"Sculptor-ing" the Galaxy? The Chemical Compositions of Red Giants in the Sculptor Dwarf Spheroidal Galaxy

Abstract

We used high-resolution, high signal-to-noise ratio spectra obtained with the Very Large Telescope and the UVVisual Echelle Spectrograph to determine abundances of 17 elements in four red giants in the Sculptor (Scl ) dwarf spheroidal galaxy. Our ½Fe=H -values range from À2.10 to À0.97, confirming previous findings of a large metallicity spread. We combined our data with similar data for five Scl giants studied recently by Shetrone et al. to form one of the largest samples of high-resolution abundances yet obtained for a dwarf spheroidal galaxy, covering essentially the full known metallicity range in this galaxy. These properties allow us to establish trends of ½X=Fe with ½Fe=H for many elements X. The trends are significantly different from the trends seen in Galactic halo and globular cluster stars. This conclusion is evident for most of the elements from oxygen to manganese. We compare our Scl sample with the most similar Galactic counterparts and find substantial differences remain even with these stars. The many discrepancies in the relationships between ½X=Fe as seen in Scl compared with Galactic field stars indicate that our halo cannot be made up in bulk of stars similar to those presently seen in dwarf spheroidal galaxies like Scl, corroborating similar conclusions reached by Shetrone et al., Fulbright, and Tolstoy et al. These results have serious implications for the Searle-Zinn and hierarchical galaxy formation scenarios. We also find that the most metal-rich star in our sample is a heavy element-rich star. This star and the ½Ba=Eu trend we see indicate that asymptotic giant branch stars must have played an important role in the evolution of the s-process elements in Scl. A very high percentage of such heavy-element stars are now known in dwarf spheroidals compared with the halo, further mitigating against the formation of the halo from such objects.

The dynamical and chemical evolution of dwarf spheroidal galaxies

“…The spread in stellar chemical abundances, and in particular the low [α/Fe] values compared to Galactic halo stars at equal metallicity, are hardly compatible with an early termination of star formation at the epoch of reionization (e.g., Harbeck et al 2001; Shetrone et al 1998Shetrone et al , 2001Shetrone et al , 2003Tolstoy et al 2003;Geisler et al 2005; Koch et al 2008, examples taken in relation to the galaxies studied in this work). Whilst some dSphs are indeed consistent with rather short star formation episodes, such as Sextans (Lee et al 2003) or Sculptor (Babusiaux et al 2005), others are characterized by much more extended periods, like Carina (Smecker-Hane et al 1996;Hurley-Keller et al 1998) or Fornax (Coleman & de Jong 2008).…”

Neutron-capture element deficiency of the Hercules dwarf spheroidal galaxy

“…also include, for comparison, Galactic stars from the compilation of Venn et al (2004;and references therein) and Milky Way dSph satellites including the more luminous dSphs (Shetrone et al 2001(Shetrone et al , 2003(Shetrone et al , 2009Fulbright et al 2004;Sadakane et al 2004;Geisler et al 2005;Aoki et al 2009;Cohen & Huang 2009;Frebel et al 2010a;Letarte et al 2010;Tafelmeyer et al 2010;Lemasle et al 2012;Venn et al 2012;and Starkenburg et al 2012) and the most recent measurements in a few ultrafaint dSphs (K08; Feltzing et al 2009;Frebel et al 2010b;Norris et al 2010b;Simon et al 2010;and Gilmore et al 2013). The Ba abundances and limits are very low, at a mean [Ba/H] of −3.97 from the three EW measurements, and [Ba/H] < −3.84 (2σ), <−3.55 (3σ), respectively.…”

VLT/FLAMES spectroscopy of red giant branch stars in the Carina dwarf spheroidal galaxy

“…The shaded areas represent the expected age ranges of star formation as indicated by photometric studies. Geisler et al, 2005;Hill et al, 2011) are shown as small green dots and Milky Way halo stars as small grey dots. …”