Segue 1: An unevolved fossil galaxy from the early universe

Journal Article · Thu May 01 04:00:00 EDT 2014 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/786/1/74· OSTI ID:22357007

Frebel, Anna ^[1]; Simon, Joshua D. ^[2]; Kirby, Evan N. ^[3]

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1. Kavli Institute for Astrophysics and Space Research and Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139 (United States)
2. Observatories of the Carnegie Institution of Washington, Pasadena, CA 91101 (United States)
3. Center for Galaxy Evolution, Department of Physics and Astronomy, University of California, Irvine, CA 92697 (United States)

We present Magellan/MIKE and Keck/HIRES high-resolution spectra of six red giant stars in the dwarf galaxy Segue 1. Including one additional Segue 1 star observed by Norris et al., high-resolution spectra have now been obtained for every red giant in Segue 1. Remarkably, three of these seven stars have metallicities below [Fe/H] = –3.5, suggesting that Segue 1 is the least chemically evolved galaxy known. We confirm previous medium-resolution analyses demonstrating that Segue 1 stars span a metallicity range of more than 2 dex, from [Fe/H] = –1.4 to [Fe/H] = –3.8. All of the Segue 1 stars are α-enhanced, with [α/Fe] ∼ 0.5. High α-element abundances are typical for metal-poor stars, but in every previously studied galaxy [α/Fe] declines for more metal-rich stars, which is typically interpreted as iron enrichment from supernova Ia. The absence of this signature in Segue 1 indicates that it was enriched exclusively by massive stars. Other light element abundance ratios in Segue 1, including carbon enhancement in the three most metal-poor stars, closely resemble those of metal-poor halo stars. Finally, we classify the most metal-rich star as a CH star given its large overabundances of carbon and s-process elements. The other six stars show remarkably low neutron-capture element abundances of [Sr/H] < –4.9 and [Ba/H] < –4.2, which are comparable to the lowest levels ever detected in halo stars. This suggests minimal neutron-capture enrichment, perhaps limited to a single r-process or weak s-process synthesizing event. Altogether, the chemical abundances of Segue 1 indicate no substantial chemical evolution, supporting the idea that it may be a surviving first galaxy that experienced only one burst of star formation.

• OSTI ID: 22357007
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 1 Vol. 786; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
CAPTURE
CARBON
COMPARATIVE EVALUATIONS
ELEMENT ABUNDANCE
GALAXIES
IRON
METALLICITY
NEUTRON REACTIONS
R PROCESS
RED GIANT STARS
RESOLUTION
S PROCESS
SPECTRA
TRITIUM
UNIVERSE
VISIBLE RADIATION