Star formation history of And XVIII: a dwarf spheroidal galaxy in isolation

TL;DR

This study investigates the star formation history of the isolated dwarf spheroidal galaxy Andromeda XVIII, revealing an ancient starburst 12-14 Gyr ago and no recent star formation, with implications for galaxy evolution in isolation.

Contribution

First detailed star formation history analysis of Andromeda XVIII using HST data, showing its isolated evolution and lack of recent star formation compared to other dSphs.

Findings

Ancient star formation burst 12-14 Gyr ago.

No recent star formation in last 1.5 Gyr.

Total stellar mass of 4.2x10^6 Msun.

Abstract

We present a photometric study of the Andromeda XVIII dwarf spheroidal galaxy associated with M31, and situated well outside of the virial radius of the M31 halo. The galaxy was resolved into stars with Hubble Space Telescope/Advanced Camera for Surveys revealing the old red giant branch and red clump. With the new observational data we determined the Andromeda XVIII distance to be D = 1.33+-0.08 Mpc using the tip of red giant branch method. Thus, the dwarf is situated at the distance of 579 kpc from M31. We model the star formation history of Andromeda XVIII from the stellar photometry and Padova theoretical stellar isochrones. An ancient burst of star formation occurred 12-14 Gyr ago. There is no sign of recent/ongoing star formation in the last 1.5 Gyr. The mass fractions of the ancient and intermediate age stars are 34 and 66 per cent, respectively, and the total stellar mass is 4.2x10^6 Msun. It is probable that the galaxy has not experienced an interaction with M31 in the past. We also discuss star formation processes of dSphs KKR 25, KKs 03, as well as dTr KK 258. Their star formation histories were uniformly measured by us from HST/ACS observations. All the galaxies are situated well beyond the Local Group and the two dSphs KKR 25 and KKs 03 are extremely isolated. Evidently, the evolution of these objects has proceeded without influence of neighbours.