VLT Spectroscopy of Globular Cluster Systems, I. The Photometric and Spectroscopic Dataset (original) (raw)
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VLT spectroscopy of globular cluster systems
Astronomy and Astrophysics, 2004
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 different 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 effective radius. Using combined optical/nearinfrared colour-colour diagrams we present a method to efficiently 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 R eff ). For the sample we find masses in the range ∼ 8.8 · 10 10 M⊙ up to ∼ 1.2 · 10 12 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.
2005
An analysis of ages, metallicities, and [alpha/Fe] ratios of globular cluster systems in early-type galaxies is presented, based on Lick index measurements summarized in Puzia et al. (2004, 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 [alpha/Fe] ratios), we find that the most reliable age indicator for our dataset is a combination of the Lick Balmer-line indices HgammaA, Hbeta, and HdeltaA. [MgFe]' is used as a spectroscopic metallicity indicator which is least affected by [alpha/Fe] variations. We introduce an interpolation routine to simultaneously derive ages, metallicities, and [alpha/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 early-type g...
FORS2/VLT survey of Milky Way globular clusters
Astronomy & Astrophysics, 2014
Context. Globular clusters trace the formation and evolution of the Milky Way and surrounding galaxies, and outline their chemical enrichment history. To accomplish these tasks it is important to have large samples of clusters with homogeneous data and analysis to derive kinematics, chemical abundances, ages and locations. Aims. We obtain homogeneous metallicities and α-element enhancement for 51 Galactic bulge, disc, and halo globular clusters that are among the most distant and/or highly reddened in the Galaxy's globular cluster system. We also provide membership selection based on stellar radial velocities and atmospheric parameters. The implications of our results are discussed. Methods. We observed R ∼ 2000 spectra in the wavelength interval 456-586 nm for over 800 red giant stars in 51 Galactic globular clusters. We applied full spectrum fitting with the code ETOILE together with libraries of observed and synthetic spectra. We compared the mean abundances of all clusters with previous work and with field stars. We used the relation between mean metallicity and horizontal branch morphology defined by all clusters to select outliers for discussion. Results. [Fe/H], [Mg/Fe], and [α/Fe] were derived in a consistent way for almost one-third of all Galactic globular clusters. We find our metallicities are comparable to those derived from high-resolution data to within σ = 0.08 dex over the interval −2.5 < [Fe/H] < 0.0. Furthermore, a comparison of previous metallicity scales with our values yields σ < 0.16 dex. We also find that the distribution of [Mg/Fe] and [α/Fe] with [Fe/H] for the 51 clusters follows the general trend exhibited by field stars. It is the first time that the following clusters have been included in a large sample of homogeneous stellar spectroscopic observations and metallicity derivation: BH 176, Djorg 2, Pal 10, NGC 6426, Lynga 7, and Terzan 8. In particular, only photometric metallicities were available previously for the first three clusters, and the available metallicity for NGC 6426 was based on integrated spectroscopy and photometry. Two other clusters, HP 1 and NGC 6558, are confirmed as candidates for the oldest globular clusters in the Milky Way. Conclusions. Stellar spectroscopy in the visible at R ∼ 2000 for a large sample of globular clusters is a robust and efficient way to trace the chemical evolution of the host galaxy and to detect interesting objects for follow-up at higher resolution and with forthcoming giant telescopes. The technique used here can also be applied to globular cluster systems in nearby galaxies with current instruments and to distant galaxies with the advent of ELTs.
Spectroscopy of globular cluster systems
Proceedings of the International Astronomical …, 2005
We compare [α/Fe], metallicity, and age distributions of globular clusters in elliptical, lenticular, and spiral galaxies, which we derive from Lick line index measurements. We find a large number of globular clusters in elliptical galaxies that reach significantly higher [α/Fe] values (> 0.5 dex) than clusters in lenticular and spiral galaxies. Most of these highly α-enriched globular clusters are old (t > 8 Gyr) and exhibit relatively high metallicities up to solar values. Given the lower [α/Fe] ratios of the diffuse stellar population in early-type galaxies, our results suggest that the extremely α-enhanced globular clusters are members of the the very first generation of star clusters formed, and that their formation epochs likely predate the formation of the majority of stars in giant early-type galaxies.
