At the survey limits: discovery of the Aquarius 2 dwarf galaxy in the VST ATLAS and the SDSS data

TL;DR

This paper reports the discovery of Aquarius 2, a distant Milky Way satellite dwarf galaxy identified through survey data, with detailed follow-up observations revealing its size, velocity dispersion, and challenging detection limits.

Contribution

The paper presents the first discovery and detailed characterization of Aquarius 2, a faint, distant dwarf galaxy near survey detection limits, using combined survey data and follow-up spectroscopy.

Findings

Aquarius 2 is approximately 110 kpc away with a half-light radius of ~160 pc.

It has a stellar velocity dispersion of about 5.4 km/s.

Its surface brightness and luminosity make it one of the hardest dwarfs to detect.

Abstract

We announce the discovery of the Aquarius~2 dwarf galaxy, a new distant satellite of the Milky Way, detected on the fringes of the VST ATLAS and the SDSS surveys. The object was originally identified as an overdensity of Red Giant Branch stars, but chosen for subsequent follow-up based on the presence of a strong Blue Horizontal Branch, which was also used to measure its distance of $\sim 110$ kpc. Using deeper imaging from the IMACS camera on the 6.5m Baade and spectroscopy with DEIMOS on Keck, we measured the satellite's half-light radius $5.1\pm 0.8$ arcmin, or $\sim 160$ pc at this distance, and its stellar velocity dispersion of $5.4^{+3.4}_{-0.9}$ km s$^{-1}$. With $\mu=30.2$ mag arcsec$^{-2}$ and $M_V=-4.36$, the new satellite lies close to two important detection limits: one in surface brightness; and one in luminosity at a given distance, thereby making Aquarius~2 one of the hardest dwarfs to find.