Exploring the Variable Sky with LINEAR. II. Halo Structure and Substructure Traced by RR Lyrae Stars to 30 kpc

TL;DR

This study analyzes approximately 5,000 RR Lyrae stars from the LINEAR dataset to map the Milky Way's halo structure, revealing its density profile, Oosterhoff types, and potential tidal streams, enhancing understanding of galactic formation.

Contribution

It provides the first large, publicly available catalog of RR Lyrae stars in the halo and identifies new halo groups and possible tidal streams using a novel group-finding algorithm.

Findings

Halo RR Lyrae density follows an oblate ellipsoid with q=0.63.

Detected seven halo groups, with some near globular clusters and streams.

Observed Oosterhoff dichotomy with a 1:4 ratio of type II to I stars.

Abstract

We present a sample of ~5,000 RR Lyrae stars selected from the recalibrated LINEAR dataset and detected at heliocentric distances between 5 kpc and 30 kpc over ~8,000 deg^2 of sky. The coordinates and light curve properties, such as period and Oosterhoff type, are made publicly available. We find evidence for the Oosterhoff dichotomy among field RR Lyrae stars, with the ratio of the type II and I subsamples of about 1:4. The number density distribution of halo RRab stars as a function of galactocentric distance can be described as an oblate ellipsoid with the axis ratio q=0.63 and with either a single or a double power law with a power-law index in the range -2 to -3. Using a group-finding algorithm EnLink, we detected seven candidate halo groups, only one of which is statistically spurious. Three of these groups are near globular clusters (M53/NGC 5053, M3, M13), and one is near a known halo substructure (Virgo Stellar Stream); the remaining three groups do not seem to be near any known halo substructures or globular clusters, and seem to have a higher ratio of Oosterhoff type II to Oosterhoff type I RRab stars than what is found in the halo. The extended morphology and the position (outside the tidal radius) of some of the groups near globular clusters is suggestive of tidal streams possibly originating from globular clusters. Spectroscopic followup of detected halo groups is encouraged.