Two Distant Halo Velocity Groups Discovered by the Palomar Transient Factory

Creators: Sesar, Branimir; Cohen, Judith G.; Levitan, David; Grillmair, Carl J.; Jurić, Mario; Kirby, Evan N.; Laher, Russ R.; Ofek, Eran O.; Surace, Jason A.; Kulkarni, Shrinivas R.; Prince, Thomas A.

Abstract

We report the discovery of two new halo velocity groups (Cancer groups A and B) traced by 8 distant RR Lyrae stars and observed by the Palomar Transient Factory (PTF) survey at R.A.~129°, Dec~20° (l~205°, b~32°). Located at 92 kpc from the Galactic center (86 kpc from the Sun), these are some of the most distant substructures in the Galactic halo known to date. Follow-up spectroscopic observations with the Palomar Observatory 5.1-m Hale telescope and W. M. Keck Observatory 10-m Keck I telescope indicate that the two groups are moving away from the Galaxy at v_(gsr) = 78.0+-5.6 km s^(-1) (Cancer group A) and v_(gsr) = 16.3+-7.1 km s^(-1) (Cancer group B). The groups have velocity dispersions of σ_(v_)gsr))=12.4+-5.0 km s^(-1) and σ _(v_(gsr))=14.9+-6.2 km s^(-1), and are spatially extended (about several kpc) making it very unlikely that they are bound systems, and are more likely to be debris of tidally disrupted dwarf galaxies or globular clusters. Both groups are metal-poor (median metallicities of [Fe/H]^A = -1.6 dex and [Fe/H]^B =-2.1 dex), and have a somewhat uncertain (due to small sample size) metallicity dispersion of ~0.4 dex, suggesting dwarf galaxies as progenitors. Two additional RR Lyrae stars with velocities consistent with those of the Cancer groups have been observed ~25 ° east, suggesting possible extension of the groups in that direction.

Additional Information

© 2012 The American Astronomical Society. Received 2012 March 7; accepted 2012 June 26; published 2012 August 2. J.G.C. and B.S. thank NSF grant AST-0908139 to J.G.C for partial support, as do S.R.K (to NSF grant AST-1009987), and C.J.G (for a NASA grant). Support for this work was provided by NASA through Hubble Fellowship grant 51256.01 awarded to E.N.K by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS 5-26555. We thank the referee for a thorough review and suggestions which led to an improved manuscript. B.S. would like to thank Ź. Ivezić, B. Willman, and K. Vivas for useful discussions. We thank I. Arcavi, A. Gal-Yam, P. Groot, A. Horesh, and D. Perley for observing at Keck and Palomar. We thank the staff at the Palomar Hale telescope for help and support with observations. We are grateful to the many people who have worked to make the Keck Telescope and its instruments a reality and to operate and maintain the Keck Observatory. The authors wish to extend special thanks to those of Hawaiian ancestry on whose sacred mountain we are privileged to be guests. Without their generous hospitality, none of the observations presented herein would have been possible. 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.

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