Two Distant Halo Velocity Groups Discovered by the Palomar Transient Factory

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 deg, Dec~20 deg (l~205 deg, b~32 deg). 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 (Cancer group A) and v_{gsr} = 16.3+-7.1 km/s (Cancer group B). The groups have velocity dispersions of σ_{v_{gsr}}=12.4+-5.0 km/s and σ_{v_{gsr}}=14.9+-6.2 km/s, 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] = -1.6 dex and [Fe/H] =-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 deg east, suggesting possible extension of the groups in that direction.

5 figures, accepted to ApJ

References in corpus (8)

• Cats and Dogs, Hair and A Hero: A Quintet of New Milky Way Companions
• Tracing Galaxy Formation with Stellar Halos II: Relating Substructure in Phase- and Abundance-Space to Accretion Histories
• Detection of a 63 Degree Cold Stellar Stream in the Sloan Digital Sky Survey
• Velocity Dispersion Profiles of Seven Dwarf Spheroidal Galaxies
• Detection of a 60 Degree-Long Dwarf Galaxy Debris Stream
• Stellar evolution through the ages: period variations in galactic RRab stars as derived from the GEOS database and TAROT telescopes
• The Origin of the Virgo Stellar Substructure
• Exploring the Variable Sky with the Sloan Digital Sky Survey

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