The ISLANDS project I: Andromeda XVI, An Extremely Low Mass Galaxy not Quenched by Reionization

TL;DR

This study presents a detailed evolutionary history of the low-mass galaxy Andromeda XVI, revealing prolonged star formation until about 6 Gyr ago, likely influenced by environmental effects rather than reionization, with new distance measurements from RR Lyrae stars.

Contribution

It provides the first comprehensive star formation history and variable star analysis of Andromeda XVI, challenging previous assumptions about early quenching mechanisms.

Findings

Star formation lasted until ~6 Gyr ago.

Star formation activity was stronger in central regions 6-8 Gyr ago.

Distance estimate from RR Lyrae stars is (m-M)0= 23.72 mag.

Abstract

Based on data aquired in 13 orbits of HST time, we present a detailed evolutionary history of the M31 dSph satellite Andromeda XVI, including its life-time star formation history, the spatial distribution of its stellar populations, and the properties of its variable stars. And XVI is characterized by prolonged star formation activity from the oldest epochs until star formation was quenched ~6 Gyr ago, and, notably, only half of the mass in stars of And XVI was in place 10 Gyr ago. And XVI appears to be a low mass galaxy for which the early quenching by either reionization or starburst feedback seems highly unlikely, and thus, is most likely due to an environmental effect (e.g., an interaction), possibly connected to a late infall in the densest regions of the Local Group. Studying the star formation history as a function of galactocentric radius, we detect a mild gradient in the star formation history: the star formation activity between 6 and 8 Gyr ago is significantly stronger in the central regions than in the external regions, although the quenching age appears to be the same, within 1 Gyr. We also report the discovery of 9 RR Lyrae stars, 8 of which belong to And XVI. The RR Lyrae stars allow a new estimate of the distance, (m-M)0= 23.72+/-0.09 mag, which is marginally larger than previous estimates based on the tip of the red giant branch.