Kinematics and Metallicities in the Bootes III Stellar Overdensity: a Disrupted Dwarf Galaxy?

TL;DR

This study uses spectroscopy to analyze the kinematics and metallicities of stars in Bootes III, suggesting it is a transitional object between a bound dwarf galaxy and an unbound tidal stream.

Contribution

It provides the first detailed spectroscopic analysis of Bootes III, revealing its velocity and metallicity properties and proposing its transitional nature.

Findings

Systemic velocity of 197.5 km/s with a dispersion of 14.0 km/s.

Evidence of a highly radial orbit indicating tidal disruption.

Morphological and abundance data support a transitional state.

Abstract

We report the results of a spectroscopic study of the Bootes III (BooIII) stellar overdensity carried out with the Hectospec multifiber spectrograph on the MMT telescope. Radial velocities have been measured for 193 BooIII candidate stars selected to have magnitudes and colors consistent with its upper main sequence and lower red giant branch, as well as a number of horizontal branch candidates. From 20 identified candidate BooIII members, we measure a systemic velocity of V_sun=197.5+-3.8 km/s and a velocity dispersion of sigma_o=14.0+-3.2 km/s. We use the somewhat large velocity dispersion and the implied highly radial orbit, along with morphological evidence from Grillmair (2009) and stellar abundances, to argue that BooIII is likely the first known object observed in a transitional state between being a bound dwarf galaxy and a completely unbound tidal stream.