The ACS LCID Project XI. On the early time resolution of LG dwarf galaxy SFHs: Comparing the effects of reionization in models with observations

TL;DR

This study examines how observational limitations affect the perceived early star formation histories of Local Group dwarf galaxies, highlighting that none are true fossils of the pre-reionization era when accounting for these effects.

Contribution

It quantifies the impact of time resolution on SFH analysis and demonstrates that observed galaxies are unlikely to be true pre-reionization fossils.

Findings

Time resolution effects can shift SFH peaks by up to 1.25 Gyr.

None of the studied galaxies are confirmed pre-reionization fossils.

Outer regions of some galaxies may be consistent with reionization quenching.

Abstract

The analysis of the early star formation history (SFH) of nearby galaxies, obtained from their resolved stellar populations is relevant as a test for cosmological models. However, the early time resolution of observationally derived SFHs is limited by several factors. Thus, direct comparison of observationally derived SFHs with those derived from theoretical models of galaxy formation is potentially biased. Here we investigate and quantify this effect. For this purpose, we analyze the duration of the early star formation activity in a sample of four Local Group dwarf galaxies and test whether they are consistent with being true fossils of the pre-reionization era; i.e., if the quenching of their star formation occurred before cosmic reionization by UV photons was completed. Two classical dSph (Cetus and Tucana) and two dTrans (LGS-3 and Phoenix) isolated galaxies with total stellar masses between $1.3\times 10^6$ to $7.2\times 10^6$ M$_\odot$ have been studied. Accounting for time resolution effects, the SFHs peak as much as 1.25 Gyr earlier than the optimal solutions. Thus, this effect is important for a proper comparison of model and observed SFHs. It is also shown that none of the analyzed galaxies can be considered a true-fossil of the pre-reionization era, although it is possible that the {\it outer regions} of Cetus and Tucana are consistent with quenching by reionization.