A Chemical Confirmation of the Faint Boötes Ii Dwarf Spheroidal Galaxy

Abstract

We present a chemical abundance study of the brightest confirmed member star of the ultrafaint dwarf galaxy Boötes II from Keck/HIRES high-resolution spectroscopy at moderate signal-to-noise ratios. At [Fe/H] = −2.93 ± 0.03(stat.)±0.17(sys.) this star chemically resembles metal-poor halo field stars and the signatures of other faint dwarf spheroidal galaxies at the same metallicities in that it shows enhanced [α/Fe] ratios, Solar Fe-peak element abundances, and low upper limits on the neutron-capture element Ba. Moreover, this star shows no chemical peculiarities in any of the eight elements we were able to measure. This implies that the chemical outliers found in other systems remain outliers pertaining to the unusual enrichment histories of the respective environments, while Boo II appears to have experienced an enrichment history typical of its very low mass. We also re-calibrated previous measurements of the galaxy's metallicity from the calcium triplet (CaT) and find a much lower value than reported before. The resulting broad metallicity spread, in excess of one dex, the very metal poor mean, and the chemical abundance patterns of the present star imply that Boötes II is a low-mass, old, metal poor dwarf galaxy and not an overdensity associated with the Sagittarius Stream as has been previously suggested based on its sky position and kinematics. The low, mean CaT metallicity of −2.7 dex falls right on the luminosity-metallicity relation delineated over four orders of magnitude from the more luminous to the faintest galaxies. Thus Boötes II's chemical enrichment appears representative of the galaxy's original mass, while tidal stripping and other mass loss mechanisms were probably not significant as for other low-mass satellites.

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).…”

Beasts of the Southern Wild: Discovery of Nine Ultra Faint Satellites in the Vicinity of the Magellanic Clouds

“…What sets Segue 1, 2 and Bootes 2 apart from similar looking star clusters are their chemical abundances. All three objects appear to have large metallicity spreads (e.g., Kirby et al 2013;Koch & Rich 2014;Frebel et al 2014), previously unnoticed in star clusters.…”

Abundance ratios of red giants in low-mass ultra-faint dwarf spheroidal galaxies

“…This determination relies on an old calibration of the Ca triplet, which was later revised. Koch & Rich (2014) made a detailed chemical analysis of the brightest confirmed member star in Boo II using Keck/Hires and derived a very low metallicity of [Fe/H] = −2.93 dex using an updated Ca triplet calibration. They also found a high [α/Fe] ratio that is compatible with the α-enhanced plateau value of the galactic halo.…”

“…As the second star has been already observed by Koch & Rich (2014), we put the results from both studies into Table A.7. The results are in general good agreement.…”