Field Ellipticals Research Articles

Cluster and field elliptical galaxies atz~ 1.3

We compared the properties of 56 elliptical galaxies selected from three clusters at $1.2<z<1.4$ with those of field galaxies in the GOODS-S (~30), COSMOS (~180) and CANDELS (~220) fields. We studied the relationships among effective radius, surface brightness, stellar mass, stellar mass density $\Sigma_{Re}$ and central mass density $\Sigma_{1kpc}$ within 1 kpc radius. We find that cluster ellipticals do not differ from field ellipticals: they share the same structural parameters at fixed mass and the same scaling relations. On the other hand, the population of field ellipticals at $z\sim1.3$ shows a significant lack of massive ($M_*> 2\times 10^{11}$ M$_\odot$) and large (R$_e > 4-5$ kpc) ellipticals with respect to the cluster. Nonetheless, at $M*<2\times 10^{11}$ M$_\odot$, the two populations are similar. The size-mass relation of ellipticals at z~1.3 defines two different regimes, above and below a transition mass $m_t\sim 2-3\times10^{10}$ M$_\odot$: at lower masses the relation is nearly flat (R$_e\propto M_*^{-0.1\pm 0.2}$), the mean radius is constant at ~1 kpc and $\Sigma_{Re}\sim \Sigma_{1kpc}$ while, at larger masses, the relation is R$_e\propto M*^{0.64\pm0.09}$. The transition mass marks the mass at which galaxies reach the maximum $\Sigma_{Re}$. Also the $\Sigma_{1kpc}$-mass relation follows two different regimes, $\Sigma_{1kpc}\propto M*^{0.64\ >m_t}_{1.07\ <m_t}$, defining a transition mass density $\Sigma_{1kpc}\sim 2-3\times10^3$ M$_\odot$ pc$^{-2}$. The mass density $\Sigma_{Re}$ does not correlate with mass, dense/compact galaxies can be assembled over a wide mass regime, independently of the environment. The central mass density, $\Sigma_{1kpc}$, besides to be correlated with the mass, is correlated to the age of the stellar population: the higher the central stellar mass density, the higher the mass, the older the age of the stellar population. [Abridged]

The XXL survey XV: evidence for dry merger driven BCG growth in XXL-100-GC X-ray clusters

The growth of brightest cluster galaxies is closely related to the properties of their host cluster. We present evidence for dry mergers as the dominant source of BCG mass growth at $z\lesssim1$ in the XXL 100 brightest cluster sample. We use the global red sequence, H$\alpha$ emission and mean star formation history to show that BCGs in the sample possess star formation levels comparable to field ellipticals of similar stellar mass and redshift. XXL 100 brightest clusters are less massive on average than those in other X-ray selected samples such as LoCuSS or HIFLUGCS. Few clusters in the sample display high central gas concentration, rendering inefficient the growth of BCGs via star formation resulting from the accretion of cool gas. Using measures of the relaxation state of their host clusters, we show that BCGs grow as relaxation proceeds. We find that the BCG stellar mass corresponds to a relatively constant fraction 1\%\ of the total cluster mass in relaxed systems. We also show that, following a cluster scale merger event, the BCG stellar mass lags behind the expected value from the M$_{cluster}$ - M$_{BCG}$ relation but subsequently accretes stellar mass via dry mergers as the BCG and cluster evolve towards a relaxed state.

The effects of environment on the intrinsic shape of galaxies

We measure the effect of the environment on the intrinsic shapes of spiral and elliptical galaxies by finding the 3D shape distribution and dust extinction that fits better the projected shape of galaxies in different environment. We find that spiral galaxies in groups are very similar to field spirals with similar intrinsic properties (magnitudes, sizes and colours). But for spirals in groups, those in denser environments or closer to the centre of the group tend to have a more circular disc than similar galaxies in less dense environments or far from the group centres. Also we find that central spiral galaxies in their groups tend to be thinner than other similar spirals. For ellipticals, we do not find any important dependence of their shape on their position in a group or on the local density. However, we find that elliptical galaxies in groups tend to be more spherical than field ellipticals with similar intrinsic properties. We find that, once in groups, the shape of member galaxies do not depend on group mass, regardless of their morphological type.