The Role of Cluster Mass in the Multiple Populations of Galactic and Extragalactic Globular Clusters
The Astronomical Journal
Studies based on Hubble Space Telescope (HST) photometry in F275W, F336W, and F438W have shown that the incidence and complexity of multiple populations (MPs) in Globular Clusters (GCs) depend on cluster mass. This result, however, is based on nearby, low-reddening Galactic GCs, for which we can obtain accurate F275W photometry. In this work we exploit HST photometry in F336W, F438W, and F814W to characterize MPs in 68 Galactic and extragalactic GCs by using the pseudo-color C F 336W,F 438W,F 814W , which is more efficient in terms of observation time than the pseudo-color C F 275W,F 336W,F 438W adopted in previous works. We first analyzed the Galactic GCs alone and find that the RGB color width strongly correlates with [Fe/H]. After removing the dependence from metallicity, we obtain a strong correlation with cluster mass, thus confirming previous findings. We then exploited the RGB width to compare MPs in Galactic and extragalactic GCs. Similarly to Galactic GCs, the RGB width of Magellanic Cloud GCs correlates with clusters mass, after removing the dependence from metallicity. This fact demonstrates that cluster mass is the main factor affecting the properties of MPs. Magellanic Cloud clusters exhibit, on average, narrower RGBs than Galactic GCs with similar mass and metallicity. We suggest that this difference is a signature of stellar mass loss in Galactic GCs. As an alternative, the mass-RGB-width relation would depend on the host galaxy. Finally, we used ground-based photometry to investigate Terzan 7 through the C U,B,I pseudo-color, ground-based analogous of C F 336W,F 438W,F 814W , and find that this cluster is consistent with a simple population.
2022
The presence of Multiple Stellar Populations (MSPs) in Galactic Globular Clusters (GCs) is a poorly understood phenomenon. By probing different spectral ranges and affected by different absorption lines using the multi-band photometric survey S-PLUS, we study four GCs — NGC104, NGC288, NGC3201 and NGC7089— that span a wide range of metallicities. With the combination of broad and narrow-band photometry in 12 different filters from 3485A (u) to 9114A (z), we identified MSPs along the rectified red-giant branch in colour-magnitude diagrams (CMDs) and separated them using a K-means clustering algorithm. Additionally, we take advantage of the large Field of View of the S-PLUS detector to investigate radial trends in our sample. We report on six colour combinations that can be used to successfully identify two stellar populations in all studied clusters and show that they can be characterised as Na-rich and Na-poor. For both NGC 288 and NGC 7089, their radial profiles show a clear concen...
An optical/NIR survey of globular clusters in early-type galaxies
Astronomy & Astrophysics, 2012
Context. Some photometric studies of extragalactic globular cluster (GC) systems using the optical and near-infrared colour combination have suggested the presence of a large fraction of intermediate-age (2-8 Gyrs) GCs. Aims. We investigate the age distributions of GC systems in 14 E/S0 galaxies. Methods. We carry out a differential comparison of the (g − z) vs. (g − K) two-colour diagrams for GC systems in the different galaxies in order to see whether there are indications of age differences. We also compare the different GC systems with a few simple stellar population models. Results. No significant difference is detected in the mean ages of GCs among elliptical galaxies. S0 galaxies on the other hand, show evidence for younger GCs. Surprisingly, this appears to be driven by the more metal-poor clusters. The age distribution of GCs in NGC 4365 seems to be similar to that of other large ellipticals (e.g. NGC 4486, NGC 4649). Padova SSPs with recently released isochrones for old ages (14 Gyrs) show less of an offset with respect to the photometry than previously published models. Conclusions. We suggest that E type galaxies assembled most of their GCs in a shorter and earlier period than S0 type galaxies. The latter galaxy type, seems to have a more extended period of GC formation/assembly.
Fe b 20 01 Properties of Globular Cluster Systems in Nearby Early-type Galaxies 1
2001
We present a study of globular clusters (GCs) in 17 relatively nearby early-type galaxies, based on deep F555W and F814W images from the Wide Field / Planetary Camera 2 (WFPC2) on board the Hubble Space Telescope (HST). A detailed analysis of color distributions, cluster sizes and luminosity functions is performed and compared with GCs in the Milky Way. In nearly all cases, a KMM test returns a high confidence level for the hypothesis that a sum of two Gaussians provides a better fit to the observed color distribution than a single Gaussian, although histograms of the (V −I) 0 distribution are not always obviously bimodal. The blue and red peak colors returned by the KMM test are both found to correlate with absolute host galaxy B band magnitude and central velocity dispersion (at about the 2 − 3σ level), but we see no clear correlation with host galaxy V −I or J −K color. Red GCs are generally smaller than blue GCs by about 20%. The size difference is seen at all radii and within s...