The usage of Strömgren photometry in studies of local group dwarf spheroidal galaxies

Abstract

Aims.In this paper we demonstrate how Strömgren uvby photometry can be efficiently used to: 1. identify red giant branch stars that are members in a dwarf spheroidal galaxy; 2. derive age-independent metallicities for the same stars and quantify the associated errors. Methods. Strömgren uvby photometry in a 11 × 22 arcmin field centered on the Draco dwarf spheroidal galaxy was obtained using the Isaac Newton Telescope on La Palma. Members of the Draco dSph galaxy were identified using the surface gravity sensitive c 1 index which discriminates between red giant and dwarf stars. Thus enabling us to distinguish the (red giant branch) members of the dwarf spheroidal galaxy from the foreground dwarf stars in our galaxy. The method is evaluated through a comparison of our membership list with membership classifications in the literature based on radial velocities and proper motions. The metallicity sensitive m 1 index was used to derive individual and age-independent metallicities for the members of the Draco dSph galaxy. The derived metallicities are compared to studies based on high resolution spectroscopy and the agreement is found to be very good. Results. We present metallicities for 169 members of the red giant branch in the Draco dwarf spheroidal galaxy (the largest sample to date). The metallicity distribution function for the Draco dSph galaxy shows a mean [Fe/H] = -1.74 dex with a spread of 0.24 dex. The correlation between metallicity and colour for the stars on the red giant branch is consistent with a dominant old, and coeval population. There is a possible spatial population gradient over the field with the most metal-rich stars being more centrally concentrated than the metal-poor stars.

A photometric and spectroscopic study of the new dwarf spheroidal galaxy in Hercules

“…This classification is adopted from Schuster et al (2004). We also show the region occupied by the RGB stars in the Draco dSph galaxy (Faria et al 2007). Schuster et al (2004) were mainly concerned with high velocity dwarf stars and to a lesser extent interested in the redder • indicates objects with a stellarity index greater than 0.5.…”

“…MS: the main sequence; SG: sub-giant stars; RG: red giant stars; RHB-AGB: the red horizontal-branch-asymptotic-giant-branch transition; HB: the horizontal branch. The shaded region marked DRACO indicates the region occupied by the RGB stars in the Draco dSph galaxy (Faria et al 2007). the dwarf and giant star regions also in the red.…”

“…A review of the Strömgren metallicity calibrations for RGB stars, available at the time, is provided by Faria et al (2007). They adopt the Hilker (2000) calibration for their RGB stars in the Draco dSph galaxy.…”

Cleaning spectroscopic samples of stars in nearby dwarf galaxies

“…Muñoz et al (2006) photometrically preselected their spectroscopic targets using the M51 filter and find probable members of the Carina dSph out to very large distances from its center, where genuine members would have otherwise been buried in the foreground. Faria et al (2007) used Strömgren photometry on the Draco dSph, showing that the Strömgren c 1 index, which is sensitive to luminosity, can be used together with the (b − y) 0 color to separate stars that are in various evolutionary stages. While photometric techniques provide broad information on the star formation history of the dwarf galaxy and in some cases on the metallicity distribution of its stars (e.g.…”

“…While photometric techniques provide broad information on the star formation history of the dwarf galaxy and in some cases on the metallicity distribution of its stars (e.g. Faria et al 2007), information on the l.o.s. velocity of the individual stars appears crucial to assess membership in some cases even when using Washington photometry (e.g.…”