The redistribution of matter in the cores of galaxy clusters

We present cosmological N-body resimulations of the assembly of the Brightest Cluster Galaxies (BCGs) in rich clusters. At z = 2 we populate dark matter subhalos with self-gravitating stellar systems whose abundance and structure match observed high-redshift galaxies. By z = 0, mergers have built much larger galaxies at cluster centre. Their dark matter density profiles are shallower than in corresponding darkmatter-only simulations, but their total mass density profiles (stars + dark matter) are quite similar. Differences are found only at radii where the effects of central black holes may be significant. Dark matter density slopes shallower than = 1.0 occur for r/r200 < 0.015, close to the half-light radii of the BCGs. Our experiments support earlier suggestions that NFW-like profiles are an attractor for the hierarchical growth of structure in collisionless systems – total mass density profiles asymptote to the solution found in dark-matter-only simulations over the radial range where mergers produce significant mixing between stars and dark matter. Simulated dark matter fractions are substantially higher in BCGs than in field ellipticals, reaching 80% within the half-light radius. We also estimate that supermassive black hole mergers should create BCG cores as large as rc � 3kpc. The good agreement of all these properties with recent observational studies of BCG structure suggests that dissipational processes have not played a dominant role in the assembly of the observed systems.

3D Spectroscopy of Dwarf Elliptical Galaxies

I present some results of 3D spectroscopy for a small sample of dwarf elliptical galaxies, mostly members of small groups. The galaxies under consideration have a typical absolute magnitude of -18 (B-band), and at the Kormendy's relation they settle within a transition zone between the main cloud of giant ellipticals and the sequence of diffuse ellipticals. By measuring Lick indices and investigating radial profiles of the SSP-equivalent ages and metallicities of the stellar populations in their central parts, I have found evolutionary distinct cores in all of them. Typically, the ages of these cores are 2-4 Gyr, and the metallicities are higher than the solar one. Outside the cores, the stellar populations are always old, T>12 Gyr, and the metallicities are subsolar. This finding implies that the well-known correlation between the stellar age and the total mass (luminosity) of field ellipticals (Trager et al. 2000, Caldwell et al. 2003, Howell 2005) may be in fact a direct consequence of a larger contribution of nuclear starbursts into the integrated stellar population in dwarfs with respect to giants, and does not relate to `downsizing'.

STELLAR POPULATIONS OF ELLIPTICAL GALAXIES IN THE LOCAL UNIVERSE

We study the stellar populations of 1,923 elliptical galaxies at z<0.05 selected from the Sloan Digital Sky Survey as a function of velocity dispersion, sigma, and environment. We construct average, high signal-to-noise spectra and find the following: (1) lower-sigma galaxies have a bluer optical continuum and stronger (but still weak) emission lines; (2) at fixed sigma, field ellipticals have a slightly bluer stellar continuum, especially at wavelengths \lesssim 4000 \AA, and have stronger (but still weak) emission lines compared to their group counterparts, although this environmental dependence is strongest for low-sigma ellipticals. Based on Lick indices measured from both the individual and average spectra, we find that: (1) at a given sigma, elliptical galaxies in groups have systematically weaker Balmer absorption than their field counterparts, although this environmental dependence is most pronounced at low sigma; (2) there is no clear environmental dependence of <Fe>, while the alpha-element absorption indices such as Mgb are only slightly stronger in galaxies belonging to rich groups. An analysis based on simple stellar populations (SSPs) reveals that more massive elliptical galaxies are older, more metal-rich and more strongly alpha-enhanced. We also find that: (1) the SSP-equivalent ages of galaxies in rich groups are, on average, ~1 Gyr older than in the field, although once again this effect is strongest at low sigma; (2) galaxies in rich groups have slightly lower [Fe/H] and are marginally more strongly alpha-enhanced; and (3) there is no significant environmental dependence of total metallicity, [Z/H]. Our results are generally consistent with stronger low-level recent star formation in field ellipticals at low sigma, similar to recent results based on ultraviolet and infrared observations. (Abridged)

THE NATURE OF FOSSIL GALAXY GROUPS: ARE THEY REALLY FOSSILS?

