Stacking the Invisibles: A Guided Search for Low-Luminosity Milky Way Satellites

Abstract

Almost every known low-luminosity Milky Way dwarf spheroidal (dSph) satellite galaxy contains at least one RR Lyrae star. Assuming that a fraction of distant (60 < d_{helio} < 100 kpc) Galactic halo RR Lyrae stars are members of yet to be discovered low-luminosity dSph galaxies, we perform a {\em guided} search for these low-luminosity dSph galaxies. In order to detect the presence of dSph galaxies, we combine stars selected from more than 123 sightlines centered on RR Lyrae stars identified by the Palomar Transient Factory. We find that this method is sensitive enough to detect the presence of Segue 1-like galaxies (M_V= -1.5^{+0.6}_{-0.8}, r_h=30 pc) even if only ~20 sightlines were occupied by such dSph galaxies. Yet, when our method is applied to the SDSS DR10 imaging catalog, no signal is detected. An application of our method to sightlines occupied by pairs of close (<200 pc) horizontal branch stars, also did not yield a detection. Thus, we place upper limits on the number of low-luminosity dSph galaxies with half-light radii from 30 pc to 120 pc, and in the probed volume of the halo. Stronger constraints on the luminosity function may be obtained by applying our method to sightlines centered on RR Lyrae stars selected from the Pan-STARRS1 survey, and eventually, from LSST. In the Appendix, we present spectroscopic observations of an RRab star in the Boötes 3 dSph and a light curve of an RRab star near the Boötes 2 dSph.

emulateapj format, 10 pages, accepted to ApJ

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• RR Lyrae From Binary Evolution: Abundant, Young and Metal-Rich
• The Outer Halo of the Milky Way as Probed by RR Lyr Variables from the Palomar Transient Facility
• Identifying RR Lyrae Variable Stars in Six Years of the Dark Energy Survey
• Serendipitous discovery of RR Lyrae stars in the Leo V ultra-faint galaxy
• Identification of RR Lyrae stars in multiband, sparsely-sampled data from the Dark Energy Survey using template fitting and Random Forest classification
• Palomar Transient Factory and RR Lyrae: Metallicity-Light Curve Relation Based on ab-Type RR Lyrae in Kepler Field
• Identifying RR Lyrae in the ZTF DR3 dataset
• RR Lyrae Stars in the Field of Sagittarius II
• The Hierarchical Structure of Galactic Haloes: Classification and characterisation with Halo-OPTICS
• The mass-metallicity relation at z>3 down to M_*= 10^4 M_Sun. A local perspective using the metallicity distribution of RR Lyrae stars
• rrlfe: Software for Generating and Applying Metallicity Calibrations for RR Lyrae Variable Stars Across a Wide Range of Phases and Temperatures