Aquarius 2 is a Difficult to Detect Dwarf Galaxy

Torrealba et al. (2016) present the discovery of Aquarius 2, a dwarf galaxy that is a distant satellite galaxy of the Milky Way. Aquarius 2 was identified based on an overdensity of Red Giant Branch (RGB) stars and an overpopulation of Blue Horizontal Branch (BHB) stars. RGB and BHB stars are stars that have evolved off the main sequence and they are many times more luminous than their main sequence progenitors. These luminous evolved stars can indicate the position of what would otherwise be an invisible galaxy.

The estimated distance of Aquarius 2 from the Milky Way is ~360,000 light years and its estimated half-light radius is ~500 light years. The half-light radius of a galaxy is the size of the volume around the center of the galaxy which accounts for half the galaxy's brightness. The half-light radius of Aquarius 2 is estimated to enclose ~3 million times the Sun's mass.

Aquarius 2 has a low surface brightness of only 30.4 mag/arcsec², and the total luminosity of this dwarf galaxy is only ~4000 times the Sun's luminosity. Aquarius 2 has a large mass-to-light ratio of ~1300, indicating it is a dark matter dominated galaxy. The low surface brightness and low overall luminosity of Aquarius 2 makes it a particularly difficult galaxy to detect. The detection of Aquarius 2 suggests the presence of more dwarf galaxies lurking out there.

The red solid line denotes the elliptical half-light contour of the best fit model for Aquarius 2. The orange, red and blue circles/squares mark the locations of stars confirmed as foreground, RGB and BHB stars, respectively. Torrealba et al. (2016)

Absolute magnitude versus half-light radius for Milky Way satellite galaxies (red open circles), M31 satellite galaxies (black unfilled triangles), globular clusters (black dots), extended objects larger than ~300 light years in size (grey dots) and Local Group/nearby galaxies (grey crosses). Torrealba et al. (2016)

Reference:
Torrealba et al. (2016), "At the survey limits: discovery of the Aquarius 2 dwarf galaxy in the VST ATLAS and the SDSS data", arXiv:1605.05338 [astro-ph.GA]