We use recently derived ages for 61 Milky Way (MW) globular clusters (GCs) to show that their age-metallicity relation (AMR) can be divided into two distinct, parallel sequences at [Fe/H] $ga -1.8$. Approximately one-third of the clusters form an off
set sequence that spans the full range in age ($sim 10.5$--13 Gyr), but is more metal rich at a given age by $sim 0.6$ dex in [Fe/H]. All but one of the clusters in the offset sequence show orbital properties that are consistent with membership in the MW disk. They are not simply the most metal-rich GCs, which have long been known to have disk-like kinematics, but they are the most metal-rich clusters at all ages. The slope of the mass-metallicity relation (MMR) for galaxies implies that the offset in metallicity of the two branches of the AMR corresponds to a mass decrement of 2 dex, suggesting host galaxy masses of $M_{*} sim 10^{7-8} msol$ for GCs that belong to the more metal-poor AMR. We suggest that the metal-rich branch of the AMR consists of clusters that formed in-situ in the disk, while the metal-poor GCs were formed in relatively low-mass (dwarf) galaxies and later accreted by the MW. The observed AMR of MW disk stars, and of the LMC, SMC and WLM dwarf galaxies are shown to be consistent with this interpretation, and the relative distribution of implied progenitor masses for the halo GC clusters is in excellent agreement with the MW subhalo mass function predicted by simulations. A notable implication of the bifurcated AMR, is that the identical mean ages and spread in ages, for the metal rich and metal poor GCs are difficult to reconcile with an in-situ formation for the latter population.
Ages have been derived for 55 globular clusters (GCs) from overlays of isochrones onto the turnoff photometry, assuming distances based on fits of zero-age horizontal branch (ZAHB) models to the lower bound of the observed distributions of HB stars.
The error bar arising just from the fitting of ZAHBs and isochrones is ~ +/- 0.25 Gyr, while that associated with distance and chemical abundance uncertainties is ~ +/- 1.5-2 Gyr. Ages vary from mean values of ~12.5 Gyr at [Fe/H] < -1.7 to ~11 Gyr at [Fe/H] > -1.0. At intermediate metallicities, the age-metallicity relation (AMR) appears to be bifurcated: one branch apparently contains clusters with disk-like kinematics, whereas the other branch is populated by clusters with halo-type orbits. There is no apparent dependence of age on Galactocentric distance (R_G) nor is there a clear correlation of HB type with age. Subtle variations in the subgiant branch (SGB) slopes of [Fe/H] < -1.5 GCs are tentatively attributed to helium abundance differences. Curiously, GCs with steep M13-like SGBs tend to be massive systems, located at small R_G, that show the strongest evidence for multiple stellar populations. The others are typically low-mass systems that, at the present time, should not be able to retain the matter lost by mass-losing stars. The apparent separation of the two groups in terms of their present-day gas retention properties is difficult to understand if all GCs were initial ~20 times their current masses. The lowest mass systems may have never been able to retain enough gas to produce a significant population of second-generation stars; in this case, the observed light element abundance variations were presumably present in the gas out of which the observed cluster stars formed.
Building on our previous spectroscopic and photometric analysis of the isolated Local Group dwarf irregular (dIrr) galaxy WLM, we present a comparison of the metallicities of its RGB stars with respect to the well studied Local Group dwarf spheroidal
galaxies (dSphs) and Magellanic Clouds. We calculate a mean metallicity of [Fe/H]$ = -1.28 pm 0.02$, and intrinsic spread in metallicity of $sigma = 0.38 pm 0.04$ dex, similar to the mean and spread observed in the massive dSph Fornax and the Small Magellanic Cloud. Thus, despite its isolated environment the global metallicity still follows expectations for WLMs mass and its global chemical evolution is similar to other nearby luminous dwarf galaxies (gas-rich or gas-poor). The data also show a radial gradient in [Fe/H] of $rm{d[Fe/H]/dr_{c}} = -0.04 pm 0.04$ dex $rm{r_{c}^{-1}}$, which is flatter than that seen in the unbiased and spatially extended surveys of dSphs. Comparison of the spatial distribution of [Fe/H] in WLM, the Magellanic Clouds, and a sample of Local Group dSphs, shows an apparent dichotomy in the sense that the dIrrs have statistically flatter radial [Fe/H] gradients than the low angular momentum dSphs. The correlation between angular momentum and radial metallicity gradient is further supported when considering the Local Group dEs. This chemodynamic relationship offers a new and useful constraint for environment driven dwarf galaxy evolution models in the Local Group.
