One large blob and many streams frosting the nearby stellar halo in Gaia DR2

TL;DR

This study analyzes the phase-space structure of nearby halo stars from Gaia DR2, revealing substructures like streams and clumps that support cosmological models of recent accretion events in the stellar halo.

Contribution

It provides new insights into the velocity and integrals of motion distribution of halo stars, identifying substructures and confirming predictions of stream-frosting from cosmological simulations.

Findings

Identification of a retrograde 'blob' in halo stars

Detection of small velocity clumps with narrow stellar populations

Confirmation that substructure is prominent among fast-moving stars

Abstract

We explore the phase-space structure of nearby halo stars identified kinematically from Gaia DR2 data. We focus on their distribution in velocity and in "integrals of motion" space as well as on their photometric properties. Our sample of stars selected to be moving at a relative velocity of at least 210 km/s with respect to the Local Standard of Rest, contains an important contribution from the low rotational velocity tail of the disk(s). The $V_R$-distribution of these stars depicts a small asymmetry similar to that seen for the faster rotating thin disk stars near the Sun. We also identify a prominent, slightly retrograde "blob", which traces the metal-poor halo main sequence reported by Gaia Collaboration et al. (2018d). We also find many small clumps especially noticeable in the tails of the velocity distribution of the stars in our sample. Their HR diagrams disclose narrow sequences characteristic of simple stellar populations. This stream-frosting confirms predictions from cosmological simulations, namely that substructure is most apparent amongst the fastest moving stars, typically reflecting more recent accretion events.