Abstract
We use VLT/FLAMES intermediate resolution (R~6500) spectra of individual red giant branch stars in the near-infrared CaII triplet (CaT) region to investigate the wide-area metallicity properties and internal kinematics of the Sextans dwarf spheroidal galaxy (dSph). Our final sample consists of 174 probable members of Sextans with accurate line-of-sight velocities (+- 2 km/s) and CaT [Fe/H] measurements (+- 0.2 dex). We use the MgI line at 8806.8 \AA\, as an empirical discriminator for distinguishing between probable members of the dSph (giant stars) and probable Galactic contaminants (dwarf stars). Sextans shows a similar chemo-dynamical behaviour to other Milky Way dSphs, with its central regions being more metal rich than the outer parts and with the more metal-rich stars displaying colder kinematics than the more metal-poor stars. Hints of a velocity gradient are found along the projected major axis and along an axis at P.A.=191 deg, however a larger and more spatially extended sample may be necessary to pin down the amplitude and direction of this gradient. We detect a cold kinematic substructure at the centre of Sextans, consistent with being the remnant of a disrupted very metal poor stellar cluster. We derive the most extended line-of-sight velocity dispersion profile for Sextans, out to a projected radius of 1.6 deg. From Jeans modelling of the observed line-of-sight velocity dispersion profile we find that this is consistent with both a cored dark matter halo with large core radius and cuspy halo with low concentration. The mass within the last measured point is in the range 2-4 x 10^8 M_sun, giving very large mass-to-light ratios, from 460 to 920 (M/L)_(V,sun).
Highlights
The Sextans dwarf spheroidal galaxy is a satellite of the Milky Way (MW; heliocentric distance of 86 kpc from Irwin et al 1990; Mateo, Fischer & Krzeminski 1995; 95.5 kpc from Lee et al 2003)
In this paper we use a spectroscopic sample of line-of-sight velocities and [Fe/H] measurements for 174 red giant branch (RGB) stars probable members of Sextans, obtained at Very Large Telescope (VLT)/Fibre Large Array Multi Element Spectrograph (FLAMES), to analyse both the wide-field metallicity properties and the internal kinematics of this galaxy
In order to derive the errors in [Fe/H], we follow the approach of Starkenburg et al (2010), i.e. we consider that the error due to the photometry is negligible with respect to the error in the determination of the Ca II triplet (CaT) equivalent widths (EW) and we input in equation (1) the values of W − 1σ W and W + 1σ W and derive the corresponding 1σ lower and upper limit in [Fe/H]
Summary
The Sextans dwarf spheroidal galaxy (dSph) is a satellite of the Milky Way (MW; heliocentric distance of 86 kpc from Irwin et al 1990; Mateo, Fischer & Krzeminski 1995; 95.5 kpc from Lee et al 2003). The mass estimates are not directly comparable as they refer to different distances from the Sextans centre, they all point to mass-to-light ratios so large as to require considerable amounts of DM, if the hypothesis of dynamical equilibrium holds. These spectroscopic samples do not reach the nominal tidal radius and the masses determined are likely to be lower limits. In this paper we use a spectroscopic sample of line-of-sight velocities and [Fe/H] measurements for 174 RGB stars probable members of Sextans, obtained at Very Large Telescope (VLT)/Fibre Large Array Multi Element Spectrograph (FLAMES), to analyse both the wide-field metallicity properties and the internal kinematics of this galaxy. C 2010 The Authors, MNRAS 411, 1013–1034 Monthly Notices of the Royal Astronomical Society C 2010 RAS perform a mass determination in Section 5 and present discussion and conclusions in Sections 6 and 7
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