A quiescent galaxy at the position of the long GRB 050219A

AbstraktAbstract

Context. Long-duration gamma-ray bursts (LGRBs) are produced by the collapse of very massive stars. Because of the short life time of their progenitors, LGRBs pinpoint star-forming galaxies. Recent studies demonstrate that LGRBs populate all types of star-forming galaxies from sub-luminous, blue compact dwarfs to luminous infrared galaxies. Aims: We present here a multi-band search for the host galaxy of the long dark GRB 050219A within the enhanced Swift/XRT error circle. We aim to characterise the properties of its host galaxy and compare them with those of other LGRB host galaxies. Methods: We used spectroscopic observations acquired with VLT/X-Shooter to determine the redshift and star-formation rate of the most probable host galaxy identified on the basis of a chance probability criterion. We compared the results with the optical and infrared spectral energy distribution obtained with Swift/UVOT, the seven-channel imager GROND at the 2.2-m telescope on La Silla and the Herschel Space Observatory, supplemented by archival observations obtained with FORS2 at the ESO/VLT, the Spitzer Space Telescope, and the GALEX survey. Results: The most probable host galaxy of the genuine long-duration GRB 050219A is a 3 Gyr-old early-type galaxy at z = 0.211. It is characterised by a ratio of star-formation rate to stellar mass (specific star-formation rate) of ~ 6 × 10-12 yr-1 that is unprecedentedly low when compared to all known LGRB host galaxies. Its properties resemble those of post-starburst galaxies. Conclusions: GRB 050219A might be the first known long burst to explode in a quiescent early-type galaxy. This would be further evidence that GRBs can explode in all kinds of galaxies, with the only requirement being an episode of high-mass star formation.

Context. Long-duration gamma-ray bursts (LGRBs) are produced by the collapse of very massive stars. Because of the short life time of their progenitors, LGRBs pinpoint star-forming galaxies. Recent studies demonstrate that LGRBs populate all types of star-forming galaxies from sub-luminous, blue compact dwarfs to luminous infrared galaxies. Aims: We present here a multi-band search for the host galaxy of the long dark GRB 050219A within the enhanced Swift/XRT error circle. We aim to characterise the properties of its host galaxy and compare them with those of other LGRB host galaxies. Methods: We used spectroscopic observations acquired with VLT/X-Shooter to determine the redshift and star-formation rate of the most probable host galaxy identified on the basis of a chance probability criterion. We compared the results with the optical and infrared spectral energy distribution obtained with Swift/UVOT, the seven-channel imager GROND at the 2.2-m telescope on La Silla and the Herschel Space Observatory, supplemented by archival observations obtained with FORS2 at the ESO/VLT, the Spitzer Space Telescope, and the GALEX survey. Results: The most probable host galaxy of the genuine long-duration GRB 050219A is a 3 Gyr-old early-type galaxy at z = 0.211. It is characterised by a ratio of star-formation rate to stellar mass (specific star-formation rate) of ~ 6 × 10-12 yr-1 that is unprecedentedly low when compared to all known LGRB host galaxies. Its properties resemble those of post-starburst galaxies. Conclusions: GRB 050219A might be the first known long burst to explode in a quiescent early-type galaxy. This would be further evidence that GRBs can explode in all kinds of galaxies, with the only requirement being an episode of high-mass star formation.