Maren Hempel

VLT spectroscopy of globular cluster systems - II. Spectroscopic ages, metallicities and [α/Fe] ratios of globular clusters in early-type galaxies

An analysis of ages, metallicities, and [α/Fe] ratios of globular cluster systems in early-type galaxies is presented, based on Lick index measurements summarized in Puzia et al. (2004, A&A, 415, 123, Paper I of this series). In the light of calibration and measurement uncertainties, age-metallicity degeneracy, and the relative dynamic range of Lick indices, as well as systematics introduced by abundance ratio variations (in particular variations of [α/Fe] ratios), we find that the most reliable age indicator for our dataset is a combination of the Lick Balmer-line indices HγA, Hβ, and HδA. [MgFe] is used as a spectroscopic metallicity indicator which is least affected by [α/Fe] variations. We introduce an interpolation routine to simultaneously derive ages, metallicities, and [α/Fe] ratios from diagnostic grids constructed from Lick indices. From a comparison of high-quality data with SSP model predictions, we find that ∼2/3 of the globular clusters in earlytype galaxies are older than 10 Gyr, up to 1/3 have ages in the range ∼5−10 Gyr, and only a few cluster are younger than ∼5 Gyr. Our sample of globular clusters covers metallicities from [Z/H] ≈ −1.3 up to ∼0.5 dex. We find that metal-rich globular clusters show on average a smaller mean age and a larger age scatter than their metal-poor counterparts. [α/Fe] diagnostic plots show that globular cluster systems in early-type galaxies have super-solar α/Fe abundance ratios with a mean [α/Fe] = 0.47 ± 0.06 dex and a dispersion of ∼0.3 dex. We find evidence for a correlation between [α/Fe] and metallicity, in the sense that more metal-rich clusters exhibit lower α-element enhancements. A discussion of systematics related to the Lick index system shows that the method suffers to some extent from uncertainties due to unknown horizontal branch morphologies at high metallicities. However, these systematics still allow us to make good qualitative statements. A detailed investigation of indices as a function of data quality reveals that the scatter in Balmer index values decreases for higher-quality data. In particular, extremely low Balmer index values that are lower than any SSP model prediction tend to disappear. Furthermore, we find that observed photometric colors are in good agreement with computed SSP colors using ages and metallicities as derived from the spectroscopic line indices.

VLT spectroscopy of globular cluster systems - I:the photometric and spectroscopic data set

We present Lick line-index measurements of extragalactic globular clusters in seven early-type galaxies (NGC 1380, 2434, 3115, 3379, 3585, 5846, and 7192) with dierent morphological types (E–S0) located in field and group/cluster environments. High-quality spectra were taken with the FORS2 instrument at ESO’s Very Large Telescope. 50% of our data allows an age resolution t=t 0:3 and a metallicity resolution 0:25−0:4 dex, depending on the absolute metallicity. Globular cluster candidates are selected from deep B;V; R; I; K FORS2/ISAAC photometry with 80−100% success rate inside one eective radius. Using combined optical/near-infrared colour-colour diagrams we present a method to eciently reduce fore-/background contamination down to <10%. We find clear signs for bi-modality in the globular cluster colour distributions of NGC 1380, 3115, and 3585. The colour distributions of globular clusters in NGC 2434, 3379, 5846, and 7192 are consistent with a broad single-peak distribution. For the analysed globular cluster systems the slopes of projected radial surface density profiles, of the form Σ(R) ∼R −Γ vary between 0:8 and 2.6. Blue and red globular cluster sub-populations show similar slopes in the clearly bi-modal systems. For galaxies with single-peak globular cluster colour distributions, there is a hint that the blue cluster system seems to have a more extended radial distribution than the red one. Using globular clusters as a tracer population we determine total dynamical masses of host galaxies out to large radii (∼1:6−4:8 Reff). For the sample we find masses in the range 8:8 X1010 M⊙ up to 1:2X1012 M⊙. The line index data presented here will be used in accompanying papers of this series to derive ages, metallicities and abundance ratios. A compilation of currently available high-quality Lick index measurements for globular clusters in elliptical, lenticular, and late-type galaxies is provided and will serve to augment the current data set.