We use SDSS-DR4 photometric and spectroscopic data out to redshift z ~ 0.1 combined with ROSAT All Sky Survey X-ray data to produce a sample of 25 fossil groups (FGs), defined as bound systems dominated by a single, luminous elliptical galaxy with extended X-ray emission. We examine possible biases introduced by varying the parameters used to define the sample, and the main pitfalls are also discussed. The spatial density of FGs, estimated via the V/V MAX test, is 2.83 × 10–6 h 3 75 Mpc–3 for LX > 0.89 × 1042 h –2 75 erg s–1 consistent with Vikhlinin et al., who examined an X-ray overluminous elliptical galaxy sample (OLEG). We compare the general properties of FGs identified here with a sample of bright field ellipticals generated from the same data set. These two samples show no differences in the distribution of neighboring faint galaxy density excess, distance from the red sequence in the color-magnitude diagram, and structural parameters such as a 4 and internal color gradients. Furthermore, examination of stellar populations shows that our 25 FGs have similar ages, metallicities, and α-enhancement as the bright field ellipticals, undermining the idea that these systems represent fossils of a physical mechanism that occurred at high redshift. Our study reveals no difference between FGs and field ellipticals, suggesting that FGs might not be a distinct family of true fossils, but rather the final stage of mass assembly in the universe.

The Evolution and Structure of Early‐Type Field Galaxies: A Combined Statistical Analysis of Gravitational Lenses

We introduce a framework for simultaneously investigating the structure and luminosity evolution of early-type gravitational lens galaxies. The method is based on the fundamental plane, which we interpret using the aperture mass-radius relations derived from lensed image geometries. We apply this method to our previous sample of 22 lens galaxies with measured redshifts and excellent photometry. Modeling the population with a single mass profile and evolutionary history, we find that early-type galaxies are nearly isothermal (logarithmic density slope n = 2.06 +/- 0.17, 68% C.L.), and that their stars evolve at a rate of dlog(M/L)_B/dz = -0.50 +/- 0.19 (68% C.L.) in the rest frame B band. For a Salpeter IMF and a concordance cosmology, this implies a mean star formation redshift of > 1.5 at 95% confidence. While this model can neatly describe the mean properties of early-type galaxies, it is clear that the scatter of the lens sample is too large to be explained by observational uncertainties alone. We therefore consider statistical models in which the galaxy population is described by a distribution of star formation redshifts. We find that stars must form over a significant range of redshifts (Delta z_f > 1.7, 68% C.L.), which can extend as low as z_f = 1 for some acceptable models. However, the typical galaxy will still have an old stellar population ( > 1.5). The lens sample therefore favors early star formation in field ellipticals -- even if we make no a priori assumption regarding the shape of the mass distribution in lenses, and include the range of possible deviations from homology in the uncertainties. Our evolution results call into question several recent claims that early-type galaxies in low-density environments have much younger stars than those in rich clusters.

Age and metallicity estimations in old stellar populations from Strömgren photometry

We present a new technique to determine age and metallicity of old stellar populations (globular clusters and elliptical galaxies) using an iterative principal component analysis on narrow band (Strömgren) colors. Our technique is capable of reproducing globular cluster [Fe/H] values to 0.02 dex and CMD ages to 1.0 Gyrs. We also present preliminary results on the application of our technique to a sample of high mass, field ellipticals and low mass, cluster dwarf ellipticals. We confirm the results of earlier studies which find that globular clusters increase in metallicity with age and that age and metallicity increase with galaxy mass. However, we find that dwarf ellipticals deviate from the elliptical sequence by having little to no correlation between age and metallicity.

A Sample of Field Ellipticals

Using well-defined selection criteria derived from Zaritsky et al. applied to the LEDA galaxy catalog, we have constructed a sample of elliptical galaxies that can be taken to lie in the field. Such criteria can easily be applied to theoretical simulations for direct comparison with observations. The variation of the number of ellipticals with selection criteria is also investigated. A preliminary study of the environment of the field ellipticals shows that, in the mean, they are surrounded by a population of dwarf galaxies, out to projected radii of at least 500 kpc, with a radial density profile of r-0.6±0.2 and a luminosity function slope of α ~ -1.8. The results are compared and contrasted to the satellite population around isolated spiral galaxies.

