Bibcode
Kordopatis, G.; Hill, V.; Irwin, M.; Gilmore, G.; Wyse, R. F. G.; Tolstoy, E.; de Laverny, P.; Recio-Blanco, A.; Battaglia, G.; Starkenburg, E.
Bibliographical reference
Astronomy and Astrophysics, Volume 555, id.A12, 16 pp.
Advertised on:
7
2013
Journal
Citations
30
Refereed citations
26
Description
Context. We aim to understand the accretion history of the Milky Way by
exploring the vertical and radial properties of the Galactic thick disc.
Aims: We study the chemical and kinematic properties of roughly a
thousand spectra of faint magnitude foreground Galactic stars observed
serendipitously during extra-galactic surveys in four lines-of-sight:
three in the southern Galactic hemisphere (surveys of the Carina, Fornax
and Sculptor dwarf spheroidal galaxies) and one in the northern Galactic
hemisphere (a survey of the Sextans dwarf spheroidal galaxy). The
foreground stars span distances up to ~3 kpc from the Galactic plane and
Galactocentric radii up to 11 kpc. Methods: The stellar
atmospheric parameters (effective temperature, surface gravity,
metallicity) are obtained by an automated parameterisation pipeline and
the distances of the stars are then derived by a projection of the
atmospheric parameters on a set of theoretical isochrones using a
Bayesian approach. The metallicity gradients are estimated for each
line-of-sight and compared with predictions from the Besançon
model of the Galaxy, in order to test the chemical structure of the
thick disc. Finally, we use the radial velocities in each line-of-sight
to derive a proxy for either the azimuthal or the vertical component of
the orbital velocity of the stars. Results: Only three
lines-of-sight have a sufficient number of foreground stars for a robust
analysis. Towards Sextans in the Northern Galactic hemisphere and
Sculptor in the South, we measure a consistent decrease in mean
metallicity with height from the Galactic plane, suggesting a chemically
symmetric thick disc. This decrease can either be due to an intrinsic
thick disc metallicity gradient, or simply due to a change in the thin
disc/thick disc population ratio and no intrinsic metallicity gradients
for the thick disc. We favour the latter explanation. In contrast, we
find evidence of an unpredicted metal-poor population in the direction
of Carina. This population was earlier detected, but our more detailed
analysis provides robust estimates of its location (|Z| < 1 kpc),
metallicity (-2 < [M/H] < -1 dex) and azimuthal orbital velocity
(Vφ ~ 120 km s-1). Conclusions: Given
the chemo-dynamical properties of the over-density towards the Carina
line-of-sight, we suggest that it represents the metal-poor tail of the
canonical thick disc. In spite of the small number of stars available,
we suggest that this metal-weak thick disc follows the often suggested
canonical thick disc velocity-metallicity correlation of
∂Vφ/∂ [M/H] ~ 40-50 km s-1
dex-1.
Based on observations collected at the European Southern Observatory at
Paranal, Chile, ESO Large Programme 171.B-0588 (DART) and
171.B-0520(A).Full Tables 2 and 4 are only available at the CDS via
anonymous ftp to http://cdsarc.u-strasbg.fr
(ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/555/A12