The ages of globular clusters in NGC 4365 revisited with deep HST observations

We present new Hubble Space Telescope (HST) NIC3, near-infrared H-band photometry of globular clusters (GCs) around NGC 4365 and NGC 1399 in combination with archival HST WFPC2 and ACS optical data. We find that NGC 4365 has a number of globular clusters with bluer optical colors than expected for their red optical-to-near-infrared colors and an old age. The only known way to explain these colors is with a significant population of intermediate-age (2-8 Gyr) clusters in this elliptical galaxy. On the other hand, our result for NGC 1399 is in agreement with previous spectroscopic work that suggests that its clusters have a large metallicity spread and are nearly all old. In the literature, there are various results from spectroscopic studies of modest samples of NGC 4365 globular clusters. The spectroscopic data allow for either the presence or absence of a significant population of intermediate-age clusters, given the index uncertainties indicated by comparing objects in common between these studies and the few spectroscopic candidates with optical-to-near-IR colors indicative of intermediate ages. Our new near-IR data of the NGC 4365 GC system with a much higher signal-to-noise ratio agree well with earlier published photometry, and both give strong evidence of a significant intermediate-age component. The agreement between the photometric and spectroscopic results for NGC 1399 and other systems lends further confidence to this conclusion and to the effectiveness of the near-IR technique.

Extragalactic globular clusters in the near infrared III. NGC 5846 and NGC 7192 - Quantifying the age distribution of sub-populations

In this third paper of our series on near-IR and optical photometry of globular cluster systems in early-type galaxies we concentrate on the photometric results for NGC 5846 and NGC 7192, two group ellipticals, and on a first comparison between the globular cluster systems investigated so far. In NGC 5846 the colour-colour diagram shows clear bi-modality in (V - K), which is confirmed by a KMM test. The mean colour of both peaks were estimated to be (V - K) b l u e = 2.57 ′ 0.06 and (V - K) r e d = 3.18 ′ 0.06. The situation in NGC 7192 is different, in that the colour-colour diagram gives no evidence for a distinct second population of globular clusters. Using simulated colour distributions of globular cluster systems, we make a first step in quantifying the cumulative age distribution in globular cluster systems. Also here the result for NGC 5846 leads us to the conclusion that its metal-rich globular cluster population contains two globular cluster populations which differ in age by several Gyr. The age structure for NGC 7192 shows instead strong similarity with a single-age population.

The star cluster population in the tidal tails of NGC 6872

We present a photometric analysis of the rich star cluster population in the tidal tails of NGC 6872. We find star clusters with ages between 1−100 Myr distributed in the tidal tails, while the tails themselves have an age of less than 150 Myr. Most of the young massive (10 4 ≤ M/M� ≤ 10 7 ) clusters are found in the outer regions of the galactic disk or the tidal tails. The mass distribution of the cluster population can be well described by a power-law of the form N(m) ∝ m −α ,w here α = 1.85 ± 0.11, in very good agreement with other young cluster populations found in a variety of different environments. We estimate the star formation rate for three separate regions of the galaxy, and find that the eastern tail is forming stars at ∼2 times the rate of the western tail and ∼5 times the rate of the main body of the galaxy. By comparing our observations with published N-body models of the fate of material in tidal tails in a galaxy cluster potential, we see that many of these young clusters will be lost into the intergalactic medium. We speculate that this mechanism may also be at work in larger galaxy clusters such as Fornax, and suggest that the so-called ultra-compact dwarf galaxies could be the most massive star clusters that have formed in the tidal tails of an ancient galactic merger.

Intermediate-age globular clusters in early-type galaxies: Better age determinations by adding U-band observations to the V, I, K datasets ,

This paper represents an extension to a series of publications on combined optical and near-infrared photometry of globular cluster systems in early-type galaxies. Based on color-color diagrams the cumulative age distributions of their globular cluster systems have been derived and revealed intermediate age clusters in two galaxies, namely NGC 4365 and NGC 5846. The extension of this observations towards the blue wavelength range was performed in order to increase the age resolution of photometric studies. In this paper we present the results of U-band observations of NGC 4365 and NGC 5846 and their combination with previously obtained V, I and Ks-band photometry. (U − I )v s. (V − Ks) color-color diagrams are used to derive the cumulative age distribution. The later is compared to simulated globular cluster systems of known age and size composition in order to set constrains on relative ages and size of the sub-populations.

