Two Distant Halo Velocity Groups Discovered by the Palomar Transient Factory

TL;DR

This paper reports the discovery of two distant, metal-poor halo velocity groups traced by RR Lyrae stars, likely remnants of tidally disrupted dwarf galaxies or globular clusters, at about 92 kpc from the Galactic center.

Contribution

The discovery of two new distant halo velocity groups using RR Lyrae stars and spectroscopic follow-up, revealing their kinematic and metallicity properties and suggesting their origin as disrupted dwarf galaxies.

Findings

Located at 92 kpc from the Galactic center.

Velocity groups have distinct velocities and dispersions.

Likely progenitors are dwarf galaxies or globular clusters.

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 \sigma_{v_{gsr}}=12.4+-5.0 km/s and \sigma_{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.