Low-luminosity Ellipticals Research Articles

SDSS J124155.33+114003.7 – a missing link between compact elliptical and ultracompact dwarf galaxies

Abstract We report the discovery of a compact object (Re= 32 pc, MB=−12.34 mag) at a projected distance of 9 kpc from Messier 59, a giant elliptical in the Virgo cluster. Using HST imaging and SDSS spectroscopy, both available in the Virtual Observatory, we find that this object has a blue core containing one-quarter of the light, and a redder n= 1 Sérsic envelope, as well as luminosity-weighted age of 9.3 ± 1.4 Gyr, a metallicity of −0.03 ± 0.04 dex and a velocity dispersion of 48 ± 5 km s−1. While ultracompact dwarf galaxies (UCDs) in the face-on view of the Fundamental Plane are found to form a sequence connecting the highest-luminosity globular clusters with the lowest-luminosity dwarf ellipticals, the compact object near M59 lies in between this UCD sequence and the positions of compact ellipticals. Its stellar age, metallicity, and effective surface brightness are similar to low-luminosity ellipticals and lenticulars, suggesting that SDSS J124155.33+114003.7 is a result of the tidal stripping of such an object.

Luminosity dependence in the Fundamental Plane projections of elliptical galaxies

We analyse the Fundamental Plane projections of elliptical galaxies as a function of luminosity, using a sample of ≈80 000 galaxies drawn from Data Release 4 (DR4) of the Sloan Digital Sky Survey (SDSS). We separate brightest cluster galaxies (BCGs) from our main sample and reanalyse their photometry due to a problem with the default pipeline sky subtraction for BCGs. The observables we consider are effective radius (Re), velocity dispersion (σ), dynamical mass (Mdyn∝Reσ2), effective density (σ2/R2e) and effective surface brightness (μe). With the exception of the L–Mdyn correlation, we find evidence of variations in the slope (i.e. the power-law index) of the Fundamental Plane projections with luminosity for our normal elliptical galaxy population. In particular, the radius–luminosity and Faber–Jackson relations are steeper at high luminosity relative to low luminosity, and the more luminous ellipticals become progressively less dense and have lower surface brightnesses than lower luminosity ellipticals. These variations can be understood as arising from differing formation histories, with more luminous galaxies having less dissipation. Data from the literature and our reanalysis of BCGs show that BCGs have radius–luminosity and Faber–Jackson relations steeper than the brightest non-BCG ellipticals in our sample, consistent with significant growth of BCGs via dissipationless mergers. The variations in slope we find in the Faber–Jackson relation of non-BCGs are qualitatively similar to that reported in the black hole mass–velocity dispersion (MBH–σ) correlation. This similarity is consistent with a roughly constant value of MBH/M★ over a wide range of early-type galaxies, where M★ is the stellar mass.

Radial kinematics of isolated elliptical galaxies

Ellipticals in very low-density environments are extremely rare but hold important clues about galaxy formation and evolution. In this paper, we continue our study of isolated elliptical galaxies, presenting results on the radial stellar kinematics for 13 isolated early-type galaxies. We derive radial rotation velocity, velocity dispersion and hermite terms to ∼1 effective radius. We observe a dichotomy in kinematic properties similar to that in the elliptical population as a whole, where low-luminosity ellipticals tend to be rotationally supported. For all galaxies the V/σ ratio increases with radius. We find kinematically distinct cores (KDCs), or velocity substructure, in ∼40 per cent of the galaxies for which we have major axis spectra. Such a fraction is similar to that observed for ellipticals in higher-density environments. Most galaxies in the sample reveal kinematic evidence for a nuclear disc. The non-relaxed kinematics in several galaxies suggests that they have undergone a merger or accretion event. Isolated ellipticals generally follow the Fundamental Plane defined by cluster ellipticals – exceptions being those galaxies with evidence for young stellar populations. Overall, we find isolated ellipticals have similar kinematic properties to their counterparts in higher-density environments.

