Guy Worthey

The Stellar Population Histories of Early-Type Galaxies. II. Controlling Parameters of the Stellar Populations

This paper analyzes single stellar population (SSP)–equivalent parameters for 50 local elliptical galaxies as a function of their structural parameters. The galaxy sample is drawn from the high-quality spectroscopic surveys of Gonzalez (1993) and Kuntschner (1998). The basic data are central values of SSP-equivalent ages, t, metallicities, [Z/H], and enhancement ratios, [E/Fe], derived in Paper I, together with global structural parameters including velocity dispersions, radii, surface brightnesses, masses, and luminosities. The galaxies fill a two-dimensional plane in the four-dimensional space of [Z/H], log t, log σ, and [E/Fe]. SSP age, t, and velocity dispersion, σ, can be taken as the two independent parameters that specify a galaxys location in this hyperplane. The hyperplane can be decomposed into two subrelations: (1) a Z-plane, in which [Z/H] is a linear function of log σ and log t and (2) a relation between [E/Fe] and σ in which [E/Fe] is larger in high-σ galaxies. Velocity dispersion is the only structural parameter that is found to modulate the stellar populations; adding other structural variables such as Ie or re does not predict [Z/H] or [E/Fe] more accurately. Cluster and field ellipticals follow the same hyperplane, but their (σ,t) distributions within it differ. Most Fornax and Virgo cluster galaxies are old, with a only a small sprinkling of galaxies to younger ages. The field ellipticals span a larger range in SSP age, with a tendency for lower σ galaxies to be younger. The present sample thus suggests that the distribution of local ellipticals in the (σ,t) plane may depend on environment. Since the (σ,t) distribution affects all two-dimensional projections involving SSP parameters, many of the familiar scaling laws attributed to ellipticals may also depend on environment. Some evidence for this is seen in the current sample. For example, only Fornax ellipticals show the classic mass-metallicity relation, whereas other subsamples do not. The tight Mg-σ relations of these ellipticals can be understood as two-dimensional projections of the metallicity hyperplane showing it edge-on. At fixed σ, young age tends to be offset by high [Z/H], preserving Mg nearly constant. The tightness of the Mg-σ relations does not necessarily imply a narrow range of ages at fixed σ. Although SSP parameters are heavily weighted by young stars, modeling them still places tight constraints on the total star formation history of elliptical galaxies. The relation between [E/Fe] and σ is consistent with a higher effective yield of Type II SNe elements at higher σ. This might occur if the IMF is enhanced in massive stars at high σ, or if more SNe II–enriched gas is retained by deeper galactic potential wells. Either way, modulating Type II yields versus σ seems to fit the data better than modulating Type Ia yields. 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 (assuming no change in σ). If the frosting abundances are close to or slightly greater than the base population, simple two-component models run along lines of constant σ 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.

The Distribution of Heavy Elements in Spiral and Elliptical Galaxies

In large disk and spheroidal galaxies, spatially resolved abundance information can be extracted by analysis of either emission lines, absorption lines, or both, depending on the situation. This review recaps signiÐcant results as they apply to nondwarf galaxies, including the Milky Way, spiral disks and bulges, and elliptical and lenticular galaxies. Methods for determining abundances are explained in appendices. Conclusions that span the galaxy types treated here are as follows. All galaxies, on average, have heavy- element abundances (metallicities) that systematically decrease outward from their galactic centers while their global metallicities increase with galaxy mass. Abundance gradients are steepest in normal spirals and are seen to be progressively Natter going in order from barred spirals to lenticulars to ellipticals. The distribution of abundances N(Z) versus Z is strongly peaked compared with simple closed-box model predictions of chemical enrichment in all galaxy types. That is, a G dwarf problem, II commonly known in the solar cylinder, exists for all large galaxies. For spiral galaxies, local metallicity appears to be correlated with total (disk)bulge) surface density. Examination of N/O versus O/H in spiral disks indicates that production of N is dominated by primary processes at low metallicity and secondary processes at high metallicity. Carbon production increases with increasing metallicity. Abundance ratios Ne/O, S/O, and Ar/O appear to be universally constant and independent of metallicity, which argues either that the initial mass function (IMF) is universally constant or that these ratios are not sensitive to IMF variations. In the Milky Way, there is a rough age-metallicity trend with much scatter, in the sense that older stars are more metal poor. In elliptical galaxies, nuclear abundances are in the range (Z/H) 0.0E0.4, but the element mixture is not scaled-solar. In large elliptical galaxies (Mg/Fe) is in the range 0.3E0.5, decreasing to B0 in smaller elliptical galaxies. Other light elements track the Mg enhancement, but the heavier Ca tracks Fe. Velocity dispersion appears to be a key parameter in the modulation of (Mg/Fe), but the cause of the connection is unclear.

