Elemental abundance investigation of two candidate extragalactic globular clusters (NGC 5024, NGC 5466)

• Main Author: Chutter, Ashley
• Other Authors: Venn, Kim
• Language: English; en
• Published: 2009
• Subjects: globular cluster; elemental abundances; hierarchical clustering; accretion; dwarf galaxy; Milky Way; galaxy; star; stellar atmosphere; spectra; metallicity; extragalactic; NGC 5466; NGC 5024; Sagittarius; tidal tail; UVic Subject Index::Sciences and Engineering::Physics::Astronomy
• Online Access: http://hdl.handle.net/1828/1355

High resolution spectra have been analyzed for two and three stars respectively in the candidate extragalactic globular clusters, NGC 5024 and NGC 5466, with the High-Resolution Spectrograph on the 9.2 m Hobby-Eberly Telescope. The goal of this investigation is to evaluate the proposed extragalactic origins of these two globular clusters. Evidence of a tidal tail in NGC 5466 (Belokurov et al., 2006) and the association of NGC 5024 with the Sagittarius stream (Martinez-Delgado et al., 2004) targeted the clusters as likely remnants of recent accretion events and thus potentially of extragalactic origin. Determination of their chemical abundance patterns could provide unique evidence to either support or dispute these claims. NGC 5024 has been associated with a proposed wrap in the Sagittarius stream which could be supported if the chemistry of NGC 5024 is similar to other clusters associated with the stream. NGC 5466 has the longest tidal tail known, which hints at an origin in a now dispersed dwarf spheroidal galaxy. Additional evidence for these clusters' capture origins has been compiled by Yoon & Lee (2002), demonstrating that these two low metallicity clusters, along with five others, are aligned in a single highly inclined plane in the outer halo. Confirmation that these clusters are remnants of dwarf galaxies would support a Galactic history which includes recent accretion events. Such evidence may bolster the cold dark matter hierarchical clustering scenario, which postulates the presence of a significant amount of substructure in the Milky Way. Unfortunately, at the metallicity of the target clusters ([Fe/H] = -1.9), the chemical distinction between Galactic stars and known dSph stars is not significant. The low [alpha/Fe] of dSph stars seen at higher metallicity is not apparent at [Fe/H] = -1.9 in either Galactic or dSph stars. Aside from a few mild discrepancies, NGC 5024 and NGC 5466 appear chemically similar to the Galactic field stars and globular clusters compiled by Pritzl et al. (2005). A moderate enhancement in the [Ba/Y] ratios relative to the halo field stars is the only positively detected chemical signature that is typically observed in dSph stars. Comparisons with Galactic GCs of similar age, metallicity and horizontal branch morphology (NGC 2298, NGC 6397 and NGC 5897) reveal a few other differences, but these could be attributed to systematic effects in the different analysis techniques. Although NGC 5024 has a similar metallicity to the GC Arp 2 that was stripped from the merging Sagittarius dwarf, neither Arp 2 (Mottini et al., 2008) nor the clusters in this study show any particularly unusual chemical abundance patterns. Thus, no conclusive evidence in support of or in opposition to the target clusters' proposed extragalactic origins has been discovered.