A new study of stellar substructures in the Fornax dwarf spheroidal galaxy

TL;DR

This study uses deep photometry to analyze stellar over-densities in the Fornax dwarf galaxy, revealing recent star formation and gas re-accretion processes through detailed age-metallicity relations and discovery of a very young, metal-rich stellar over-density.

Contribution

It provides new insights into the formation and evolution of stellar structures in Fornax by identifying a recently formed, metal-rich over-density and analyzing its properties in detail.

Findings

Inner shell-like structure has a peak age of ~1.5 Gyr and [Fe/H]=-0.6 dex.

Discovered a 100 Myr old, solar metallicity over-density 0.7 kpc from the center.

The young over-density may represent the last star formation activity in Fornax.

Abstract

Using deep V, B-V wide-field photometry, we have conducted a new study of stellar over-densities in the Fornax dwarf spheroidal galaxy by determining detailed Star Formation Histories from Colour-Magnitude Diagram analysis. We have concentrated on the relatively young stellar component (<4 Gyr old), and compared this to the underlying Fornax field population. We have studied in more detail the previously known inner shell-like structure and shown that it has a well-defined age-metallicity relation with a peak at ~1.5 Gyr, [Fe/H]=-0.6 dex. Comparison to the Fornax centre shows that the over-dense feature is consistent with the age-metallicity relation of young field stars, and likely formed from Fornax gas. This is consistent with a scenario in which the over-density was formed by the re-accretion of previously expelled gas. We have also discovered a new stellar over-density, located 0.3 degrees (0.7 kpc) from the centre, which is only 100 Myr old, with solar metallicity. This feature constitutes some of the youngest, most metal-rich stars stars observed in Fornax to date. It is unclear how the young over-density was formed, although the age and metallicity of stars suggest this feature may represent the last star formation activity of the Fornax dSph.