The Ages, Metallicities, and Alpha Element Enhancements of Globular Clusters in the Elliptical NGC 5128: A Homogeneous Spectroscopic Study With Gemini/Gemini Multi-Object Spectrograph

Abstract

We present new integrated light spectroscopy of globular clusters in NGC 5128, a nearby giant elliptical galaxy less than 4 Mpc away, in order to measure radial velocities and derive ages, metallicities, and alpha-element abundance ratios. Using the Gemini South 8-meter telescope with the instrument GMOS, we obtained spectroscopy in the range of ∼ 3400 − 5700Å for 72 globular clusters with signal-to-noise greater than 30Å −1 and we have also discovered 35 new globular clusters within NGC 5128 from our radial velocity measurements. We measured and compared the Lick indices from Hδ A through Fe5406 with the single stellar population models of Thomas et al. (2003) and Thomas et al. (2004) in order to derive age, metallicity and [α/Fe] values. We also measure Lick indices for 41 Milky Way globular clusters from Puzia et al. (2002) and Schiavon et al. (2005) with the same methodology for direct comparison. Our results show that 68% of the NGC 5128 GCs have old ages (> 8 Gyr), 14% have intermediate ages (5 − 8 Gyr), and 18% have young ages (< 5 Gyr). However, when we look at the metallicity of the globular clusters as a function of age, we find 92% of metalpoor GCs and 56% of metal-rich GCs in NGC 5128 have ages > 8 Gyr, indicating that the majority of both metallicity subpopulations of globular clusters formed early, with a significant population of young and metal-rich globular clusters forming later. Our metallicity distribution function generated directly from spectroscopic Lick indices is clearly bimodal, as is the color distribution of the same set of globular clusters. Thus the metallicity bimodality is real and not an artifact of the color to metallicity conversion. However, the metallicity distribution function obtained from comparison with the single stellar population models is consistent with a unimodal, bimodal, or multimodal shape. The [α/Fe] values are supersolar with a mean value of 0.14 ± 0.04, indicating a fast formation timescale. However, the GCs in NGC 5128 are not as [α/Fe] enhanced as the Milky Way GCs also examined in this study. Our measured indices also indicate that the globular clusters in NGC 5128 may have a slight overabundance in nitrogen and a wider range of calcium strength compared to the Milky Way globular clusters. Our results support a rapid, early formation of the globular cluster system in NGC 5128, with subsequent major accretion and/or GC and star forming events in more recent times.

How old are the stars in the halo of NGC 5128 (Centaurus A)?

“…In the next section we explore whether the fits are improved by adding a second age component, hence simulating a two bursts star formation history. This is motivated also from other observations: NGC 5128 is likely to have experienced a history of satellite accretions (minor mergers), but also some previous observations of resolved stellar populations and globular clusters have implied a smaller population of younger stars with ages close to ∼3−5 Gyr (Soria et al 1996;Marleau et al 2000;Rejkuba et al 2003;Woodley et al 2010b). …”

“…Further observations of the stellar populations since that time have continued to support its underlying normality. Direct spectroscopic measurements of the ages and compositions of its globular clusters throughout the bulge and halo (Peng et al 2004a;Beasley et al 2008;Woodley et al 2010b) show that their age distribution has a clearly wider range than is the case for the Milky Way clusters. But the great majority of them are older than τ > ∼ 8 Gy, with a small fraction that may have arisen in later formation events.…”

Two formation channels of ultra-compact dwarf galaxies in Hickson compact groups

“…Globular clusters, on average, have super-solar [α/Fe], being "alpha enhanced" (Puzia et al 2006;Colucci et al 2009;Woodley et al 2010;Taylor et al 2010), reflecting their short formation time-scales. UCDs present a broader range of [α/Fe] abundances which varies as a function of environment (Mieske et al 2007;Hau et al 2009).…”

Massive star clusters in galaxies

“…The possibility that most of the metal-rich clusters in elliptical galaxies formed through major mergers (Ashman & Zepf 1992) does not, by itself, solve the original 'S N problem' because such large amounts of incoming gas are needed to make the thousands of GCs in giant ellipticals at any plausible formation efficiency that this version of their origin becomes similar to basic hierarchical merging (see Harris 2001). The Woodley et al 2010) and for a sample of halo field stars (dotted line, Rejkuba et al 2005).…”