Abundance Ratio Trends and Nucleosynthesis in Elliptical Galaxies and Spheroids

Model and observed spectral feature indices indicate that, in galaxies of all types with velocity dispersions larger than about 225 km s–1, [Mg/Fe] progressively drifts greater than zero, until it reaches about 0.3 in the largest ellipticals. For elliptical galaxies that have absorption-line data for more elements, the abundances of Na and N elevate in a similar fashion, relative to both Ca and Fe ([Ca/Fe] ≈ 0; [Na, N/Fe] progressively > 0). Titanium may also share this apparently generic light-element behavior. The abundance pattern in elliptical galaxies matches neither the disk, the halo, nor the bulge of our own Galaxy, although the bulge appears to come the closest. N is depleted in the Galactic bulge but elevated in M31, M31s metal-rich globular clusters, and large elliptical galaxies. If all measured elements are considered, the abundance patterns in our own Galaxy and in external galaxies requires the presence of at least three sources of chemical enrichment whose relative contributions can vary from environment to environment. These three sources of enrichment may correspond to Type Ia supernovae (SNe), Type II SNe, and N-rich mass loss from intermediate-mass stars, but the behavior of O, Ca, Si, Sc, V, and Ti in the Galactic bulge, disk, and halo seems to require at least one additional supernova flavor.Abundance ratio effects represent a barrier to the estimation of mean ages from integrated light, a barrier that is greater than that of isochrone error. Isochrone grids allowing for the variation of individual elements are needed, but relative changes in isochrone temperatures computed as a function of abundance pattern need to be accurate to roughly 7 K if 5% age estimates are desired.

Evidence for a Young Stellar Population in NGC 5018

Two absorption line indices, Ca II and Hδ/λ4045, measured from high-resolution spectra are used with evolutionary synthesis models to verify the presence of a young stellar population in NGC 5018. The derived age of this population is ~2.8 Gyr with a metallicity roughly solar, and it completely dominates the integrated light of the galaxy near 4000 A.

Color‐Color Relations for Red Giants in Star Clusters

New Johnson-Cousins UBVRI photometry of giants in globular clusters is combined with JHK photometry on the CIT system to produce color sequences for giants from the globular clusters M3, M5, M13, and M92. UBVRI data are also presented for giants in the metal-rich open cluster NGC 6791. These data fill a gap in the literature, especially for the R and I bands. We provide the empirical relations between broadband colors for various (Fe/H) values for metal-poor giants. The color sequences for and show clear U 2 BB 2 V separations for different (Fe/H) values. We also find weak, although unexpected, metallicity dependences of , , and colors. is metal insensitive. The above colors are plotted as a function of V 2 RV 2 IJ 2 KH 2 KV 2 , and a literature relation is given. K (V 2 K)-T eff

Elliptical Galaxy Chemical Evolution

Conclusions about elliptical and other early-type galaxies come from studies of integrated starlight and studies of giant stars in the nearest galaxies. The abundance distribution for all galaxy types at all radii appears to exhibit a “G dwarf problem” in that the distribution is more strongly peaked than the one-zone closed-box constantyield homogeneous instantaneous-recycling analytic model predicts. The peak of the distribution is near solar for galaxies as small as M32 up to giant elliptical galaxies. Much (and perhaps all) of the line-strength variation between small and large galaxies is due to increased light element abundances in larger galaxies. In large elliptical galaxies [N/Fe], [Na/Fe], and [Mg/Fe] appear to be about a factor of 2 supersolar, but [Ca/Fe] ≈0. The fact that the larger ellipticals show enhancement but not small ones leads to various conjectures about the ratio of chemical enrichment contributed by Type Ia supernovae versus Type II, and how this ratio can be made to vary with environment. At least a plurality of elliptical galaxies show an abundance gradient of about ∆ log Z ≈ -0.3∆ log R. This is about a factor of 2 more shallow than that reported for the disks of spiral galaxies, and at least some ellipticals seem to have gradients even more shallow than that. [Mg/Fe] appears approximately constant with radius, implying that Mg enrichment is an approximately global phenomenon.