Segue 1: An Unevolved Fossil Galaxy From the Early Universe

Abstract

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 metalrich 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.

TOPoS

“…Other local galaxies with at least one such star are Draco (Shetrone et al 2001;Fulbright et al 2004;Cohen & Huang 2009), Sagittarius (Zaggia et al 2004;Bonifacio et al 2006;Sbordone et al 2015;and Monaco et al, in prep. ), Fornax (Tafelmeyer et al 2010), Ursa Minor (Kirby & Cohen 2012;Ural et al 2015), Sextans (Aoki et al 2009b;Tafelmeyer et al 2010), Coma Ber (Frebel et al 2010b), Ursa Maior II (Frebel et al 2010b), Segue 1 (Roederer & Kirby 2014;Frebel et al 2014), Hercules (Koch et al 2008;Adén et al 2011), Boo I (Gilmore et al 2013;Ishigaki et al 2014b), and Boo II (Koch & Rich 2014). Particularly relevant to the present discussion is the discovery of a CEMP-no star in the Sculptor dwarf spheroidal (Skúladóttir et al 2015).…”

r-PROCESS PRODUCTION SITES AS INFERRED FROM Eu ABUNDANCES IN DWARF GALAXIES

“…We do not consider Sagitarius, since this galaxy has been shown to be considerably tidally disrupted (Ibata et al 1995;Mateo et al 1996;Majewski et al 2003), and its mass is therefore very uncertain. Table 1 shows the V-band luminosities, L V , the velocity dispersions, σ, and the half light radii, r 1/2 taken from Walker et al (2009Walker et al ( , 2015, and the average Fe and Eu abundances (Shetrone et al 2001(Shetrone et al , 2003Frebel et al 2010;Letarte et al 2010;Frebel et al 2014;Ji et al 2016a,b;Roederer & Kirby 2014;Tsujimoto et al 2015;Roederer et al 2016). The errors of the average abundances shown in …”

“…Recently, Ji et al (2016a) and Roederer et al (2016) reported the first discovery of highly r-process enriched stars in a UFD, Reticulum II, while there are only strong upper limits on the abundances of r-process elements for some other UFDs (Frebel et al 2010(Frebel et al , 2014Roederer & Kirby 2014). These measurements that show large fluctuations in the r-process abundance imply that only a single r-process event took place in Reticulum II.…”

OBSERVATIONAL CONSTRAINTS ON FIRST-STAR NUCLEOSYNTHESIS. II. SPECTROSCOPY OF AN ULTRA METAL-POOR CEMP-no STAR*

“…We used the compilation of line lists by Frebel et al (2014), based on lines from Aoki et al (2002), Barklem et al (2005), Lawler et al (2009), and the VALD database (Kupka et al 1999). The neutron-capture absorption lines in the blue spectral region, particularly close to strong CH or CN features, need to be carefully synthesized, since these are intrinsically weak in CEMP-no stars.…”