminosities. Furthermore, GRBs are not only an effective tracer of SF but are perhaps ideal tracers of typical galaxies undergoing SF at any epoch, making them perhaps our best hope of locating the earliest galaxies at z > 7 [17].

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