HI in very metal-poor galaxies: the SBS 0335-052 system

TL;DR

This study uses GMRT HI 21cm observations to reveal that the extremely metal-poor dwarf galaxy pair SBS 0335-052 is a major gas-rich merger with complex HI morphology and star formation driven by superbubble expansion.

Contribution

First detailed HI imaging of the metal-poor dwarf pair SBS 0335-052, showing merger dynamics, tidal features, and star formation processes linked to superbubble activity.

Findings

SBS 0335-052 is a gas-rich major merger with tidal tails.

Star formation occurs mainly in superstar clusters with an age gradient.

Superbubble expansion influences star formation and HI distribution.

Abstract

We present Giant Metrewave Radio Telescope (GMRT), HI 21cm observations of SBS 0335-052E and SBS 0335-052W, a close pair of dwarf galaxies, which are further unusual in being the most metal-poor star-forming galaxies known. We present images at several angular resolutions, ranging from ~40 to 4 arcsec. These images show that SBS 0335-052 is a strongly interacting system, with a faint diffuse HI bridge seen at low resolution, and elongated tails seen at the higher resolutions. The overall morphology suggests that the pair represents a major merger of extremely gas-rich galaxies. The low-resolution velocity field is dominated by the velocity difference between the two galaxies and the velocity gradient along the tidal features. However, for SBS 0335-052W at least, at high angular resolution, one sees a central velocity field that could be associated with the spin of the original undisturbed disc. The highest angular resolution HI images show that the ionized superbubble, identified by Thuan, Izotov & Lipovetsky (1997), in the Hubble Space Telescope (HST) images of SBS 0335-052E, is extended along one of the diffuse tidal features, and that there is a high-density HI clump at the other end of the superbubble. The star formation in SBS 0335-052E occurs mainly in a group of superstar clusters (SSCs) with a clear age gradient; the age decreases as one approaches the dense HI clump. We suggest that this propagating star formation is driven by the superbubble expanding into a medium with a tidally-produced density gradient. The high pressures associated with the compressed material would also naturally explain why current star formation is mainly concentrated in superstar clusters.