Ultraluminous Infrared Galaxies: QSOs in Formation?

We present new near-infrared (NIR) Keck and Very Large Telescope (VLT) spectroscopic data on the stellar dynamics in late-stage, ultraluminous infrared galaxy (ULIRG) mergers. We now have information on the structural and kinematic properties of 18 ULIRGs, eight of which contain QSO-like active galactic nuclei (AGNs). The host properties (σ, reff, μeff, MK) of AGN-dominated and star formation dominated ULIRGs are similar. ULIRGs fall remarkably close to the fundamental plane of early-type galaxies. They populate a wide range of the plane, are on average similar to L* rotating ellipticals, but are well offset from giant ellipticals and optically/UV-bright, low-z QSOs/radio galaxies. ULIRGs and local QSOs/radio galaxies are very similar in their distributions of bolometric and extinction-corrected NIR luminosities, but ULIRGs have smaller effective radii and velocity dispersions than the local QSO/radio galaxy population. Hence, their host masses and inferred black hole masses are correspondingly smaller. The latter are more akin to those of local Seyfert galaxies. ULIRGs thus resemble local QSOs in their NIR and bolometric luminosities because they are (much more) efficiently forming stars and/or feeding their black holes, and not because they have QSO-like, very massive black holes. We conclude that ULIRGs as a class cannot evolve into optically bright QSOs. They will more likely become quiescent, moderate mass field ellipticals or, when active, might resemble the X-ray-bright, early-type galaxies that have recently been found by the Chandra observatory.

Old elliptical galaxies at z = 1.5 and the Kormendy relation

Deep spectroscopy of the two mJy radio galaxies LBDS 53W069 and 53W091 has previously shown them to have old (>3 Gyr) stellar populations at z=1.5. Here we present the results of HST observations of these galaxies. 53W069 has an r^1/4 profile in both the F814W & F110W data with an effective radius of 0.30 arcsec (2.7 kpc). The restframe U-B colour gradient requires a stellar population of super-solar (3Z_sun) metallicity that formed on a very short timescale at high redshift (z>5). 53W091 has a regular r^1/4 profile in F110W with an effective radius of 0.32 arcsec (2.9 kpc). The F814W profile is more extended and is consistent with a blue exponential disk that contributes 20% of the flux within r_e. The restframe U-B colour gradient is significantly larger than that observed in field ellipticals at z<1, implying a stellar population of mixed metallicity (1-3Z_sun) that formed in a high-redshift rapid burst. We have compared these two LBDS radio galaxies with the Kormendy relation of ten 3CR radio galaxies at z=0.8. The LBDS galaxies follow the same relation as the more radio-luminous 3CR galaxies, assuming passive evolution of their stellar populations, although they are smaller than the 3CR galaxies whose mean effective radius is 12 kpc. Their sizes and radio luminosities are consistent with scaling relations applied to the 3CR galaxies, in which both radio power and effective radius scale with galaxy mass. Compared with a sample of z=0.4 cluster ellipticals, 53W069 & 53W091 lie well within the scatter of the Kormendy relation. We conclude that the hosts of these mJy radio sources at z=1.5 are passively-evolving elliptical galaxies that will evolve into ordinary L* ellipticals by the present day. (Abridged)

