VISTA's view of the Sagittarius dwarf spheroidal galaxy and southern Galactic Bulge

MCDONALD, ; ZIJLSTRA, A. A.; I.;; SLOAN, G. C.; KERINS, E. J.; LAGADEC, E.; MINNITI, D.; SANTUCHO, M. V.; GUROVICH, S.; DOMÍNGUEZ ROMERO, M. J. DE L.

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY

WILEY-BLACKWELL PUBLISHING, INC

Lugar: Londres; Año: 2013 vol. 436 p. 413 - 429

0035-8711

We present the deepest near-infrared (ZJKs) photometry yet obtained of the Sagittarius dwarf spheroidal (Sgr dSph), using VISTA to survey 11 square degrees centred on its core. We list locations and ZJKs-band magnitudes for over 2.9 million sources in the field. We discuss the isolation of the Sgr dSph from the foreground and Galactic Bulge populations, identify the Sgr dSph´s horizontal branch in the near-infrared for the first time, and map the density of the galaxy´s stars. We present isochrones for the Sgr dSph and Bulge populations. These are consistent with the previously-reported properties of the Sgr dSph core: namely that it is dominated by a population between [Fe/H] ~ -1 dex and solar, with a significant [alpha/Fe] versus [Fe/H] gradient. While strong contamination from the Galactic Bulge prevents accurate measurement of the (Galactic) north side of the Sgr dSph, the dwarf galaxy can be well-approximated by a roughly ovaloid projection of characteristic size 4 x 2 degrees, beyond which the projected stellar density is less than half that of the region surrounding the core. The galaxy´s major axis is perpendicular to the Galactic Plane, as in previous studies. We find slight evidence to confirm a metallicity gradient in the Sgr dSph and use isochrones to fit a distance of 24.3 +/- 2.3 kpc. We were unable to fully constrain the metallicity distribution of the Sgr dSph due to the Bulge contamination and strong correlation of [alpha/Fe] with metallicity, however we find that metal-poor stars ([Fe/H] <~ --1) make up <~29 per cent of the Sgr dSph´s upper-RGB population. The Bulge population is best fit by a younger population with [Fe/H] ~ 0 and [alpha/Fe] ~ 0 or slightly higher. We find no evidence for a split, peanut- or X-shaped Bulge population in this line of sight (l = 5.6 +/- ~1 deg, b = -14.1 +/- ~3 deg).