A Deep Photometric Look at Two of Andromeda's Dwarf Spheroidals: X and XVII

TL;DR

This study uses deep photometry to accurately determine the distances and properties of Andromeda X and XVII dwarf galaxies, revealing their true locations, metallicities, and stellar populations, and highlighting the importance of horizontal branch observations.

Contribution

It provides revised distances and detailed stellar properties of Andromeda X and XVII, emphasizing the significance of HB measurements over RGB tip methods for sparse systems.

Findings

And X is closer at 621 kpc, not previous estimates.

And XVII is one of the closest satellites at 734 kpc.

Both galaxies are metal-poor with intrinsic metallicity spreads.

Abstract

We use deep wide-field photometry from the Large Binocular Camera to study the stellar and structural properties of the recently discovered Andromeda X and Andromeda XVII (And X and And XVII) dwarf galaxies. Using the mean apparent magnitude of the horizontal branch (HB), we derive distances of 621 +- 20 kpc to And X and 734+- 23 kpc to And XVII, closer by >60 kpc than the previous estimates which were based on red giant branch (RGB) observations. Thus our results warrant against the use of the RGB tip method for determining distances to systems with sparsely populated RGBs, and show how crucial HB observations are in obtaining accurate distances in systems such as these. We find that And X is a relatively faint (MV = -7.36), highly elongated (e = 0.48) system at a distance of 174 +- 62 kpc from Andromeda. And XVII is brighter (MV = -8.61) with an M31-centric distance of 73 kpc which makes it one of the closest satellites to Andromeda. Both galaxies are metal-poor: we derive <[Fe/H]>=-2.2 for And X, while And XVII shows <[Fe/H]> = -2.0, consistent with the relation of higher luminosity dwarfs being more metal- rich. Additionally, both galaxies show considerable intrinsic spreads in metallicity (0.2 and 0.3 dex for And X and And XVII respectively), consistent with multiple stellar populations.