HOST GALAXY PROPERTIES OF THE SUBLUMINOUS GRB 120422A/SN 2012bz*

摘要

GRB 120422A is a nearby (z = 0.283) long-duration gamma-ray burst (LGRB) detected by Swift with E γ, iso ~ 4.5 × 1049 erg. It is also associated with the spectroscopically confirmed broad-lined Type Ic SN 2012bz. These properties establish GRB 120422A/SN 2012bz as the sixth and newest member of the class of subluminous GRBs supernovae (SNe). Observations also show that GRB 120422A/SN 2012bz occurred at an unusually large offset (~8?kpc) from the host galaxy nucleus, setting it apart from other nearby LGRBs and leading to speculation that the host environment may have undergone prior interaction activity. Here, we present spectroscopic observations using the 6.5?m Magellan telescope at Las Campanas. We extract spectra at three specific locations within the GRB/SN host galaxy, including the host nucleus, the explosion site, and the "bridge" of diffuse emission connecting these two regions. We measure a metallicity of log(O/H) + 12 = 8.3 ± 0.1 and a star formation rate (SFR) per unit area of 0.08 M ☉?yr–1 kpc–2 at the host nucleus. At the GRB/SN explosion site we measure a comparable metallicity of log(O/H) + 12 = 8.2 ± 0.1 but find a much lower SFR per unit area of 0.01 M ☉?yr–1 kpc–2. We also compare the host galaxy of this event to the hosts of other LGRBs, including samples of subluminous LGRBs and cosmological LGRBs, and find no systematic metallicity difference between the environments of these different subtypes.