Super Star Cluster NGC 1705-1: A Local Analog to the Birth Site of Long-Duration γ-Ray Bursts


Recent observations suggest that global properties of the host galaxies for long-duration γ-ray bursts (GRBs) are particularly well suited for creating the massive star progenitors of these GRBs. Motivated by the hypothesis that massive young star clusters located in metal-poor, low- metallicity galaxies are a natural birth site for GRB progenitors, we present a comparison study of the ISM properties along the sight line toward the super star cluster NGC 1705-1 and those in distant GRB hosts. Using the same set of metal transitions in the UV and applying known ISM structures in NGC 1705, we find that NGC 1705-1 resembles distant GRB host galaxies in its high neutral gas column, low molecular gas fraction, low metallicity, α-element enhancement, and low dust depletion. The lack of molecular gas is due to the enhanced UV radiation field in the starburst environment, consistent with the expectations for GRB progenitors. In addition, the known presence of dense neutral gas clouds at r<åisebox-0.5ex 500 pc from NGC 1705-1 provides a natural reservoir of C$^+$, Si$^+$, and Fe$^+$ ions that may be subsequently excited by the afterglow UV radiation field to produce excited lines commonly seen in GRB host ISM. We further argue that the apparent offset in the velocity profiles of low- and high-ionization transitions from absorption-line observations alone already offers important clues for related starburst episodes in GRB host galaxies. Our study shows that a statistical comparison between the ISM around star clusters and high-redshift GRB progenitors is important for identifying the key physical parameters that facilitate the formation of GRBs.

Astrophysical Journal