Host Galaxy Properties of the Subluminous GRB 120422A/SN 2012bz

TL;DR

This paper studies the host galaxy of the subluminous GRB 120422A/SN 2012bz, analyzing its metallicity, star formation, and environment, revealing similarities with other LGRB hosts despite its unusual offset from the galaxy center.

Contribution

It provides detailed spectroscopic measurements of the host galaxy at multiple locations, highlighting environmental properties of a subluminous GRB/SN and comparing them to other LGRB hosts.

Findings

Metallicity at explosion site: log(O/H)+12 = 8.2 +/- 0.1

Star formation rate per area at explosion site: 0.01 M/yr/kpc^2

No systematic metallicity difference between subluminous and cosmological LGRB hosts

Abstract

GRB 120422A is a nearby (z = 0.283) long-duration GRB (LGRB) detected by Swift with E(gamma,iso) ~ 4.5\times1049 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 GRB/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 6.5m 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 per unit area of 0.08 Mo/yr/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 star formation rate per unit area of 0.01 M/yr/kpc^2. We also compare the host galaxy of this event to the hosts other LGRBs, including samples of subluminous LGRBs and cosmological LGRBs, and find no systematic metallicity difference between the environments of these different subtypes.