Full spectral fitting of Milky Way and M 31 globular clusters: ages and metallicities (original) (raw)
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The ages and metallicities of galaxies in the local universe
Monthly Notices of …, 2005
We derive stellar metallicities, light-weighted ages and stellar masses for a magnitudelimited sample of 175,128 galaxies drawn from the Sloan Digital Sky Survey Data Release Two (SDSS DR2). We compute median-likelihood estimates of these parameters using a large library of model spectra at medium-high resolution, covering a comprehensive range of star formation histories. The constraints we derive are set by the simultaneous fit of five spectral absorption features, which are well reproduced by our population synthesis models. By design, these constraints depend only weakly on the α/Fe element abundance ratio. Our sample includes galaxies of all types spanning the full range in star formation activity, from dormant early-type to actively starforming galaxies. By analysing a subsample of 44,254 high-quality spectra, we show that, in the mean, galaxies follow a sequence of increasing stellar metallicity, age and stellar mass at increasing 4000Å-break strength. For galaxies of intermediate mass, stronger Balmer absorption at fixed 4000Å-break strength is associated with higher metallicity and younger age. We investigate how stellar metallicity and age depend on total galaxy stellar mass. Low-mass galaxies are typically young and metal-poor, massive galaxies old and metal-rich, with a rapid transition between these regimes over the stellar mass range 3 × 10 9 M * 3 × 10 10 M ⊙ . Both high-and low-concentration galaxies follow these relations, but there is a large dispersion in stellar metallicity at fixed stellar mass, especially for low-concentration galaxies of intermediate mass.
Age and metallicity effects in single stellar populations: application to M 31 clusters
Astronomy and Astrophysics, 1997
We have recently calculated (Borges et al. 1995) integrated metallicity indices for single stellar populations (SSP). Effects of age, metallicity and abundances were taken into account. In particular, the explicit dependence of the indices Mg 2 and NaD respectively on the ratios [Mg/Fe] and [Na/Fe] was included in the calibration. We report in this work an application of those models to a sample of 12 globular clusters in M 31. A fitting procedure was used to obtain age, metallicity and the [Mg/Fe] ratio for each object, which best reproduce the data. The mean age of the sample is 15±2.8 Gyr and the mean [Mg/Fe] ratio is 0.35±0.10. These values and the derived metallicity spread are comparable to those found in galactic counterparts.
Metallicity distribution of bulge globular clusters\ in M 31
Astronomy and Astrophysics
This paper presents 3600-5400Å integrated spectra of 19 globular clusters (or candidates) projected on the central regions of M 31, r ≤ 5.3 (≈1.2 kpc). We check the cluster nature of these objects, and derive their ages, metallicities and reddenings. From the initial sample, 16 objects turn out to be true star clusters, two are Galactic dwarf stars, and one is a high redshift background galaxy. Only two clusters are found to be super metal-rich, suggesting that this phenomenon is not very common. For some clusters, we cannot rule out the possibility that they are of intermediate age; this requires confirmation by observations at the calcium triplet. We also present the metallicity histogram of this central bulge sample and discuss possible scenarios to explain its properties.
The horizontal branch luminosity vs. metallicity in M 31 globular clusters
Astronomy & Astrophysics, 2012
Context. Thanks to the outstanding capabilites of the HS T , our current knowledge about the M31 globular clusters (GCs) is similar to our knowledge of the Milky Way GCs in the 1960s-1970s, which set the basis for studying the halo and galaxy formation using these objects as tracers, and established their importance in defining the cosmic distance scale. Aims. We intend to derive a new calibration of the M V (HB)-[Fe/H] relation by exploiting the large photometric database of old GCs in M31 in the HS T archive. Methods. We collected the BVI data for 48 old GCs in M31 and analysed them by applying the same methods and procedures to all objects. We obtained a set of homogeneous colour-magnitude diagrams (CMDs) that were best-fitted with the fiducial CMD ridge lines of selected Milky Way template GCs. Reddening, metallicity, Horizontal Branch (HB) luminosity and distance were determined self-consistently for each cluster.
