The chemical evolution of subpopulations in the Carina dwarf spheroidal

Abstract

Abstract. We present the observed metallicity distribution for 437 red giants in the Carina dwarf spheroidal (dSph) galaxy that was derived from FLAMES medium-resolution spectroscopy of the near-infrared Ca triplet (CaT). We find a mean [Fe/H] of −1.7 dex and a full spread in metallicities of at least one dex. Most of this width is due to the occurrence of three populations at different [Fe/H], which are likely to be related to the three star forming (SF) episodes that governed Carina's evolution and also correlate with peaks in the age-distribution. Both the lack of any correlation between stellar colour and metallicity and a comparison of the metallicities with chemical evolution models, which suggest a major role of infalling gas, contribute to the detailed understanding of Carina's evolutionary history. Moreover, we find a mild dependence of metallicity on radial location, reflected in a shift of the distribution's peak towards the metal poor end in the outer regions.Keywords. galaxies: abundances, galaxies: evolution, galaxies: dwarf Stellar populations in CarinaPhotometric analyses of the faint Carina dSph have disclosed the presence of various stellar populations (e.g., Monelli et al. 2003), exhibiting a prominent old (>11 Gyr), intermediate-age (5-6 Gyr), and a young (3 Gyr) population. This implies that Carina must have undergone several SF episodes with at least three significant pulses. In apparent defiance of the resulting spread in ages, the galaxy's colour-magnitude diagram (CMD) features a remarkably narrow red giant branch (RGB). A reason for that can be a counteracting spread in metallicities, where metal rich, young stars have colours comparable to the older, more metal poor ones. Such an undesired age-metallicity degeneracy can be resolved if independent, accurate [Fe/H] measurements are available so that stellar ages as the remaining parameter can be obtained from isochrones. Moreover, the overall shape and possible spatial variations of the metallicity distribution functions (MDFs) themselves provide vital ingredients for analysing Carina's unusual SF history.With the aim of studying the chemical and kinematical properties of Carina's separate populations we launched an ESO Large Programme. In this vein, spectroscopic observations of five fields in Carina were performed, comprising 1257 red giants out to the tidal radius, thus providing a wide spatial coverage and good number statistics. Our observations were performed during 23 nights with the multi-object spectrograph FLAMES at the VLT in medium-resolution mode (grating LR 8), centered at the near-infrared CaT around 860 nm. For details on the data reduction and calibration see Koch et al., 134 at https://www.cambridge.org/core/terms. https://doi

Milky Way versus Andromeda: a tale of two disks

“…On the other hand, Koch et al (2005) found that for the Carina dSph, there is a larger age spread (from 2 Gyr to more than 11 Gyr), while its colour-magnitude diagram shows a narrow RGB distribution. The large age spread implies that Carina dSph must have undergone various episodes of star formation process.…”

Dwarf spheroidals in MOND

“…2). In any case, Koch et al (2007) make it clear from their analysis of Leo II that there is compelling evidence for velocity anisotropy in dwarfs.…”

“…Note that although there are two parameters for the anisotropy, the profile required is virtually the same for 6/8 dwarfs (see fig 2). In any case, Koch et al (2007) make it clear from their analysis of Leo II that there is compelling evidence for velocity anisotropy in dwarfs.…”

Baryonic Properties of the Darkest Galaxies

“…The last few years have seen a substantial increase in kinematic data for dSphs, largely thanks to large optical telescopes such as ESO's Very Large Telescope (VLT), Keck, Gemini, and Magellan. Programs such as our VLT Large Programme Wilkinson et al 2006;Koch et al 2006b) concentrated on obtaining radial velocity dispersion profiles for as many stars as possible out to the optical boundaries of the target dSphs. Results of our program are also described in the contribution by Wyse & Gilmore in these proceedings.…”