Chemical Abundances in NGC 5053: A Very Metal-Poor and Dynamically Complex Globular Cluster

Abstract

NGC 5053 provides a rich environment to test our understanding of the complex evolution of globular clusters (GCs). Recent studies have found that this cluster has interesting morphological features beyond the typical spherical distribution of GCs, suggesting that external tidal effects have played an important role in its evolution and current properties. Additionally, simulations have shown that NGC 5053 could be a likely candidate to belong to the Sagittarius dwarf galaxy (Sgr dSph) stream. Using the Wisconsin-Indiana-Yale-NOAO-Hydra multi-object spectrograph, we have collected high quality (signal-to-noise ratio ∼ 75-90), medium-resolution spectra for red giant branch stars in NGC 5053. Using these spectra we have measured the Fe, Ca, Ti, Ni, Ba, Na, and O abundances in the cluster. We measure an average cluster [Fe/H] abundance of −2.45 with a standard deviation of 0.04 dex, making NGC 5053 one of the most metal-poor GCs in the Milky Way (MW). The [Ca/Fe], [Ti/Fe], and [Ba/Fe] we measure are consistent with the abundances of MW halo stars at a similar metallicity, with alphaenhanced ratios and slightly depleted [Ba/Fe]. The Na and O abundances show the Na-O anti-correlation found in most GCs. From our abundance analysis it appears that NGC 5053 is at least chemically similar to other GCs found in the MW. This does not, however, rule out NGC 5053 being associated with the Sgr dSph stream.

Chemical abundances of giant stars in NGC 5053 and NGC 5634, two globular clusters associated with the Sagittarius dwarf spheroidal galaxy?

“…The other element that is distinctly different between the two analyses is Ca. Boberg et al (2015) [Si/Fe] in this star (also derived from nine lines), we believe that our result is very robust. Given that no details on the lines used are given in the preprint, we cannot make a hypothesis about the reason of this discrepancy.…”

“…The high Na abundance in star NGC 5053-69 is also not consistent with the value expected in a first-generation star, indicatingin agreement with Boberg et al (2015) -that NGC 5053 hosts multiple stellar populations as well.…”

“…While the present paper was undergoing the refereeing process, an independent analysis of NGC 5053 based on medium resolution spectra (R ∼ 13 000) appeared as a preprint (Boberg et al 2015). They observed star NGC 5053-69, (star 6 in their list).…”

“…We did not use the same lines: they used the 616.2 nm line, while we used the 568.2 nm and 568.8 nm lines. We also adopted different NLTE corrections; Boberg et al (2015) used those of Gratton et al (1999). The other element that is distinctly different between the two analyses is Ca.…”

A Model for Clumpy Self-enrichment in Globular Clusters

“…However, the observational data themselves show a more nuanced story. Observations of internal iron spreads are plotted in Figure 5(a) in blue and red for metal-poor and metal-rich clusters respectively, from the compilation of Willman & Strader (2012) supplemented by more recent observations (Kacharov et al 2013;Da Costa et al 2014;Boberg et al 2015;Marino et al 2015;Dalessandro et al 2016 , and for the mean MMR among the metal-poor clusters around giant elliptical galaxies in the ACS Fornax Cluster Survey (Mieske et al 2010;yellow). The black diamonds denote the particular model clusters whose MDFs are shown in Figure 3.…”