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

Abstract

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