The shell game: a panoramic view of Fornax

TL;DR

This panoramic study of the Fornax dwarf galaxy reveals a second red giant branch population and clarifies that a previously observed shell structure is actually a background galaxy overdensity, challenging merger hypotheses.

Contribution

The study provides the first wide-area panoramic view of Fornax, identifying a new stellar population and correcting the nature of a shell structure, thus offering new insights into its formation history.

Findings

Discovery of a second red giant branch population.

Identification of a shell structure as background galaxies.

No evidence supporting recent gas-rich merger.

Abstract

We present a panoramic study of the Fornax dwarf spheroidal galaxy, using data obtained as part of the VLT Survey Telescope (VST) ATLAS Survey. The data presented here -- a subset of the full survey -- uniformly cover a region of 25 square degrees centred on the galaxy, in $g$, $r$ and $i$-bands. This large area coverage reveals two key differences to previous studies of Fornax. First, data extending beyond the nominal tidal radius of the dwarf highlight the presence of a second distinct red giant branch population. This bluer red giant branch appears to be coeval with the horizontal branch population. Second, a shell structure located approximately 1.4 degrees from the centre of Fornax is shown to be a mis-identified background overdensity of galaxies. This last result casts further doubt on the hypothesis that Fornax underwent a gas-rich merger in its relatively recent past.