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

Creators: Sesar, Branimir; Banholzer, Sophianna R.; Cohen, Judith G.; Martin, Nicolas F.; Grillmair, Carl J.; Levitan, David; Laher, Russ R.; Ofek, Eran O.; Surace, Jason A.; Kulkarni, Shrinivas R.; Prince, Thomas A.; Rix, Hans-Walter

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 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 Sloan Digital Sky Survey Data Release 10 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 the Large Synoptic Survey Telescope. In Appendix A, 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.

Additional Information

© 2014 American Astronomical Society. Received 2014 April 24; accepted 2014 August 1; published 2014 September 16. We thank Željko Ivezić, Erik Tollerud, and Colin Slater for comments, suggestions, and useful discussions. B.S. and J.G.C. acknowledge the NSF grant AST-0908139 to J.G.C. for partial support, as do S.R.K. (NSF grant AST-1009987), and C.J.G. (for a NASA grant). B.S. acknowledges funding from the European Research Council under the European Union's Seventh Framework Programme (FP 7) ERC grant agreement No. 321035. S.R.B. thanks the Caltech Summer Undergraduate Research Fellowship (SURF) for support. E.O.O. is incumbent of the Arye Dissentshik career development chair and is grateful for support by grants from the Willner Family Leadership Institute Ilan Gluzman (Secaucus NJ), Israeli Ministry of Science, Israel Science Foundation, Minerva foundation, Weizmann-UK foundation and the I-CORE Program of the Planning and Budgeting Committee and The Israel Science Foundation. This work has made use of BaSTI web tools. This article is based on observations obtained with the Samuel Oschin Telescope as part of the Palomar Transient Factory project, a scientific collaboration between the California Institute of Technology, Columbia University, Las Cumbres Observatory, the Lawrence Berkeley National Laboratory, the National Energy Research Scientific Computing Center, the University of Oxford, and the Weizmann Institute of Science. It is also partially based on observations obtained as part of the Intermediate Palomar Transient Factory project, a scientific collaboration among the California Institute of Technology, Los Alamos National Laboratory, the University of Wisconsin, Millwakee, the Oskar Klein Center, the Weizmann Institute of Science, the TANGO Program of the University System of Taiwan, the Kavli Institute for the Physics and Mathematics of the Universe, and the Inter-University Centre for Astronomy and Astrophysics. Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, and the U.S. Department of Energy Office of Science. The SDSS-III Web site is http://www.sdss3.org/. SDSS-III is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS-III Collaboration including the University of Arizona, the Brazilian Participation Group, Brook haven National Laboratory, Carnegie Mellon University, University of Florida, the French Participation Group, the German Participation Group, Harvard University, the Instituto de Astrofisica de Canarias, the Michigan State/Notre Dame/JINA Participation Group, Johns Hopkins University, Lawrence Berkeley National Laboratory, Max Planck Institute for Astrophysics, Max Planck Institute for Extraterrestrial Physics, New Mexico State University, New York University, Ohio State University, Pennsylvania State University, University of Portsmouth, Princeton University, the Spanish Participation Group, University of Tokyo, University of Utah, Vanderbilt University, University of Virginia, University of Washington, and Yale University.

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