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Register a new userThe Stellar Population Histories of Local Early-Type Galaxies. I. Population Parameters
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S. C. Trager
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We present single stellar population (SSP) equivalent ages, metallicities, and abundance ratios for local elliptical galaxies derived from Hbeta, Mgb, and <Fe> absorption line strengths. We use an extension of the Worthey (1994) stellar population models that incorporates non-solar line-strength response functions by Tripicco & Bell (1995), allowing us to correct the models for the enhancements of Mg and other alpha-like elements relative to the Fe-peak elements. SSP-equivalent ages of local ellipticals from Gonzalez (1993) are found to vary widely, 1.5 < t < 18 Gyr, while metallicities [Z/H] and enhancement ratios [E/Fe] are strongly peaked around <[Z/H]>=+0.26 and <[E/Fe]>=+0.20 (in an aperture of radius Re/8). The enhancement ratios are milder than previous estimates, owing to the application of non-solar abundance corrections to both Mgb and <Fe> for the first time. Gradients in stellar populations within galaxies are found to be mild, with SSP-equivalent age decreasing by 25%, metallicity decreasing by <[Z/H]>=0.20 dex, and [E/Fe] remaining nearly constant out to an aperture of radius Re/2 for nearly all systems. Our ages have an overall zeropoint uncertainty of at least 25% due to uncertainties in the stellar evolution prescription, the oxygen abundance, the effect of non-solar abundances on the isochrones, and other unknowns. However, the relative age rankings of stellar populations should be largely unaffected by these errors. In particular, the large spread in ages appears to be real and cannot be explained by contamination of Hbeta by blue stragglers or hot horizontal branch stars, or by fill-in of Hbeta by emission. Correlations between these derived SSP-equivalent parameters and other galaxy observables will be discussed in future papers. (Abridged)
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We analyze single-stellar-population (SSP) equivalent parameters for 50 local elliptical galaxies as a function of their structural parameters. These galaxies fill a two-dimensional plane in the four-dimensional space of [Z/H], log t, log $sigma$, and [E/Fe]. SSP age and velocity dispersion can be taken as the two independent parameters that specify a galaxys location in this ``hyperplane. The hyperplane can be decomposed into two sub-relations: (1) a ``Z-plane, in which [Z/H] is a linear function of log $sigma$ and log t; and (2) a relation between [E/Fe] and $sigma$ in which [E/Fe] is larger in high-$sigma$ galaxies. Cluster and field ellipticals follow the same hyperplane, but their ($sigma$,t) distributions within it differ. Nearly all cluster galaxies are old; the field ellipticals span a large range in SSP age. The tight Mg--$sigma$ relations of these ellipticals can be understood as two-dimensional projections of the metallicity hyperplane showing it edge-on; the tightness of these relations does not necessarily imply a narrow range of ages at fixed $sigma$. The relation between [E/Fe] and $sigma$ is consistent with a higher effective yield of Type II SNe elements at higher $sigma$. The Z-plane is harder to explain and may be a powerful clue to star formation in elliptical galaxies if it proves to be general. Present data favor a ``frosting model in which low apparent SSP ages are produced by adding a small frosting of younger stars to an older base population. If the frosting abundances are close to or slightly greater than the base population, simple two-component models run along lines of constant $sigma$ in the Z-plane, as required. This favors star formation from well-mixed pre-enriched gas rather than unmixed low-metallicity gas from an accreted object. (Abridged)
We present stellar population parameters of twelve early-type galaxies (ETGs) in the Coma Cluster based on spectra obtained using the Low Resolution Imaging Spectrograph on the Keck II Telescope. Our data allow us to examine in detail the zero-point and scatter in their stellar population properties. Our ETGs have SSP-equivalent ages of on average 5-8 Gyr with the models used here, with the oldest galaxies having ages of ~10 Gyr old. This average age is identical to the mean age of field ETGs. Our ETGs span a large range in velocity dispersion but are consistent with being drawn from a population with a single age. Specifically, ten of the twelve ETGs are consistent within their formal errors of having the same age, 5.2+/-0.2 Gyr, over a factor of more than 750 in mass. We therefore find no evidence for downsizing of the stellar populations of ETGs in the core of the Coma Cluster. We suggest that Coma Cluster ETGs may have formed the majority of their mass at high redshifts but suffered small but detectable star formation events at z~0.1-0.3. Previous detections of downsizing from stellar populations of local ETGs may not reflect the same downsizing seen in lookback studies of RSGs, as the young ages of the local ETGs represent only a small fraction of their total masses. (abridged)
