Stealth Galaxies in the Halo of the Milky Way

TL;DR

This paper predicts a hidden population of ultra-faint, low-surface-brightness dwarf galaxies in the Milky Way halo, which are difficult to detect but could significantly impact our understanding of galaxy formation and dark matter distribution.

Contribution

It introduces the concept of stealth galaxies, theorizing their existence and properties, and discusses implications for galaxy formation thresholds and observational strategies.

Findings

Stealth galaxies have surface brightness fainter than 30 mag/arcsec^2.

Their properties resemble ultrafaint dwarfs but with more extended stellar distributions.

The total number of stealth galaxies depends on galaxy formation thresholds.

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 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.