Super Star Cluster NGC1705-1: A Local Analogue to the Birthsite of Long-duration Gamma-ray Bursts

TL;DR

This study compares the interstellar medium properties of the super star cluster NGC1705-1 with distant GRB host galaxies, revealing similarities in gas composition, metallicity, and radiation environment, supporting the idea that such clusters are GRB birthsites.

Contribution

It provides the first detailed comparison of local star cluster ISM properties with those of distant GRB hosts, highlighting key physical parameters involved in GRB progenitor formation.

Findings

NGC1705-1 resembles GRB hosts in gas column density and metallicity

Enhanced UV radiation explains low molecular gas in starburst environments

Dense neutral gas clouds near NGC1705-1 may produce GRB afterglow features

Abstract

Recent observations suggest that global properties of the host galaxies for long-duration gamma-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 birthsite for GRB progenitors, we present a comparison study of the ISM properties along the sightline toward the super star cluster, NGC1705-1, and those in distant GRB hosts. Using the same set of metal transitions in the UV and applying known ISM structures in NGC1705, we find that NGC1705-1 resembles distant GRB host galaxies in its high neutral gas column, low molecular gas fraction, low metallicity, alpha-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<500 pc from NGC1705-1 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.