Chemical Abundances in the Ultra-faint Dwarf Galaxies Grus I and Triangulum II: Neutron-capture Elements as a Defining Feature of the Faintest Dwarfs*

Abstract

We present high-resolution spectroscopy of four stars in two candidate ultra-faint dwarf galaxies (UFDs), Grus I (Gru I) and Triangulum II (Tri II). Neither object currently has a clearly determined velocity dispersion, placing them in an ambiguous region of parameter space between dwarf galaxies and globular clusters. No significant metallicity difference is found for the two Gru I stars, but both stars are deficient in neutron-capture elements. We verify previous results that Tri II displays significant spreads in metallicity and [α/Fe]. Neutron-capture elements are not detected in our Tri II data, but we place upper limits at the lower envelope of Galactic halo stars, consistent with previous very low detections. Stars with similarly low neutron-capture element abundances are common in UFDs, but rare in other environments. This signature of low neutron-capture element abundances traces chemical enrichment in the least massive star-forming dark matter halos, and further shows that the dominant sources of neutron-capture elements in metal-poor stars are rare. In contrast, all known globular clusters have similar ratios of neutron-capture elements to those of halo stars, suggesting that globular cluster do not form at the centers of their own dark matter halos. The low neutron-capture element abundances may be the strongest evidence that Gru I and Tri II are (or once were) galaxies rather than globular clusters, and we expect future observations of these systems to robustly find non-zero velocity dispersions or signs of tidal disruption. However, the nucleosynthetic origin of this low neutron-capture element floor remains unknown.

Chemical Abundance Analysis of Tucana III, the Second r-process Enhanced Ultra-faint Dwarf Galaxy*

“…Finally, while it is true that globular clusters generally have neutron-capture enhancement similar to that observed here (and very different from other ultra-faint dwarf galaxies, see Ji et al 2018, for example), we do not feel that this commonality is enough to claim that Tuc III is more likely to be a globular cluster. Furthermore, Tuc III's average metallicity, [Fe/H]∼ −2.49 (Li et al 2018a), would place it on the extreme low end of the distribution of globular cluster metallicities: the lowest metallicity globular cluster in the Harris (1996Harris ( , 2010 catalog is NGC 7078 with [Fe/H]=−2.37.…”

“…Interestingly, two of the recently discovered southern hemisphere ultra-faint dwarf galaxies, Ret II and Tuc III, have now been shown to have multiple stars enhanced in r-process elements to a greater or lesser extent. The other DESdiscovered ultra-faint dwarfs that have been studied chemically to date, Tuc II (Ji et al 2016b), Gru I (Ji et al 2018), and Hor I (Nagasawa et al 2018), do not show r-process enhancement. Additional ultra-faint dwarfs have member stars that are bright enough to be studied chemically, and may further add to the census of r-process enhanced galaxies.…”

High-resolution Optical Spectroscopy of Stars in the Sylgr Stellar Stream*

“…Stars in dwarf galaxies, on the other hand, exhibit a wide range of metallicities and ratios among their metals. A quick scan of figures 2 and 4 of Ji et al (2019), for example, reveals that it is rare-but not unprecedented-for two randomly-selected stars within a given UFD galaxy to have the same composition. We explore in this section how the chemistry of the two Sylgr stream stars compares to that of metal-poor field stars and surviving GCs, UFD galaxies, and dSph galaxies.…”

The southern stellar stream spectroscopic survey (S5): Overview, target selection, data reduction, validation, and early science

“…We adopt the standard deviation of Fe I abundances as the [Fe/H] uncertainty. Typical systematic uncertainties are 150 K, 0.3 dex, 0.2 km s −1 , and 0.1 dex for effective temperature, surface gravity, microturbulence, and metallicity, respectively (see Ji et al 2019 for details).…”