Abstract
Molecular species, most frequently H_2, are present in a small, but growing, number of gamma-ray burst (GRB) afterglow spectra at redshifts z~2-3, detected through their rest-frame UV absorption lines. In rare cases, lines of vibrationally excited states of H_2 can be detected in the same spectra. The connection between afterglow line-of-sight absorption properties of molecular (and atomic) gas, and the observed behaviour in emission of similar sources at low redshift, is an important test of the suitability of GRB afterglows as general probes of conditions in star formation regions at high redshift. Recently, emission lines of carbon monoxide have been detected in a small sample of GRB host galaxies, at sub-mm wavelengths, but no searches for H_2 in emission have been reported yet. In this paper we perform an exploratory search for rest-frame K band rotation-vibrational transitions of H_2 in emission, observable only in the lowest redshift GRB hosts (z<0.22). Searching the data of four host galaxies, we detect a single significant rotation-vibrational H_2 line candidate, in the host of GRB 031203. Re-analysis of Spitzer mid-infrared spectra of the same GRB host gives a single low significance rotational line candidate. The (limits on) line flux ratios are consistent with those of blue compact dwarf galaxies in the literature. New instrumentation, in particular on the JWST and the ELT, can facilitate a major increase in our understanding of the H_2 properties of nearby GRB hosts, and the relation to H_2 absorption in GRBs at higher redshift.
Highlights
Gamma-ray burst (GRB) afterglow spectroscopy has shown great promise as a probe of gas and dust properties within, and near, star forming regions in distant galaxies
There is evidence that most of the gamma-ray burst (GRB) sightlines that pass by significant column densities of molecules will contain large dust column densities and that such sightlines are underrepresented in the subset of GRBs with well-detected optical afterglows (Prochaska et al 2009; Kruhler et al 2013)
A single rovibrational H2 emission line candidate is detected at the position of the 1–0 S(3) transition in the host of GRB 031203
Summary
Gamma-ray burst (GRB) afterglow spectroscopy has shown great promise as a probe of gas and dust properties within, and near, star forming regions in distant galaxies (see e.g. Schady 2015 for a review). Tumlinson et al 2007), the low detection rate, the excitation state of the detected H2, and occasionally the association of the H2 absorber with excited atomic metal fine structure lines, have shown that in several cases the H2 absorbers are likely located far from the star forming region in which the GRB progenitor resided (e.g. D’Elia et al 2014). The use of CO as a tracer molecule for H2 is a well established technique, though evidence that the CO to H2 conversion factor in GRB host sightlines is comparable to Galactic translucent clouds, is limited to a single case (Prochaska et al 2009): the only afterglow spectrum so far with a detection of CO absorption lines. In this paper we perform a first exploratory search for ro-vibrational H2 lines in a sample of four, low redshift, long GRB host galaxies, to inform more sensitive searches with future observatories
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