A spectroscopic study of type Ibc supernova host galaxies from untargeted surveys

We present the first spectroscopic study of the host environments of TypeIbc supernovae (SNeIbc) discovered exclusively by untargeted SN searches. Past studies of SNIbc host environments have been biased toward high-mass, high-metallicity galaxies by focusing on SNe discovered in galaxy-targeted SN searches. Our new observations more than double the total number of spectroscopic stellar population age and metallicity measurements published for untargeted SNIbc host environments. For the 12 SNeIb and 21 SNeIc in our metallicity sample, we find median metallicities of 0.62 Z_⊙ and 0.83 Z _⊙, respectively, but determine that the discrepancy in the full distribution of metallicities is not statistically significant. This median difference would correspond to only a small difference in the mass loss via metal-line-driven winds (≲ 30%), suggesting this does not play the dominant role in distinguishing SNeIb and Ic progenitors. However, the median metallicity of the seven broad-lined SNeIc (SNeIc-BL) in our sample is significantly lower, 0.45 Z _⊙. The age of the young stellar population of SNIc-BL host environments also seems to be lower than for SNeIb and Ic, but our age sample is small. Combining all SNIbc host environment spectroscopy from the literature to date does not reveal a significant difference in SNIb and Ic metallicities, but reinforces the significance of the lower metallicities for SNeIc-BL. This combined sample demonstrates that galaxy-targeted SN searches introduce a significant bias for studies seeking to infer the metallicity distribution of SN progenitors, and we identify and discuss other systematic effects that play smaller roles. We discuss the path forward for making progress on SNIbc progenitor studies in the Large Synoptic Survey Telescope era. This paper includes data gathered with the 6.5m Magellan Telescopes located at Las Campanas Observatory, Chile.