The Astrophysical Journal, 2011
One of the conundrums in extragalactic astronomy is the discrepancy in observed metallicity distribution functions (MDFs) between the two prime stellar components of early-type galaxies-globular clusters (GCs) and halo field stars. This is generally taken as evidence of highly decoupled evolutionary histories between GC systems and their parent galaxies. Here we show, however, that new developments in linking the observed GC colors to their intrinsic metallicities suggest nonlinear color-to-metallicity conversions, which translate observed color distributions into strongly peaked, unimodal MDFs with broad metal-poor tails. Remarkably, the inferred GC MDFs are similar to the MDFs of resolved field stars in nearby elliptical galaxies and those produced by chemical evolution models of galaxies. The GC MDF shape, characterized by a sharp peak with a metal-poor tail, indicates a virtually continuous chemical enrichment with a relatively short timescale. The characteristic shape emerges across three orders of magnitude in the host galaxy mass, suggesting a universal process of chemical enrichment among various GC systems. Given that GCs are bluer than field stars within the same galaxy, it is plausible that the chemical enrichment processes of GCs ceased somewhat earlier than that of the field stellar population, and if so, GCs preferentially trace the major, vigorous mode of star formation events in galactic formation. We further suggest a possible systematic age difference among GC systems, in that the GC systems in more luminous galaxies are older. This is consistent with the downsizing paradigm whereby stars of brighter galaxies, on average, formed earlier than those of dimmer galaxies; this additionally supports the similar nature shared by GCs and field stars. Although the sample used in this study (the Hubble Space Telescope Advanced Camera for Surveys/Wide Field Channel, WFPC2, and WFC3 photometry for the GC systems in the Virgo galaxy cluster) confines our discussion to R R e for giant ellipticals and 10 R e for normal ellipticals, our findings suggest that GC systems and their parent galaxies have shared a more common origin than previously thought, and hence greatly simplify theories of galaxy formation.
Evidence for metallicity spreads in three massive M 31 globular clusters
Astronomy and Astrophysics, 2008
Aims. We quantify the intrinsic width of the red giant branches of three massive globular clusters in M31 in a search for metallicity spreads within these objects. Methods. We present HST/ACS observations of three massive clusters in M31, G78, G213, and G280. A thorough description of the photometry extraction and calibration is presented. After derivation of the color-magnitude diagrams, we quantify the intrinsic width of the red giant branch of each cluster. Results. This width translates into a metallicity dispersion that indicates a complex star formation history for this type of system. For G78, σ [Fe/H] = 0.86 ± 0.37; for G213, 0.89 ± 0.20; and for G280, 1.03 ± 0.26. We find that the metallicity dispersion of the clusters does not scale with mean metallicity. We also find no trend with the cluster mass. We discuss some possible formation scenarios that would explain our results.
2009
We present a new homogeneous set of metallicity estimates based on Lick indices for 245 old globular clusters of the M31 galaxy comprised in the Revised Bologna Catalog. The metallicity distribution of the M31 globular clusters is briefly discussed and compared with that of the Milky Way. Simple parametric statistics suggests that the [Fe/H] distribution is likely not unimodal. The strong correlation between metallicity and kinematics found in previous studies is confirmed. The most metal-rich GCs tend to be packed at the center of the system and share the galactic rotation as traced by the HI disk. Although the velocity dispersion around the curve increases with decreasing metallicity, also clusters with [Fe/H]<-1.0 display a clear rotational pattern, at odds with their Milky Way counterparts.
An updated survey of globular clusters in M 31
Astronomy and Astrophysics, 2009
Aims. We present a new homogeneous set of metallicity estimates based on Lick indices for the old globular clusters of the M31 galaxy. The final aim is to add homogeneous spectroscopic metallicities to as many entries as possible of the Revised Bologna Catalog of M31 clusters ⋆⋆ , by reporting Lick indices measurements from any source (literature, new observations, etc.) into the same scale. Methods. New empirical relations of [Fe/H] as a function of [MgFe] and Mg2 indices, as defined by Trager et al. (1998), are based on well studied Galactic Globular Clusters, complemented with theoretical model predictions for −0.2 ≤ [Fe/H] ≤ +0.5. Lick indices for M31 clusters from various literature sources (225 clusters) and from new observations by our team (71 clusters) have been transformed into the Trager et al. (1998) system, yielding new metallicity estimates for 245 globular clusters of M31. Results. Our values are in good agreement with recent estimates based on detailed spectral fitting and with those obtained from Color Magnitude Diagrams of clusters imaged with the Hubble Space Telescope. The typical uncertainty on individual estimates is ≃ ±0.25 dex, as resulted from the comparison with metallicities derived from Color Magnitude Diagrams of individual clusters. Conclusions. The metallicity distribution of M31 globular cluster is briefly discussed and compared with that of the Milky Way. Simple parametric statistical tests suggest that the distribution is likely not unimodal. The strong correlation between metallicity and kinematics found in previous studies is confirmed. The most metal-rich GCs tend to be packed at the center of the system and to cluster tightly around the galactic rotation curve defined by the HI disk, while the velocity dispersion about the curve increases with decreasing metallicity. However, also the clusters with [Fe/H] < −1.0 display a clear rotation pattern, at odds with their Milky Way counterparts.