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Register a new userThe extended star formation history of omega Centauri
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Michael Hilker
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For the first time, the abundances of a large sample of subgiant and turn-off region stars in omega Centauri have been measured, the data base being medium resolution spectroscopy from FORS2 at the VLT. Absolute iron abundances were derived for about 400 member stars from newly defined line indices with an accuracy of +/-0.15 dex. The abundances range between -2.2<[Fe/H]<-0.7 dex, resembling the large metallicity spread found for red giant branch stars. The combination of the spectroscopic results with the location of the stars in the colour magnitude diagram has been used to estimate ages for the individual stars. Whereas most of the metal-poor stars are consistent with a single old stellar population, stars with abundances higher than [Fe/H]=-1.3 dex are younger. The total age spread in omega Cent is about 3 Gyr. The monotonically increasing age-metallicity relation seems to level off above [Fe/H]=-1.0 dex. Whether the star formation in omega Cen occured continuously or rather episodically has to be shown by combining more accurate abundances with highest quality photometry.
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We present metallicity measurements based on GIRAFFE@VLT spectra of 80 subgiant-branch stars of the Galactic globular cluster Omega Centauri. The VLT spectroscopic data are complemented by ACS/HST and [email protected] high-accuracy color-magnitude diagrams. We have obtained the [Fe/H] abundance for each of the 80 target stars, and the abundances of C, N, Ca, Ti, and Ba for a subset of them, by comparison with synthetic spectra. We show that stars with [Fe/H]<-1.25 have a large magnitude spread on the flat part of the SGB. We interpret this as empirical evidence for an age spread. We have identified four distinct stellar groups within the SGB region: (i) an old, metal-poor group ([Fe/H]~-1.7); (ii) an old, metal-rich group ([Fe/H]~-1.1); (iii) a young (up to 4--5 Gyr younger than the old component) metal-poor group ([Fe/H]~-1.7); (iv) a young, intermediate-metallicity ([Fe/H]~-1.4) group, on average 1--2 Gyr younger than the old metal-poor population, and with an age spread that we cannot properly quantify with the present sample. In addition, a group of SGB stars are spread between the intermediate-metallicity and metal-rich branches of the SGB. The spread in age within each population establishes that the progenitor of Omega Cen system must have had a composite nature.
We use the SDSS-Gaia catalogue to search for substructure in the stellar halo. The sample comprises 62,133 halo stars with full phase space coordinates and extends out to heliocentric distances of $sim 10$ kpc. As actions are conserved under slow changes of the potential, they permit identification of groups of stars with a common accretion history. We devise a method to identify halo substructures based on their clustering in action space, using metallicity as a secondary check. This is validated against smooth models and numerical constructed stellar halos from the Aquarius simulations. We identify 21 substructures in the SDSS-Gaia catalogue, including 7 high significance, high energy and retrograde ones. We investigate whether the retrograde substructures may be material stripped off the atypical globular cluster $omega$~Centauri. Using a simple model of the accretion of the progenitor of the $omega$~Centauri, we tentatively argue for the possible association of up to 5 of our new substructures (labelled Rg1, Rg3, Rg4, Rg6 and Rg7) with this event. This sets a minimum mass of $5 times 10^8 M_odot$ for the progenitor, so as to bring $omega$~Centauri to its current location in action -- energy space. Our proposal can be tested by high resolution spectroscopy of the candidates to look for the unusual abundance patterns possessed by $omega$~Centauri stars.
