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Register a new userA 2dF spectroscopic study of globular clusters in NGC 5128: Probing the formation history of the nearest giant Elliptical
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We have performed a spectroscopic study of globular clusters (GCs) in the giant elliptical NGC 5128 using the 2dF facility at the Anglo-Australian telescope. We obtained integrated optical spectra for a total of 254 GCs, 79 of which are newly confirmed on the basis of their radial velocities and spectra. In addition, we obtained an integrated spectrum of the galaxy starlight along the southern major axis. We derive an empirical metallicity distribution function (MDF) for 207 GCs (~14 of the estimated total GC system) based upon Milky Way GCs. This MDF is multimodal at high statistical significance with peaks at [Z/H]~-1.3 and -0.5. A comparison between the GC MDF and that of the stellar halo at 20 kpc (~4 Reff) reveals close coincidence at the metal-rich ends of the distributions. However, an inner 8 kpc stellar MDF shows a clear excess of metal-rich stars when compared to the GCs. We compare a higher S/N subsample (147 GCs) with two stellar population models which include non-solar abundance ratio corrections. The vast majority of our sample (~90%) appears old, with ages similar to the Milky Way GC system. There is evidence for a population of intermediate-age (~4-8 Gy) GCs (<15% of the sample) which are on average more metal-rich than the old GCs. We also identify at least one younger cluster (~1-2 Gy) in the central regions of the galaxy. Our observations are consistent with a picture where NGC 5128 has undergone at least two mergers and/or interactions involving star formation and limited GC formation since z=1, however the effect of non-canonical hot stellar populations on the integrated spectra of GCs remains an outstanding uncertainty in our GC age estimates.
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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.
The distance to NGC 5128, the central galaxy of the Centaurus group and the nearest giant elliptical to us, has been determined using two independent distance indicators: the Mira period-luminosity (PL) relation and the luminosity of the tip of the red giant branch (RGB). The data were taken at two different locations in the halo of NGC 5128 with the ISAAC near-IR array on ESO VLT. From more than 20 hours of observations with ISAAC a very deep Ks-band luminosity function was constructed. The tip of the RGB is detected at Ks=21.24 pm 0.05 mag. Using an empirical calibration of the K-band RGB tip magnitude, and assuming a mean metallicity of [M/H]=-0.4 dex and reddening of E(B-V)=0.11, a distance modulus of NGC 5128 of (m-M)_0=27.87 pm 0.16 was derived. The comparison of the H-band RGB tip magnitude in NGC 5128 and the Galactic Bulge implies a distance modulus of NGC 5128 of (m-M)_0=27.9 pm 0.2 in good agreement with the K-band RGB tip measurement. The population of stars above the tip of the RGB amounts to 2176 stars in the outer halo field and 6072 stars in the inner halo field. The large majority of these sources belong to the asymptotic giant branch (AGB) population in NGC 5128 with numerous long period variables. Mira variables were used to determine the distance of NGC 5128 from a period-luminosity relation calibrated using the Hipparcos parallaxes and LMC Mira period-luminosity relation in the K-band. This is the first Mira period-luminosity relation outside the Local Group. A distance modulus of 27.96 pm 0.11 was derived, adopting the LMC distance modulus of 18.50 pm 0.04. The mean of the two methods yields a distance modulus to NGC 5128 of 27.92 pm 0.19 corresponding to D=3.84 pm 0.35 Mpc.
Globular clusters (GCs) are thought to be ancient relics from the early formative phase of galaxies, although their physical origin remains uncertain. GCs are most numerous around massive elliptical galaxies, where they can exhibit a broad colour dispersion, suggesting a wide metallicity spread. Here, we show that many thousands of compact and massive (~5$times$10$^{rm 3}-$3$times$ 10$^{rm 6} M_{odot}$) star clusters have formed at an approximately steady rate over, at least, the past ~1Gyr around NGC 1275, the central giant elliptical galaxy of the Perseus cluster. Beyond ~1Gyr, these star clusters are indistinguishable in broadband optical colours from the more numerous GCs. Their number distribution exhibits a similar dependence with luminosity and mass as the GCs, whereas their spatial distribution resembles a filamentary network of multiphase gas associated with cooling of the intracluster gas. The sustained formation of these star clusters demonstrates that progenitor GCs can form over cosmic history from cooled intracluster gas, thus contributing to both the large number and broad colour dispersion$-$owing to an age spread, in addition to a spread in metallicity$-$of GCs in massive elliptical galaxies. The progenitor GCs have minimal masses well below the maximal masses of Galactic open star clusters, affirming a common formation mechanism for star clusters over all mass scales irrespective of their formative pathways.
NGC 4649 (M60) is one of a handful of giant Virgo ellipticals. We have obtained Gemini/GMOS spectra for 38 GCs associated with this galaxy. Applying the multi-index chi^2 minimisation technique of Proctor & Sansom (2002) with the single stellar population models of Thomas, Maraston & Korn (2004) we derive ages, metallicities and alpha-element abundance ratios. We find several young (2--3 Gyr old) super-solar metallicity GCs, while the majority are old (>10 Gyrs), spanning a range of metallicities from solar to [Z/H]=-2. At least two of these young GCs are at large projected radii of 17-20 kpc. The galaxy itself shows no obvious signs of a recent starburst, interaction or merger. A trend of decreasing alpha-element ratio with increasing metallicity is found.
We perform a detailed abundance analysis on integrated-light spectra of 20 globular clusters (GCs) in the early-type galaxy NGC 5128 (Centaurus A). The GCs were observed with X-Shooter on the VLT. The cluster sample spans a metallicity range of $-1.92 < $ [Fe/H] $< -0.13$ dex. Using theoretical isochrones we compute synthetic integrated-light spectra and iterate the individual abundances until the best fit to the observations is obtained. We measured abundances of Mg, Ca, and Ti, and find a slightly higher enhancement in NGC 5128 GCs with metallicities [Fe/H] < $-$0.75 dex, of the order of $sim$0.1 dex, than in the average values observed in the MW for GCs of the same metallicity. If this $alpha$-enhancement in the metal-poor GCs in NGC 5128 is genuine, it could hint at a chemical enrichment history different than that experienced by the MW. We also measure Na abundances in 9 out of 20 GCs. We find evidence for intra-cluster abundance variations in 6 of these clusters where we see enhanced [Na/Fe] > $+$0.25 dex. We obtain the first abundance measurements of Cr, Mn, and Ni for a sample of the GC population in NGC 5128 and find consistency with the overall trends observed in the MW, with a slight enhancement ($<$0.1 dex) in the Fe-peak abundances measured in the NGC 5128.
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