Star clusters are known to have smaller intrinsic metallicity spreads than dwarf galaxies due to their shorter star formation timescales. Here we use individual spectroscopic [Fe/H] measurements of stars in 19 Local Group dwarf galaxies, 13 Galactic
open clusters, and 49 globular clusters to show that star cluster and dwarf galaxy linear metallicity distributions are binomial in form, with all objects showing strong correlations between their mean linear metallicity $bar{Z}$ and intrinsic spread in metallicity $sigma(Z)^{2}$. A plot of $sigma(Z)^{2}$ versus $bar{Z}$ shows that the correlated relationships are offset for the dwarf galaxies from the star clusters. The common binomial nature of these linear metallicity distributions can be explained with a simple inhomogeneous chemical evolution model (e.g., Oey 2000), where the star cluster and dwarf galaxy behaviour in the $sigma(Z)^{2}-bar{Z}$ diagram is reproduced in terms of the number of enrichment events, covering fraction, and intrinsic size of the enriched regions. The inhomogeneity of the self-enrichment sets the slope for the observed dwarf galaxy $sigma(Z)^{2}-bar{Z}$ correlation. The offset of the star cluster sequence from that of the dwarf galaxies is due to pre-enrichment, and the slope of the star cluster sequence represents the remnant signature of the self-enriched history of their host galaxies. The offset can be used to separate star clusters from dwarf galaxies without a priori knowledge of their luminosity or dynamical mass. The application of the inhomogeneous model to the $sigma(Z)^{2}-bar{Z}$ relationship provides a numerical formalism to connect the self-enrichment and pre-enrichment between star clusters and dwarf galaxies using physically motivated chemical enrichment parameters.
We present spectroscopic data for 180 red giant branch stars in the isolated dwarf irregular galaxy WLM. Observations of the Calcium II triplet lines in spectra of RGB stars covering the entire galaxy were obtained with FORS2 at the VLT and DEIMOS on
Keck II allowing us to derive velocities, metallicities, and ages for the stars. With accompanying photometric and radio data we have measured the structural parameters of the stellar and gaseous populations over the full galaxy. The stellar populations show an intrinsically thick configuration with $0.39 leq q_{0} leq 0.57$. The stellar rotation in WLM is measured to be $17 pm 1$ km s$^{-1}$, however the ratio of rotation to pressure support for the stars is $V/sigma sim 1$, in contrast to the gas whose ratio is seven times larger. This, along with the structural data and alignment of the kinematic and photometric axes, suggests we are viewing WLM as a highly inclined oblate spheroid. Stellar rotation curves, corrected for asymmetric drift, are used to compute a dynamical mass of $4.3pm 0.3times10^{8} $M$_{odot}$ at the half light radius ($r_{h} = 1656 pm 49$ pc). The stellar velocity dispersion increases with stellar age in a manner consistent with giant molecular cloud and substructure interactions producing the heating in WLM. Coupled with WLMs isolation, this suggests that the extended vertical structure of its stellar and gaseous components and increase in stellar velocity dispersion with age are due to internal feedback, rather than tidally driven evolution. These represent some of the first observational results from an isolated Local Group dwarf galaxy which can offer important constraints on how strongly internal feedback and secular processes modulate SF and dynamical evolution in low mass isolated objects.
We present the first determination of the radial velocities and metallicities of 78 red giant stars in the isolated dwarf irregular galaxy WLM. Observations of the calcium II triplet in these stars were made with FORS2 at the VLT-UT2 in two separated
fields of view in WLM, and the [Fe/H] values were conformed to the Carretta & Gratton (1997) metallicity scale. The mean metallicity is <[Fe/H]> = -1.27 +/- 0.04 dex, with a standard deviation of 0.37. We find that the stars in the inner field are more metal rich by [Fe/H] =0.30 +/- 0.06 dex. These results are in agreement with previous photometric studies that found a radial population gradient, as well as the expectation of higher metallicities in the central star forming regions. Age estimates using Victoria-Regina stellar models show that the youngest stars in the sample (< 6 Gyr) are more metal rich by [Fe/H] = 0.32 +/- 0.08 dex. These stars also show a lower velocity dispersion at all elliptical radii compared to the metal-poor stars. Kinematics for the whole red giant sample suggest a velocity gradient approximately half that of the gas rotation curve, with the stellar component occupying a thicker disk decoupled from the HI rotation plane. Taken together, the kinematics, metallicities, and ages in our sample suggest a young metal-rich, and kinematically cold stellar population in the central gas-rich regions of WLM, surrounded by a separate dynamically hot halo of older, metal poor stars.
