Discovery of a Stellar Overdensity in Eridanus-Phoenix in the Dark Energy Survey

TL;DR

The paper reports the discovery of a significant stellar overdensity in Eridanus-Phoenix from DES data, potentially linked to other Galactic structures, with implications for understanding the Milky Way's halo composition.

Contribution

This work identifies a new stellar overdensity in Eridanus-Phoenix, providing detailed spatial and stellar population analysis, and suggests possible associations with known Galactic structures.

Findings

Overdensity spans at least 30° by 10° in sky.

Poisson significance of detection is at least 9 sigma.

Distance estimate of about 16 kpc from the Sun.

Abstract

We report the discovery of an excess of main sequence turn-off stars in the direction of the constellations of Eridanus and Phoenix from the first year data of the Dark Energy Survey (DES). The Eridanus-Phoenix (EriPhe) overdensity is centered around l~285 deg and b~-60 deg and spans at least 30 deg in longitude and 10 deg in latitude. The Poisson significance of the detection is at least 9 sigma. The stellar population in the overdense region is similar in brightness and color to that of the nearby globular cluster NGC 1261, indicating that the heliocentric distance of EriPhe is about d~16 kpc. The extent of EriPhe in projection is therefore at least ~4 kpc by ~3 kpc. On the sky, this overdensity is located between NGC 1261 and a new stellar stream discovered by DES at a similar heliocentric distance, the so-called Phoenix Stream. Given their similar distance and proximity to each other, it is possible that these three structures may be kinematically associated. Alternatively, the EriPhe overdensity is morphologically similar to the Virgo overdensity and the Hercules-Aquila cloud, which also lie at a similar Galactocentric distance. These three overdensities lie along a polar plane separated by ~120 deg and may share a common origin. Spectroscopic follow-up observations of the stars in EriPhe are required to fully understand the nature of this overdensity.