The horizontal branch of the Sculptor dwarf galaxy

¹ Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK
e-mail: M.Salaris@aljmu.ac.uk
² Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK
³ Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 AV Groningen, The Netherlands
⁴ Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna, Viale Berti Pichat 6/2, 40127 Bologna, Italy
⁵ INAF − Osservatorio Astronomico di Collurania, via M. Maggini, 64100 Teramo, Italy

Received: 19 August 2013
Accepted: 17 September 2013

Abstract

We have performed the first detailed simulation of the horizontal branch (HB) of the Sculptor dwarf spheroidal galaxy by means of synthetic modelling techniques, taking consistently into account the star formation history and metallicity evolution as determined from the main sequence and red giant branch spectroscopic observations. The only free parameter in the whole analysis is the integrated mass loss of red giant branch stars. This is the first time that synthetic HB models, consistent with the complex star formation history of a galaxy, are calculated and matched to the observed HB. We find that the metallicity range covered by the star formation history, as constrained by the red giant branch spectroscopy, plus a simple mass loss law, enable us to cover both the full magnitude and colour range of HB stars. In addition, the number count distribution along the observed HB can be also reproduced provided that the red giant branch mass loss is mildly metallicity dependent, with a very small dispersion at fixed metallicity. The magnitude, metallicity and period distribution of the RR Lyrae stars are also well reproduced. There is no excess of bright objects that require enhanced-He models. The lack of signatures of enhanced-He stars along the HB is consistent with the lack of the O-Na anticorrelation observed in Sculptor and other dwarf galaxies, and confirms the intrinsic difference between Local Group dwarf galaxies and globular cluster populations. We also compare the brightness of the observed red giant branch bump with the synthetic counterpart, and find a discrepancy. The theoretical bump is brighter than the observed one, which is similar to what is observed in Galactic globular clusters.