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Register a new userProbing the Intermediate-Age Globular Clusters in NGC 5128 from Ultraviolet Observations
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We explore the age distribution of the globular cluster (GC) system of the nearby elliptical galaxy NGC 5128 using ultraviolet (UV) photometry from Galaxy Evolution Explorer (GALEX) observations, with UV - optical colors used as the age indicator. Most GCs in NGC 5128 follow the general trends of GCs in M31 and Milky Way in UV - optical color-color diagram, which indicates that the majority of GCs in NGC 5128 are old similar to the age range of old GCs in M31 and Milky Way. A large fraction of spectroscopically identified intermediate-age GC (IAGC) candidates with ~ 3-8 Gyr are not detected in the FUV passband. Considering the nature of intermediate-age populations being faint in the far-UV (FUV) passband, we suggest that many of the spectroscopically identified IAGCs may be truly intermediate in age. This is in contrast to the case of M31 where a large fraction of spectroscopically suggested IAGCs are detected in FUV and therefore may not be genuine IAGCs but rather older GCs with developed blue horizontal branch stars. Our UV photometry strengthens the results previously suggesting the presence of GC and stellar subpopulation with intermediate age in NGC 5128. The existence of IAGCs strongly indicates the occurrence of at least one more major star formation episode after a starburst at high redshift.
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We present spectroscopy for globular clusters (GCs) in the elliptical galaxy NGC 4365, obtained with the LRIS spectrograph on the Keck I telescope. Previous studies have shown that the optical color distribution of GCs in NGC 4365 lacks the bimodal structure that is common in globular cluster systems, showing only a single broad peak. Measurements of Balmer line indices (Hbeta, Hgamma and Hdelta) on the GC spectra support recent suggestions by Puzia et al., based on optical and near-infrared photometry, that some of the clusters in NGC 4365 are intermediate-age (2-5 Gyrs) and metal-rich (-0.4<[Z/H]<0) rather than old (~10-15 Gyrs) and metal-poor. We also find some genuinely metal-poor, old clusters, suggesting that the ages and metallicities of the two populations conspire to produce the single broad distribution observed in optical colors.
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