HIGH-RESOLUTION SPECTROSCOPY OF EXTREMELY METAL-POOR STARS IN THE LEAST EVOLVED GALAXIES: URSA MAJOR II and COMA BERENICES

By A. Frebel, J. Simon, M. Geha, B. Willman. 2009

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We present spectra of six metal-poor stars in two of the ultra-faint dwarf galaxies orbiting the Milky Way (MW), Ursa Major II, and Coma Berenices obtained with the Keck/High Resolution Echelle Spectrometer (HIRES). These observations include the first high-resolution spectroscopic observations of extremely metal-poor ([Fe/H] < −3.0) stars not belonging to the MW halo field star population. We obtain abundance measurements and upper limits for 26 elements between carbon and europium. The entire sample of stars spans a range of −3.2 < [Fe/H] < −2.3, and we confirm that each galaxy contains a large intrinsic spread of Fe abundances. A comparison with MW halo stars of similar metallicities reveals substantial agreement between the abundance patterns of the ultra-faint dwarf galaxies and the MW halo for the light, α, and iron-peak elements (C to Zn). This agreement contrasts with the results of earlier studies of more metal-rich stars (−2.5 ≲ [Fe/H] ≲ −1.0) in more luminous dwarf spheroidal galaxies, which found significant abundance discrepancies with respect to the MW halo data. The abundances of neutron-capture elements (Sr to Eu) in the ultra-faint dwarf galaxies are extremely low, consistent with the most metal-poor halo stars, but not with the typical halo abundance pattern at [Fe/H] ≳ −3.0. Not only are our results broadly consistent with a galaxy formation model that predicts that massive dwarf galaxies are the source of the metal-rich component ([Fe/H]> − 2.5) of the MW halo, but they also suggest that the faintest known dwarfs may be the primary contributors to the metal-poor end of the MW halo metallicity distribution.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/6212d8c43b64d153f253183b8b32f90042b54809

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Citations in the corpus

MULTI-ELEMENT ABUNDANCE MEASUREMENTS FROM MEDIUM-RESOLUTION SPECTRA. II. CATALOG OF STARS IN MILKY WAY DWARF SATELLITE GALAXIES (2010, background): “As another example of a trend seen with the source of the HRS measurements, consider the study of Frebel et al. (2010b).”
DETAILED CHEMICAL ABUNDANCES IN THE r-PROCESS-RICH ULTRA-FAINT DWARF GALAXY RETICULUM 2 (2016, background): “A few CEMP stars have been identified in the UFD galaxies (Frebel et al. 2010; Norris et al. 2010b; Lai et al. 2011; Gilmore et al. 2013), and we add one star in Ret 2 to this small but growing inventory.”
THE QUENCHING OF THE ULTRA-FAINT DWARF GALAXIES IN THE REIONIZATION ERA (2014, result): “…2008; Martin et al. 2008a; Greco et al. 2008; Muñoz et al. 2010; Weisz et al. 2014b), while spectroscopy of their giant stars indicates low metallicities, but with a dispersion significantly larger than the measurement errors (Frebel et al. 2010; Norris et al. 2010; Kirby et al. 2008, 2011, 2013).”
MULTI-ELEMENT ABUNDANCE MEASUREMENTS FROM MEDIUM-RESOLUTION SPECTRA. III. METALLICITY DISTRIBUTIONS OF MILKY WAY DWARF SATELLITE GALAXIES (2010, background): “Since then, extremely metal-poor stars have been discovered in dSphs (Kirby et al. 2008b, 2009; Geha et al. 2009; Cohen & Huang 2009, 2010; Frebel et al. 2010a,b; Simon et al. 2010a,b; Norris et al. 2010a,b; Starkenburg et al. 2010; Tafelmeyer et al. 2010).”
THE SEGUE STELLAR PARAMETER PIPELINE. V. ESTIMATION OF ALPHA-ELEMENT ABUNDANCE RATIOS FROM LOW-RESOLUTION SDSS/SEGUE STELLAR SPECTRA (2010, background): “…2007, 2008, 2010) are quite similar to those observed for stars in the Galactic halo with [Fe/H] < −2.5, strengthening the argument of Carollo et al. (2007, 2010) and Frebel et al. (2010) that dSphs may well have contributed to (at least) the formation of the outer-halo component of the Galaxy.”
The birth of a galaxy – II. The role of radiation pressure (2012, background): “…their [α/Fe] versus [Fe/H] evolution (Tolstoy, Hill & Tosi 2009), metallicity distribution functions (Kirby et al. 2011), metallicity radial gradients (Tolstoy et al. 2004), star formation histories (Monelli et al. 2010), and individual abundance patterns (Frebel et al. 2010; Caffau et al. 2011).”
METALLICITY EVOLUTION OF DAMPED Lyα SYSTEMS OUT TO z ∼ 5 (2012, background): “…dwarf spheroidal galaxies were originally thought to lack stars with [Fe/H]∼ −3 (Helmi et al. 2006), fainter systems have sufficiently low metallicity that they could be contributors to the Galactic halo (Kirby et al. 2008; Koch et al. 2008; Norris et al. 2008; Frebel et al. 2010; Lai et al. 2011).”