On the formation of globular cluster systems in a hierarchical Universe

ABSTRA C T We have investigated the formation of globular cluster (GC) systems in the fiducial semianalytic model of galaxy formation of Cole et al., by assuming that GCs are formed at high redshifts ﷿z . 5fi in protogalactic fragments, and during the subsequent gas-rich merging of these fragments. Under these assumptions we have simulated the GC systems of 450 elliptical galaxies, and find that the majority (93 per cent) are intrinsically bimodal in metallicity. We find that, in the mean, the metal-rich GC subpopulations are younger than the metal-poor GC subpopulations, with ages of 9 and 12 Gyr respectively, and that the mean ages of the metalrich GCs are dependent upon host galaxy luminosity and environment (halo circular velocity), whereas the metal-poor GCs are not. We find that the continued gaseous merging of the protogalactic fragments leads to significant age-structure amongst the metal-rich GCs. These GCs exhibit a large age-range (5 to 12 Gyr), which increases for low-luminosity galaxies, and for galaxies in low circular velocity haloes. Moreover, the metal-rich GCs associated with low-luminosity field and/or group ellipticals are , 2 Gyr younger than the metal-rich GCs in luminous cluster ellipticals. We find that the total GC populations scale with host galaxy luminosity as NGC / L 1:25 V;gal , a result in agreement with observations of luminous elliptical galaxies. This scaling is due to a systematic increase in the M/L ratios of the galaxy haloes with luminosity for LV;gal . L * galaxies in the model. A comparison between the luminosity growth of the model ellipticals and their GC formation indicates that mergers do not significantly affect SN at z , 2. We find the mean colours of the both the metal-rich and metal-poor GCs exhibit only a weak dependence upon host galaxy luminosity, a result consistent with contemporary observations. We conclude that gaseous merging, the bulk of which occurs at 1 # z # 4i n ourLCDM model, leads to the formation of the metalrich peak of the GC systems of elliptical galaxies. We suggest that the formation and subsequent truncation of the metal-poor GCs in the protogalactic fragments is closely related to the star formation rate in these fragments, which may have been significantly higher at very early times.

The hierarchical origin of galaxy morphologies

We report first results from a series of N-body/gasdynamical simulations designed to study the origin of galaxy morphologies in a cold dark matter-dominated universe. The simulations include star formation and feedback and have numerical resolution sufficiently high to allow for a direct investigation of the morphology of simulated galaxies. We find, in agreement with previous theoretical work, that the presence of the main morphological components of galaxies—disks, spheroids, bars—is regulated by the mode of gas accretion and intimately linked to discrete accretion events. In the case we present, disks arise from the smooth deposition of cooled gas at the center of dark halos, spheroids result from the stirring of preexisting disks during mergers, and bars are triggered by tides generated by satellites. This demonstrates that morphology is a transient phenomenon within the lifetime of a galaxy and that the Hubble sequence reflects the varied accretion histories of galaxies in hierarchical formation scenarios. In particular, we demonstrate directly that disk/bulge systems can be built and rebuilt by the smooth accretion of gas onto the remnant of a major merger and that the present-day remnants of late dissipative mergers between disks are spheroidal stellar systems with structure resembling that of field ellipticals. The perplexing variety of galaxy morphologies is thus highly suggestive of—and may actually even demand—a universe where structures have evolved hierarchically.

A NICMOS view of GPS, CSS, and FR II host galaxies

We present the results of HST NICMOS observations of a sample of GPS, CSS, and large scale FR ii radio galaxies. The absolute magnitude, near-IR colors, and surface brightness profiles of the three classes of objects are similar. This is consistent with the scenario in which a powerful radio source evolves along the GPS–CSS–FR ii size sequence. Their properties are more like regular (giant) field ellipticals than Brightest Cluster Galaxies (BCG’s). This implies GPS/CSS radio sources apparently do not evolve into BCG FR i type sources.

Bulge Globular Clusters in Spiral Galaxies

There is now strong evidence that the metal-rich globular clusters (GC) near the center of our Galaxy are associated with the Galactic bulge rather than the disk as previously thought. Here we extend the concept of bulge GCs to the GC systems of nearby spiral galaxies. In particular, the kinematic and metallicity properties of the GC systems favor a bulge rather than a disk origin. The number of metal-rich GCs normalized by the bulge luminosity is roughly constant (i.e. bulge S_N ~ 1) in nearby spirals, and this value is similar to that for field ellipticals when only the red (metal--rich) GCs are considered. We argue that the metallicity distributions of GCs in spiral and elliptical galaxies are remarkably similar, and that they obey the same correlation of mean GC metallicity with host galaxy mass. We further suggest that the metal-rich GCs in spirals are the direct analogs of the red GCs seen in ellipticals. The formation of a bulge/spheroidal stellar system is accompanied by the formation of metal-rich GCs. The similarities between GC systems in spiral and elliptical galaxies appear to be greater than the differences.