There has been significant controversy over the mechanisms responsible for forming compact stellar systems like ultra compact dwarfs (UCDs), with suggestions that UCDs are simply the high mass extension of the globular cluster (GC) population, or alternatively, the liberated nuclei of galaxies tidally stripped by larger companions. Definitive examples of UCDs formed by either route have been difficult to find, with only a handful of persuasive examples of stripped-nucleus type UCDs being known. In this paper we present very deep Gemini/GMOS spectroscopic observations of the suspected stripped nucleus UCD NGC 4546-UCD1 taken in good seeing conditions (< 0.7). With these data we examine the spatially resolved kinematics and star formation history of this unusual object. We find no evidence of a rise in the central velocity dispersion of the UCD, suggesting that this UCD lacks a massive central black hole like those found in some other compact stellar systems, a conclusion confirmed by detailed dynamical modelling. Finally we are able to use our extremely high signal to noise spectrum to detect a temporally extended star formation history for this UCD. We find that the UCD was forming stars since the earliest epochs until at least 1-2 Gyr ago. Taken together these observations confirm that NGC 4546-UCD1 is the remnant nucleus of a nucleated dwarf galaxy that was tidally destroyed by NGC 4546 within the last 1-2 Gyr.
We use deep HST ACS/HRC observations of a field within M32 (F1) and an M31 background field (F2) to determine the star formation history (SFH) of M32 from its resolved stellar population. We find that 2-5Gyr old stars contribute som40%+/- 17% of M32s mass, while 55%+/-21% of M32s mass comes from stars older than 5 Gyr. The mass-weighted mean age and metallicity of M32 at F1 are <Age>=6.8+/-1.5 Gyr and <[M/H]>=-0.01+/-0.08 dex. The SFH additionally indicates the presence of young (<2 Gyr old), metal-poor ([M/H]sim-0.7) stars, suggesting that blue straggler stars contribute ~2% of the mass at F1; the remaining sim3% of the mass is in young metal-rich stars. Line-strength indices computed from the SFH imply a light-weighted mean age and metallicity of 4.9 Gyr and [M/H] = -0.12 dex, and single-stellar-population-equivalent parameters of 2.9+/-0.2 Gyr and [M/H]=0.02+/-0.01 dex at F1 (~2.7 re). This contradicts spectroscopic studies that show a steep age gradient from M32s center to 1re. The inferred SFH of the M31 background field F2 reveals that the majority of its stars are old, with sim95% of its mass already acquired 5-14 Gyr ago. It is composed of two dominant populations; sim30%+/-7.5% of its mass is in a 5-8 Gyr old population, and sim65%+/-9% of the mass is in a 8-14 Gyr old population. The mass-weighted mean age and metallicity of F2 are <Age>=9.2+/-1.2 Gyr and <[M/H]>=-0.10+/-0.10 dex, respectively. Our results suggest that the inner disk and spheroid populations of M31 are indistinguishable from those of the outer disk and spheroid. Assuming the mean age of M31s disk at F2 (sim1 disk scale length) to be 5-9 Gyr, our results agree with an inside-out disk formation scenario for M31s disk.
We present a variable star catalog of an extensive ground-based wide-field variability survey in the globular cluster omega Centauri. Using the ANU 40-inch (1m) telescope at Siding Spring Observatory, the cluster was observed with a 52x52 (0.75 deg^2) field for 25 nights. A total of 187 variable stars were identified in the field, 81 of which are new discoveries. This work comprises the widest field variability survey yet undertaken for this cluster. Here we present the V+R lightcurves and preliminary analysis of the detected variable stars, comprising 58 eclipsing binaries, 69 RR Lyrae stars, 36 long period variables (P>=2d) and 24 miscellaneous pulsators including 15 SX Phoenicis stars and two Type II Cepheids. Analysis of the eclipsing binary radial distribution has revealed an apparent lack of binaries in the 8-15 range, perhaps indicating two separate binary populations. Four detached binaries have short periods (<2.5d) and are likely composed of low-mass M-dwarf components, useful for testing stellar evolution models. One further detached system has a period of 0.8 days and due to the blueness of the system could be composed of white dwarf stars. Analysis of the RR Lyrae sample has produced a reddening corrected distance modulus (also accounting for metallicity spread) for the cluster of 13.68+-0.27, a result consistent with previously published values. This paper also presents a total stellar database comprising V and I photometry (with astrometry better than 0.25) for 203,892 stars with 12.0<V<21.0 and 25-night V+R lightcurves for 109,726 stars (14.0<V<22.0) for both the cluster and the field.
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