Observational biases against finding Milky Way ( MW ) dwarf galaxies at low Galactic latitudes ( b \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ < $ } } } 20 ^ { \circ } ) and at low surface brightnesses ( \mu _ { V, 0 } \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } % \hbox { $ > $ } } } 29 mag arcsec ^ { -2 } ) currently limit our understanding of the faintest limits of the galaxy luminosity function .
This paper is a proof-of-concept that groups of two or more RR Lyrae stars could reveal MW dwarf galaxies at d > 50 kpc in these unmined regions of parameter space , with only modest contamination from interloper groups when large halo structures are excluded .
For example , a friends-of-friends ( FOF ) search with a 2D linking length of 500 pc could reveal dwarf galaxies more luminous than M _ { V } = -3.2 mag and with surface brightnesses as faint as 31 mag arcsec ^ { -2 } ( or even fainter , depending on RR Lyrae specific frequency ) .
Although existing public RR Lyrae catalogs are highly incomplete at d > 50 kpc and/or include < 1 % of the MW halo ’ s volume , a FOF search reveals two known dwarfs ( Boötes I and Sextans ) and two dwarf candidate groups possibly worthy of follow-up .
PanSTARRS 1 ( PS1 ) may catalog RR Lyrae to 100 kpc ( in the absence of Galactic extinction ) which would include up to \sim 15 % of predicted MW dwarf galaxies .
Groups of PS1 RR Lyrae should therefore reveal very low surface brightness and low Galactic latitude dwarfs within its footprint , if they exist .
With sensitivity to RR Lyrae to d \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ > $ } } } 600 kpc , LSST is the only planned survey that will be both wide-field and deep enough to use RR Lyrae to definitively measure the Milky Way ’ s dwarf galaxy census to extremely low surface brightnesses , and through the Galactic plane .