Ultra-deep imaging of nearby dwarf irregular galaxies: stellar haloes and disk structure

TL;DR

This study uses ultra-deep imaging to analyze the stellar structure of nearby dwarf irregular galaxies, revealing insights into their disk and potential stellar halo features, and suggesting internal processes shape their extended stellar components.

Contribution

First detailed ultra-deep imaging analysis of dwarf irregular galaxies' stellar structures, highlighting the role of internal evolution over external accretion.

Findings

Extended stellar components are likely disks formed by internal processes.

No clear evidence of stellar halos beyond exponential profiles.

Outer disk colors indicate ages of 1-6 Gyr.

Abstract

We have examined the stellar structure of 10 nearby, low stellar mass (10^6 to 6 x 10^7 Msolar) dwarf irregular galaxies by fitting ellipses as a function of surface brightness on ultra-deep V images. These are compared to far ultraviolet images as tracers of the star formation. We find that the often asymmetrical distribution of large patches of star formation activity in dwarfs, even out to low disk surface brightness levels, skews the broad-band optical isophotes in these galaxies. We also looked for evidence of the presence of a stellar halo. Possible hints of such are found in several galaxies from irregularities in the ellipses, but a stack of seven of the galaxies shows a pure exponential out to a V surface brightness of 32.3 mag/arcsec^2 where the stellar surface density is 0.0013 +/- 0.0011 Msolar/pc^2. The extended stellar component, most likely a disk structure, is probably due to internal evolutionary processes rather than external accretion. The UBVI colors of the annuli are consistent with ages of 1-6 Gyr for the far outer stellar disk.