Extragalactic archeology with the GHOSTS Survey I. - Age-resolved disk structure of nearby low-mass galaxies

TL;DR

This study investigates the age-dependent structural evolution of three nearby low-mass edge-on galaxies using resolved stellar populations, revealing no significant thick disk or flaring, and showing very low vertical heating rates compared to the Milky Way.

Contribution

It provides the first detailed age-resolved structural analysis of low-mass galaxies, challenging models that predict strong flaring and significant thick disks in such systems.

Findings

No separate thick disks detected in the galaxies.

Scale heights increase with stellar age, but each population fits a single disk.

Faint halos are present but contain less than 1% of the disk mass.

Abstract

We study the individual evolution histories of three nearby low-mass edge-on galaxies (IC 5052, NGC4244, and NGC5023). Using resolved stellar populations, we constructed star count density maps for populations of different ages and analyzed the change of structural parameters with stellar age within each galaxy. We do not detect a separate thick disk in any of the three galaxies, even though our observations cover a wider range in equivalent surface brightness than any integrated light study. While scale heights increase with age, each population can be well described by a single disk. Two of the galaxies contain a very weak additional component, which we identify as the faint halo. The mass of these faint halos is lower than 1% of the mass of the disk. The three galaxies show low vertical heating rates, which are much lower than the heating rate of the Milky Way. This indicates that heating agents, such as giant molecular clouds and spiral structure, are weak in low-mass galaxies. All populations in the three galaxies exhibit no or only little flaring. While this finding is consistent with previous integrated light studies, it poses strong constraints on galaxy simulations, where strong flaring is often found as a result of interactions or radial migration.