Extragalactic Globular Clusters in the Near-Infrared

Visual and near-infrared colours are used to identify intermediate-age globular cluster in early-type galaxies. In NGC5846 and NGC4365 colour-colour diagrams (V-I vs. V-K) reveal a sub-population in the globular cluster systems with an age much younger than the expected old populations (13-15 Gyr). A comparison with SSP models dates the formation of this younger population 2-7 Gyr back. Further evidence for multiple star formation events is given by the age distribution in both systems and the comparison to models. Our photometric age determination was confirmed by spectroscopy for a small sample of the clusters in NGC4365.Visual and near-infrared colours are used to identify intermediate-age globular cluster in early-type galaxies. In NGC5846 and NGC4365 colour-colour diagrams (V-I vs. V-K) reveal a sub-population in the globular cluster systems with an age much younger than the expected old populations (13-15 Gyr). A comparison with SSP models dates the formation of this younger population 2-7 Gyr back. Further evidence for multiple star formation events is given by the age distribution in both systems and the comparison to models. Our photometric age determination was confirmed by spectroscopy for a small sample of the clusters in NGC4365.

Extragalactic globular clusters in the near infrared IV. Quantifying the age structure using Monte-Carlo simulations

In previous papers of the series, we used a combination of optical and near-infrared colours to derive constrains on the relative age structure in globular cluster systems. Here, we present the details, strength and limitations of our method based on Monte-Carlo simulations of colour-colour diagrams and cumulative age distributions. The simulations are based on general informations about the globular cluster systems (e.g. colour-ranges, the number ratios between sub-populations) and the different single stellar population models (SSPs) which are used to derive relative ages. For both the modeled systems and the observed globular cluster systems we derive the cumulative age distribution and introduce two parameters to define it, the so-called 50% age and the result of the reduced χ 2 test of the comparison between models and observations. The method was tested successfully on several systems and allowed to reveal significant intermediate age populations in two of them.

Extragalactic globular clusters in the near-infrared. V. IC 4051 and NGC 3311

We present the results of combined optical and near-infrared photometry for the globular cluster systems of the giant ellipticals IC 4051 and NGC 3311. We use the reduced age-metallicity degeneracy in (V - I) vs. (V - H) color-color diagrams to derive the cumulative age distribution within the red sub-population of globular clusters and to search for age sub-populations. The age distribution is then compared to the one determined for simulated globular cluster systems in order to set constraints on the relative age and size of these globular cluster sub-populations. In both galaxies we find a significant fraction of globular clusters with ages between 2-5 Gyr. We also investigate the metallicity distribution in both systems. Small number statistics prevent us from making any definite statements concerning NGC 3311, but we find that the derived metallicity distribution of the IC 4051 clusters strongly depends on the assumed age distribution. Based on our most likely result that finds a large number of young/intermediate age clusters (∼2 Gyr) within the selected globular cluster sample, we find metallicity peaks at ∼-0.2 for the old clusters and +0.8 for the young clusters. Only few very metal poor clusters are found. However, the metallicity distribution within the young/intermediate globular cluster population is significantly affected by our choice of the applied Single Stellar Population model. The mean metallicity of the second generation of globular clusters changes from the above mentioned and extremely high +0.8 dex to +0.2 dex. Note that the model dependency becomes less severe with an increasing age of the cluster population.

VVV Search for New Young Clusters Towards the Star Forming Regions in Our Galaxy: First Results

One of the main goals of the “VVV – Vista Variables in the Via Lactea” ESO Large Survey is to search for new star clusters of different ages. In order to trace the early stages of star clusters formation we carrying out a survey of infrared star cluster candidates and stellar groups in the directions of known massive star formation regions. To date, we have identified 47 candidates. Most of them appear compact (with small angular sizes) and very young.