A wide angle view of the Sagittarius dwarf spheroidal galaxy

Abstract

Context. The Sagittarius dwarf Spheroidal Galaxy (Sgr dSph) provides us with a unique possibility of studying a dwarf galaxy merging event while still in progress. Moving along a short-period, quasi-polar orbit in the Milky Way Halo, Sgr dSph is being tidally dispersed along a huge stellar stream. Due to its low distance (25 kpc), the main body of Sgr dSph covers a vast area in the sky (roughly 15 × 7 degrees). Available photometric and spectroscopic studies have concentrated either on the central part of the galaxy or on the stellar stream, but the overwhelming majority of the galaxy body has never been probed. Aims. The aim of the present study is twofold. On the one hand, to produce color magnitude diagrams across the extension of Sgr dSph to study its stellar populations, searching for age and/or composition gradients (or lack thereof). On the other hand, to derive spectroscopic low-resolution radial velocities for a subsample of stars to determine membership to Sgr dSph for the purpose of high resolution spectroscopic follow-up. Methods. We used VIMOS@VLT to produce V and I photometry on 7 fields across the Sgr dSph minor and major axis, plus 3 more centered on the associated globular clusters Terzan 7, Terzan 8 and Arp 2. A last field has been centered on M 54, lying in the center of Sgr dSph. VIMOS high resolution spectroscopic mode has then been used to derive radial velocities for a subsample of the observed stars, concentrating on objects having colors and magnitudes compatible with the Sgr dSph red giant branch. Results. We present photometry for 320 000 stars across the main body of Sgr dSph, one of the richest, and safely the most wide-angle sampling ever produced for this fundamental object. We also provide robust memberships for more than one hundred stars, whose high resolution spectroscopic analysis will be the object of forthcoming papers. Sgr dSph appears remarkably uniform among the observed fields. We confirm the presence of a main Sgr dSph population characterized roughly by the same metallicity of 47 Tuc, but we also found the presence of multiple populations on the peripheral fields of the galaxy, with a metallicity spanning from [Fe/H] = -2.3 to a nearly solar value.

A VIMOS spectroscopy study of photometric variables and straggler candidates inω Centauri

“…Thus, we find that the standard error Δv H in a single velocity measurement based on H β fitting is equal to 15 ± 0.5 km s −1 . A similar accuracy of velocity measurements from VIMOS spectra (10-20 km s −1 ) was reported by Giuffrida et al (2010). A quantitatively similar, but intuitive rather than rigorous estimate of Δv H follows from Fig.…”

“…Typical VIMOS spectra can be calibrated automatically by the ESO-VIMOS pipeline, which cannot, however, be entirely trusted because the spectra of some slits extend below λ = 5000 Å, where calibration lamp lines are scarce 1 , and some of them are very weak. Giuffrida et al (2010) wrote an interactive procedure within the pipeline that provided more control of each calibration step; we decided to reduce the data manually, using standard IRAF 2 procedures. We started from sci_raw frames and proceeded through object identification, slit extraction, flatfielding, science aperture extraction, lamp aperture extraction, wavelength calibration, and normalization of the spectra.…”

Cool carbon stars in the halo and in dwarf galaxies: Hα, colours, and variability

“…The best colour-magnitude diagrams of Sgr indeed show that multiple, young and intermediate-age populations exist in this galaxy with different metallicities (e.g. Siegel et al 2007;Giuffrida et al 2010). However, the Sgr histogram numbers are small.…”

The variability of carbon stars in the Sagittarius dwarf spheroidal galaxy

“…Deep Hubble observations (Siegel et al 2006) have revealed an amazing array of stellar populations of various ages and abundances. Similarly, Giuffrida et al (2010) have detected several populations of different metallicities and presumably ages in a few fields in Sgr. According to Bellazzini et al (2006) the mean age of the population in the central region of Sgr is larger than 5 Gyr with a best estimate of 8 ± 1.5 Gyr.…”

“…The fit yields a distance modulus of 17.01 (25.2 kpc), a value in reasonable agreement with the currently accepted distance for Sgr. Hundreds of giants in the direction of Sgr have published radial velocities; however, large programs such as the one of Frinchaboy et al (2012) or Giuffrida et al (2010) target K and M giants. Very few C stars have radial velocities to confirm their Sgr membership.…”