* Astronomy

Blobrana · L

RE: Sculptor Dwarf Spheroidal Galaxy

Blobrana · L

Title: A Virial Core in the Sculptor Dwarf Spheroidal Galaxy
Authors: A. Agnello (Cambridge), N.W. Evans (Cambridge)

The projected virial theorem is applied to the case of multiple stellar populations in the nearby dwarf spheroidal galaxies. As each population must reside in the same gravitational potential, this provides strong constraints on the nature of the dark matter halo. We derive necessary conditions for two populations with Plummer or exponential surface brightnesses to reside in a cusped Navarro-Frenk-White (NFW) halo. We apply our methods to the Sculptor dwarf spheroidal, and show that there is no NFW halo compatible with the energetics of the two populations. The dark halo must possess a core radius of ~ 120 pc for the virial solutions for the two populations to be consistent. This conclusion remains true, even if the effects of flattening or self-gravity of the stellar populations are included.

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Blobrana · L

Title: The Star Formation & Chemical Evolution History of the Sculptor Dwarf Spheroidal Galaxy
Authors: T. J. L. de Boer, E. Tolstoy, V. Hill, A. Saha, K. Olsen, E. Starkenburg, B. Lemasle, M. J. Irwin, G. Battaglia

We have combined deep photometry in the B,V and I bands from CTIO/MOSAIC of the Sculptor dwarf spheroidal galaxy, going down to the oldest Main Sequence Turn-Offs, with spectroscopic metallicity distributions of Red Giant Branch stars. This allows us to obtain the most detailed and complete Star Formation History to date, as well as an accurate timescale for chemical enrichment. The Star Formation History shows that Sculptor is dominated by old (>10 Gyr), metal-poor stars, but that younger, more metal-rich populations are also present. Using Star Formation Histories determined at different radii from the centre we show that Sculptor formed stars with an increasing central concentration with time. The old, metal-poor populations are present at all radii, while more metal-rich, younger stars are more centrally concentrated. We find that within an elliptical radius of 1 degree, or 1.5 kpc from the centre, a total mass in stars of 7.8 x 10^{6} solar masses was formed, between 14 and 7 Gyr ago, with a peak at 13-14 Gyr ago. We use the detailed Star Formation History to determine age estimates for individual Red Giant Branch stars with high resolution spectroscopic abundances. Thus, for the first time, we can directly determine detailed timescales for the evolution of individual chemical elements. We find that the trends in alpha-elements match what is expected from an extended, relatively uninterrupted period of star formation continuing for 6-7 Gyr. The knee in the alpha-element distribution occurs at an age of 10.9 ±1Gyr, suggesting that SNe Ia enrichment began ~2 ±1Gyr after the start of star formation in Sculptor.

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