The Relative Star Formation Histories of Spiral Bulges and Elliptical Galaxies in the Hubble Deep Fields

Hierarchical galaxy formation models make specific predictions concerning the relative assembly rates and star formation histories of spiral bulges and field ellipticals. Independently of the cosmological model and initial power spectrum of fluctuations, at all epochs the stellar populations in spiral bulges should be older and redder than those in typical ellipticals selected at the same redshift. To test this simple prediction, we analyze the internal optical colors of a complete sample of I814 < 24 mag early-type and spiral galaxies from the northern and southern Hubble Deep Fields (HDF). The subset of galaxies in the HDF-North are also investigated in the near-infrared using NICMOS photometry. We compare the central (inner 5% radius) colors of those spirals with clearly visible bulges with the integrated colors of ellipticals in our sample. Comparisons are possible to a redshift z 1, at which point well-defined bulges become difficult to locate. The reliability of determining bulge colors using central apertures is tested by considering the homogeneity of the pixel-by-pixel colors for typical cases and through comparisons based on the simulated appearance at moderate redshift of the local sample of de Jong. We show via these tests and by selecting HDF subsets chosen according to inclination that disk contamination effects should be minimal. While spiral bulges are systematically redder in their optical colors than their associated disks at all redshifts, we find that the majority are significantly bluer than the red locus occupied by most field ellipticals at similar redshifts. In the near-infrared, similar trends are found at redshifts z < 0.6, but at higher redshifts some bulges as red as the reddest ellipticals are found. We conclude that a significant rejuvenation may have occurred in the inner stellar populations of many spiral galaxies, particularly those at intermediate redshifts. We examine the optical and near-infrared colors of the HDF bulges in the context of models that include the effects of secondary star formation superimposed upon preexisting old populations and conclude the data is best fitted when this secondary activity is burstlike. We discuss the consequences for models of secular evolution in disks should these bursts have been particularly prevalent at z 0.6 as the limited HDF data seems to imply.

Detection Rate of Molecular Gas in Elliptical Galaxies: Constraints on Galaxy-Formation Theories

Abstract We calculated the detection rate of cold gas in elliptical galaxies using a semi-analytic model of galaxy formation, and compared it with observations. We have shown that a model with a long star-formation time-scale ( ∼ 20 Gyr) is inconsistent with observations. Thus, some mechanisms of reducing the mass of interstellar medium, such as the consumption of molecular gas by star formation and/or reheating from supernovae, are certainly effective in galaxies. Our model predicts that star formation is induced when galaxies in a halo collide with each other and it reduces the cold gas left until the present. However, we find that the reduction through random collisions of satellite (non-central) galaxies in the mean-free time-scale in a halo is not required to explain the observations. For cD galaxies, the predicted detection rate of cold gas is consistent with observations as long as the transformation of hot gas into cold gas is prevented in halos whose circular velocities are larger than 500;km; s−1. Moreover, we find that the cold gas brought into cDs through capturing gas-rich galaxies is small. We also show that the fraction of galaxies with observable cold gas should be small for cluster ellipticals in comparison with that for field ellipticals. Our fiducial models and the models with a large reheating efficiency can reproduce the observations well, although a comparison with a larger and complete sample of elliptical galaxies will constrain the physical parameters in galaxy-formation theories more strictly.

The stellar content of brightest cluster galaxies

We present near-infrared K-band spectroscopy of 21 elliptical or cD brightest cluster galaxies (BCGs), for which we have measured the strength of the 2.293-μm CO stellar absorption feature. We find that the strength of this feature is remarkably uniform among these galaxies, with a smaller scatter in equivalent width than for the normal elliptical population in the field or clusters. The scatter for BCGs is 0.156 nm, compared with 0.240 nm for Coma cluster ellipticals, 0.337 nm for ellipticals from a variety of other clusters, and 0.422 nm for field ellipticals. We interpret this homogeneity as being the result of a greater age, or more uniform history of star formation in BCGs than in other ellipticals; only a small fraction of the scatter can be caused by metallicity variations, even in the BCGs. Notwithstanding the small scatter, correlations are found between CO strength and various galaxy properties, including R-band absolute magnitude, which could improve the precision of these galaxies as distance indicators in measurements of cosmological parameters and velocity flows.

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 González (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 galaxy's 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.