Accurate Stellar Kinematics at Faint Magnitudes: application to the Bootes~I dwarf spheroidal galaxy

TL;DR

This paper presents a refined method for measuring stellar radial velocities using VLT/FLAMES+GIRAFFE, applied to the Bootes I dwarf galaxy, revealing a complex velocity structure with multiple stellar populations.

Contribution

Developed an enhanced data reduction technique for precise radial velocities, enabling detailed kinematic analysis of faint dwarf galaxies like Bootes I.

Findings

Intrinsic velocity dispersion of Bootes I is below 6.5 km/s.

Evidence for a two-population model with distinct velocity dispersions.

Detection of a possible velocity anisotropy reflecting formation history.

Abstract

We develop, implement and characterise an enhanced data reduction approach which delivers precise, accurate, radial velocities from moderate resolution spectroscopy with the fibre-fed VLT/FLAMES+GIRAFFE facility. This facility, with appropriate care, delivers radial velocities adequate to resolve the intrinsic velocity dispersions of the very faint dSph dwarf galaxies. Importantly, repeated measurements let us reliably calibrate our individual velocity errors ($0.2 \leq \delta_V\leq 5$ km s$^{-1}$) and directly detect stars with variable radial velocities. We show, by application to the Bootes-1 dwarf spheroidal, that the intrinsic velocity dispersion of this system is significantly below 6.5\,km/s reported by previous studies. Our data favor a two-population model of Bootes-1, consisting of a majority `cold' stellar component, with velocity dispersion $2.4^{+0.9}_{-0.5}$\,km/s, and a minority `hot' stellar component, with velocity dispersion $\sim 9$\,km/s, although we can not completely rule out a single component distribution with velocity dispersion $4.6^{0.8}_{-0.6}$\,km/s. We speculate this complex velocity distribution actually reflects the distribution of velocity anisotropy in Bootes-1, which is a measure of its formation processes.