Stealth Galaxies in the Halo of the Milky Way

Abstract

We predict that there is a population of low-luminosity dwarf galaxies orbiting within the halo of the Milky Way that have surface brightnesses low enough to have escaped detection in star-count surveys. The overall count of stealth galaxies is sensitive to the presence (or lack) of a low-mass threshold in galaxy formation. These systems have luminosities and stellar velocity dispersions that are similar to those of known ultrafaint dwarf galaxies but they have more extended stellar distributions (half light radii greater than about 100 pc) because they inhabit dark subhalos that are slightly less massive than their higher surface brightness counterparts. As a result, the typical peak surface brightness is fainter than 30 mag per square arcsec. One implication is that the inferred common mass scale for Milky Way dwarfs may be an artifact of selection bias. If there is no sharp threshold in galaxy formation at low halo mass, then ultrafaint galaxies like Segue 1 represent the high-mass, early forming tail of a much larger population of objects that could number in the hundreds and have typical peak circular velocities of about 8 km s −1 and masses within 300 pc of about 5 million solar masses. Alternatively, if we impose a low-mass threshold in galaxy formation in order to explain the unexpectedly high densities of the ultrafaint dwarfs, then we expect only a handful of stealth galaxies in the halo of the Milky Way. A complete census of these objects will require deeper sky surveys, 30m-class follow-up telescopes, and more refined methods to identify extended, self-bound groupings of stars in the halo.

SAGITTARIUS II, DRACO II AND LAEVENS 3: THREE NEW MILKY WAY SATELLITES DISCOVERED IN THE PAN-STARRS 1 3πSURVEY

“…The recent discoveries of such faint candidate DGs out to ∼70 kpc within DES confirm that they are in fact common and that they could indeed correspond to the large population of faint dark-matter dominated systems expected to inhabit the MW halo (e.g., Tollerud et al 2008;Bullock et al 2010). These new satellites, located close to the Magellanic Clouds (Bechtol et al 2015;Kirby et al 2015;Koposov et al 2015), have emphasized the tendency of these faint stellar systems to be brought into the MW surroundings in groups.…”

Beyond collisionless dark matter: Particle physics dynamics for dark matter halo structure

“…Baryonic processes such as supernova feedback and/or photoionization may play important for suppressing star formation in dwarf galaxies, explaining the observed (weak) baryon content of these small galaxies [53]. Recently, the Sloan Digital Sky Survey has discovered many faint galaxies, such that it is evident that as many as a factor of 5 − 20 of the known dwarf galaxies could be still undiscovered due to faintness, luminosity bias and limited sky coverage [54][55][56]. Consensus is thus shifting toward the view that the number of MW subhalos is not an issue for the predictions of CCDM, at least for smaller subhalos.…”

Triangulum Ii: A Very Metal-Poor and Dynamically Hot Stellar System

“…Although they are expected by the dozen in simulations (Tollerud et al 2008;Bullock et al 2010), only a handful of these faint and small stellar systems have so far been confirmed as dwarf galaxies. Any new addition to the list is particularly valuable as these objects are among the most promising for the indirect detection of the elusive dark matter particle (e.g., Geringer-Sameth et al 2015).…”