Constraining the molecular gas in the environs of a z ~ 8 gamma-ray burst host galaxy

GRB 090423 is the most distant spectroscopically confirmed source observed in the Universe. Using observations at 37.5 GHz, we place constraints on molecular gas emission in the CO (3–2) line from its host galaxy and immediate environs. The source was not detected either in line emission or in the rest-frame 850-μm continuum, yielding an upper limit of S8mm= 9.3 μJy and M(H2) <4.3 × 109 M⊙ (3σ), applying standard conversions. This implies that the host galaxy of GRB 090423 did not possess a large reservoir of warm molecular gas but was rather modest either in star formation rate or in mass. It suggests that this was not an extreme starburst and hence that gamma ray bursts at high redshift trace relatively modest star formation rates, in keeping with the behaviour seen at lower redshifts. We do, however, identify a millimetre emission-line source in the field of GRB 090423. Plausible interpretations include a CO (1–0) emitting galaxy at z= 2.1, CO (2–1) at z= 5.2 and CO (3–2) at z= 8.3. Efforts to identify a counterpart for the molecular line emitter and to further characterize this source are continuing.