We present the stellar population content of early-type galaxies from the Atlas3D survey. Using spectra integrated within apertures covering up to one effective radius, we apply two methods: one based on measuring line-strength indices and applying single stellar population (SSP) models to derive SSP-equivalent values of stellar age, metallicity, and alpha enhancement; and one based on spectral fitting to derive non-parametric star-formation histories, mass-weighted average values of age, metallicity, and half-mass formation timescales. Using homogeneously derived effective radii and dynamically-determined galaxy masses, we present the distribution of stellar population parameters on the Mass Plane (M_JAM, Sigma_e, R_maj), showing that at fixed mass, compact early-type galaxies are on average older, more metal-rich, and more alpha-enhanced than their larger counterparts. From non-parametric star-formation histories, we find that the duration of star formation is systematically more extended in lower mass objects. Assuming that our sample represents most of the stellar content of todays local Universe, approximately 50% of all stars formed within the first 2 Gyr following the big bang. Most of these stars reside today in the most massive galaxies (>10^10.5 M_sun), which themselves formed 90% of their stars by z~2. The lower-mass objects, in contrast, have formed barely half their stars in this time interval. Stellar population properties are independent of environment over two orders of magnitude in local density, varying only with galaxy mass. In the highest-density regions of our volume (dominated by the Virgo cluster), galaxies are older, alpha-enhanced and have shorter star-formation histories with respect to lower density regions.
We study the internal radial gradients of the stellar populations in a sample comprising 522 early-type galaxies (ETGs) from the SAMI (Sydney- AAO Multi-object Integral field spectrograph) Galaxy Survey. We stack the spectra of individual spaxels in radial bins, and derive basic stellar population properties: total metallicity ([Z/H]), [Mg/Fe], [C/Fe] and age. The radial gradient ($ abla$) and central value of the fits (evaluated at R$_e$/4) are compared against a set of six possible drivers of the trends. We find that velocity dispersion ($sigma$) - or, equivalently gravitational potential - is the dominant driver of the chemical composition gradients. Surface mass density is also correlated with the trends, especially with stellar age. The decrease of $ abla$[Mg/Fe] with increasing $sigma$ is contrasted by a rather shallow dependence of $ abla$[Z/H] with $sigma$ (although this radial gradient is overall rather steep). This result, along with a shallow age slope at the massive end, imposes stringent constraints on the progenitors of the populations that contribute to the formation of the outer envelopes of ETGs. The SAMI sample is split between a field sample and a cluster sample. Only weak environment-related differences are found, most notably a stronger dependence of central total metallicity ([Z/H]$_{e4}$) with $sigma$, along with a marginal trend of $ abla$[Z/H] to steepen in cluster galaxies, a result that is not followed by [Mg/Fe]. The results presented here serve as constraints on numerical models of the formation and evolution of ETGs.
We present results from a pilot study of radial stellar population trends in early-type galaxies using the VLT VIMOS integral field unit (IFU). We observe twelve galaxies in the cluster Abell 3389 (z~0.027). For each galaxy, we measure 22 line-strength indices in multiple radial bins out to at least the effective radius. We derive stellar population parameters using a grid inversion technique, and calculate the radial gradients in age, metallcity and alpha-abundance. Generally, the galaxies in our sample have flat radial trends in age and [alpha/Fe], but negative gradients in [Z/H] (-0.20 +/- 0.05 dex). Combining our targets with two similar, long-slit studies to increase sample size, we find that the gradients are not correlated with the central velocity dispersion or K-band luminosity (both proxies for galaxy mass). However, we find that the age and metallicity gradients are both anti-correlated with their respective central values (to > 4 sigma), such that galaxies with young cores have steeper positive age gradients, and those with metal-rich centres have strong negative [Z/H] gradients.
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