The redshifted selected sample of long gamma-ray burst host galaxies: the complete metallicity measurements at $z \leq 0.41$

Yuu Niino, Kentaro Aoki, Tetsuya Hashimoto, Takashi Hattori, Shoto Ishikawa, Nobunari Kashikawa, George Kosugi, Masafusa Onoue, Jun Toshikawa, Kiyoto Yabe

Published 2016-06-07Version 1

We present the complete list of host galaxy metallicities for all long GRBs whose redshifts were determined to be $\leq 0.41$ before the end of March 2014, including newly obtained spectroscopic datasets of the host galaxies of GRB 060614, 090417B, and 130427A. We compare the metallicity distribution of the redshift selected complete sample to the model predictions, and constrain the relation between metallicity and GRB occurrence. We take account of spatial variation of metallicities among star forming regions within a galaxy. We found that the models, in which only low-metallicity stars produce GRBs with a sharp cutoff of GRB production efficiency around 12+log(O/H) $\sim$ 8.2, can well reproduce the observed distribution, while the models with moderate (or no) metallicity dependence are not consitistent with the observations. This is the first fair estimate of the metallicity distribution of GRB host galaxies based on the redshift selected complete sample in the {\it Swift} era. We also discuss possible sampling biases we may suffer by collecting long GRBs whose redshifts are known, presenting the photometric observations of the host galaxy of GRB 111225A at $z = 0.297$ whose redshift has been undetermined until $\sim$ 2.3 years after the burst.