Globular cluster systems in low-luminosity early-type galaxies near the Fornax cluster centre

We present a photometric study of the globular cluster systems of the Fornax cluster galaxies NGC 1374, NGC 1379, and NGC 1387. The data consists of images from the wide-field MOSAIC Imager of the CTIO 4-m telescope, obtained with Washington C and Kron-Cousins R filters. The images cover a field of 36 x 36 arcmin, corresponding to 200 x 200 kpc at the Fornax distance. Two of the galaxies, NGC 1374 and NGC 1379, are low-luminosity ellipticals while NGC 1387 is a low-luminosity lenticular. Their cluster systems are still embedded in the cluster system of NGC 1399. Therefore the use of a large field is crucial and some differences to previous work can be explained by this. The colour distributions of all globular cluster systems are bimodal. NGC 1387 presents a particularly distinct separation between red and blue clusters and an overproportionally large population of red clusters. The radial distribution is different for blue and red clusters, red clusters being more concentrated towards the respective galaxies. The different colour and radial distributions point to the existence of two globular cluster subpopulations in these galaxies. Specific frequencies are in the range S_N= 1.4-2.4, smaller than the typical values for elliptical galaxies. These galaxies might have suffered tidal stripping of blue globular clusters by NGC 1399.

Elliptical galaxies from mergers of discs

We analyse N-body galaxy merger experiments involving disc galaxies. Mergers of disc-bulge-halo models are compared to those of bulge-less, disc-halo models to quantify the effects of the central bulge on the merger dynamics and on the structure of the remnant. Our models explore galaxy mass ratios 1:1 through 3:1, and use higher bulge mass fraction than previous studies. A full comparison of structural and dynamical properties to observations is carried out. The presence of central bulges results in longer tidal tails, oblate final intrinsic shapes, surface brightness profiles with higher Sersic index, steeper rotation curves, and oblate-rotator internal dynamics. Mergers of bulge-less galaxies do not generate long-lasting tidal tails, and their strong triaxiality seems inconsistent with observations; these remnants show shells, which we do not find in models including central bulges. Giant ellipticals with boxy isophotes and anisotropic dynamics cannot be produced by the mergers modeled here; they could be the result of mergers between lower luminosity ellipticals, themselves plausibly formed in disc-disc mergers.

A SAURON look at galaxy bulges

AbstractKinematic and population studies show that bulges are generally rotationally flattened systems similar to lowluminosity ellipticals. However, observations with state‐of‐the‐art integral field spectrographs, such as SAURON, indicate that the situation is much more complex, and allow us to investigate phenomena such as triaxiality, kinematic decoupling and population substructure, and to study their connection to current formation and evolution scenarios for bulges of early‐type galaxies. We present the examples of two S0 bulges from galaxies in our sample of nearby galaxies: one that shows all the properties expected from classical bulges (NGC5866), and another case that presents kinematic features appropriate for barred disk galaxies (NGC7332). (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Cl 1358+62: Characterizing the Physical Properties of Cluster Galaxies atz= 0.33

[Abridged] We examine the physical properties of 173 cluster members in CL1358+62 (z=0.33) from HST WFPC2 imaging taken in the F606W & F814W filters (rest-frame B & V) over a 2.2x2.2 Mpc^2 field. Structural parameters are measured for each member by fitting a PSF-convolved, two component model to their 2D surface brightness distribution. We examine bulge+disk models using three different bulge profiles (de Vaucouleurs, Sersic, and exponential), and rigorously test the robustness of our results by analyzing several thousand artificial galaxies in the same manner as the cluster data. Combining structural parameters with ground-based spectroscopy, we test for correlations between morphological characteristics, current star formation, total galaxy colors, and cluster substructure. We find that: (1) Using a de Vaucouleurs bulge, bulge-to-total ratios and Hubble type (-5<T<8) are strongly correlated (99% CL), but the scatter is large and early-type spirals are not reliably distinguished from E's and S0's based on (B/T). (2) The low luminosity ellipticals in our sample are likely to be face-on S0 galaxies. (3) High galaxy asymmetry and strong [OII] emission are strongly correlated for disk-dominated members. (4) There exists a small population (~5%) of bulge-dominated members whose significant [OII] emission (<-5A) suggest they harbor AGN. (5) At these redshifts, determining the correct Sersic index can be highly unreliable.

