Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userThe Stellar Population Histories of Early-Type Galaxies. II. Controlling Parameters of the Stellar Populations
83
0
0.0
(
0
)
Authors
S. C. Trager
Ask ChatGPT about the research
No Arabic abstract
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)
rate research
Read More
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)
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)
A series of population models are designed to explore the star formation history of gas-rich, low surface brightness (LSB) galaxies. LSB galaxies are unique in having properties of very blue colors, low H$alpha$ emission and high gas fractions that indicated a history of constant star formation (versus the declining star formation models used for most spirals and irregulars). The model simulations use an evolving multi-metallicity composite population that follows a chemical enrichment scheme based on Milky Way observations. Color and time sensitive stellar evolution components (i.e., BHB, TP-AGB and blue straggler stars) are included, and model colors are extended into the Spitzer wavelength regions for comparison to new observations. In general, LSB galaxies are well matched to the constant star formation scenario with the variation in color explained by a fourfold increase/decrease in star formation over the last 0.5 Gyrs (i.e., weak bursts). Early-type spirals, from the S$^4$G sample, are better fit by a declining star formation model where star formation has decreased by 40% in the last 12 Gyrs.
We present radial stellar population parameters for a subsample of 12 galaxies from the 36 isolated early-type galaxies of Reda et al. Using new long-slit spectra, central values and radial gradients for the stellar age, metallicity [Z/H] and alpha-element abundance [E/Fe] are measured. Similarly, the central stellar population parameters are derived for a further 5 isolated early-type galaxies using their Lick indices from the literature. On average, the seventeen isolated galaxies have mean central [Z/H]o and [E/Fe]o of 0.29+/-0.03 and 0.17+/-0.03 respectively and span a wide range of ages from 1.7 to 15 Gyrs. We find that isolated galaxies follow similar scaling relations between central stellar population parameters and galaxy velocity dispersion to their counterparts in high density environments. However, we note a tendency for isolated galaxies to have slightly younger ages, higher [Z/H] and lower [E/Fe]. Such properties are qualitatively consistent with the expectation of an extended star formation history for galaxies in lower density environments. Generally we measure constant age and [E/Fe] radial gradients. We find that the age gradients anti-correlate with the central galaxy age. Metallicity gradients range from near zero to strongly negative. For our high mass galaxies metallicity gradients are shallower with increasing mass. Such behaviour is not predicted in dissipational collapse models but might be expected in multiple mergers. The metallicity gradients correlate with the central age and metallicity, as well as to the age gradients. In conclusion, our stellar population data for isolated galaxies are more compatible with an extended merger/accretion history than early dissipative collapse.
We have acquired intermediate resolution spectra in the 3700-7000 A wavelength range for a sample of 65 early-type galaxies predominantly located in low density environments, a large fraction of which show emission lines. The spectral coverage and the high quality of the spectra allowed us to derive Lick line-strength indices and to study their behavior at different galacto-centric distances. Ages, metallicities and element abundance ratios have been derived for the galaxy sample by comparison of the line-strength index data set with our new developed Simple Stellar Population (SSP) models. We have analyzed the behavior of the derived stellar population parameters with the central galaxy velocity dispersion and the local galaxy density in order to understand the role played by mass and environment on the evolution of early-type galaxies. We find that the chemical path is mainly driven by the halo mass, more massive galaxies exhibiting the more efficient chemical enrichment and shorter star formation timescales. Galaxies in denser environments are on average older than galaxies in less dense environments. The last ones show a large age spread which is likely to be due to rejuvenation episodes.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
