Accurate Stellar Kinematics at Faint Magnitudes: Application to the Bo?tes I Dwarf Spheroidal Galaxy

摘要

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. The repeated measurements let us reliably calibrate our individual velocity errors (0.2 ≤ δV ≤ 5 km s~(-1)) and directly detect stars with variable radial velocities. We show, by application to the Bo?tes I dwarf spheroidal, that the intrinsic velocity dispersion of this system is significantly below 6.5 km s~(-1) reported by previous studies. Our data favor a two-population model of Bo?tes I, consisting of a majority 'cold' stellar component, with velocity dispersion 2.4_(-0.5)~(+0.9) km s~(-1), and a minority 'hot' stellar component, with velocity dispersion ～ 9 km s~(-1), although we can not completely rule out a single component distribution with velocity dispersion 4.6_(-0.6)~(+0.8) km s~(-1).