The Globular Cluster System of the Low-Luminosity Elliptical Galaxy NGC 1427

Fil: Forte, Juan Carlos. Universidad Nacional de la Plata. Facultad de Ciencias Astronomicas y Geofisicas; Argentina. Consejo Nacional de Investigaciones Cientificas y Tecnicas; Argentina

Line-of-sight velocity distributions of low-luminosity elliptical galaxies

The shape of the line-of-sight velocity distribution (LOSVD) is measured for a sample of 14 elliptical galaxies, predominantly low-luminosity ellipticals. The sample is dominated by galaxies in the Virgo cluster but also contains ellipticals in nearby groups and low density environments. The parameterization of the LOSVD due to Gerhard and van der Marel and Franx is adopted, which measures the asymmetrical and symmetrical deviations of the LOSVD from a Gaussian by the amplitudes h3 and h4 of the Gauss-Hermite series. Rotation, velocity dispersion, h3 and h4 are determined as a function of radius for both major and minor axes. Non-Gaussian LOSVDs are found for all galaxies along the major axes. Deviations from a Gaussian LOSVD along the minor axis are of much lower amplitude if present at all. Central decreases in velocity dispersion are found for three galaxies. Two galaxies have kinematically-decoupled cores: NGC 4458 and the well-known case of NGC 3608.

The Kinematics of 3 : 1 Merger Remnants and the Formation of Low‐Luminosity Elliptical Galaxies

We test the formation of low-luminosity elliptical galaxies through collisionless mergers of unequal-mass disk galaxies. The kinematic properties of a small survey of simulated merger end products with initial disk mass ratios of 3 : 1 are compared to a sample of seven low-luminosity galaxies by H.-W. Rix and coworkers that were chosen photometrically to be ellipticals. In this paper, we go beyond a comparison in terms of global properties (using one number to characterize a model or a galaxy, e.g., a4, ellipticity at some fixed radius, or central velocity dispersion) and examine the detailed kinematics as a function of galactocentric distance. The merger remnants are observed through a slit along the major and minor axes, using a pixel binning and slit width similar to the ones used during the spectroscopic observations. Inside each bin, we determine the line-of-sight velocity distributions and parameterize them with Gauss-Hermite functions. We compare the rotational support of the merger remnants, i.e., the ratio of the mean line-of-sight velocity v to the velocity dispersion σ along the major axis, the normalized rotation on the minor axis, and the major-axis Gauss-Hermite moments h3, to those of the galaxies. The N-body remnants are very flattened when viewed edge-on ( ~ 0.6) and should be inclined before making a fair comparison with the data set of H.-W. Rix and coworkers (which has ~ 0.3). When the merger remnants are appropriately inclined, their v/σ profiles rise slower than the ones: v/σmerger is in the range [0.1, 0.8] at 1Reff and [0.2, 1.6] at 2Reff, whereas v/σobserved spans the intervals [0.8, 2.0] at 1Reff and [1.4, 3.5] at 2Reff. Note that even when the remnants are viewed edge-on, the v/σ profiles do not match the observations. The detailed comparison of the observations with our set of purely collisionless 3 : 1 merger remnants shows that these objects and low-luminosity ellipticals do not have similar kinematic profiles. This suggests that this kind of dissipationless merger (or mergers with more even masses, e.g., mass ratios of 2 : 1 or 1 : 1) is not likely to be the dominant formation channel for low-luminosity elliptical galaxies.

A Robust Age Indicator for Old Stellar Populations

We derive new spectral Hγ index definitions that are robust age indicators for old and relatively old stellar populations and therefore have great potential for solving the age-metallicity degeneracy of galaxy spectra. To study this feature as a function of age, metallicity, and resolution, we have used a new spectral synthesis model that predicts spectral energy distributions of single-age, single-metallicity stellar populations at resolution FWHM~1.8 Å (which can be smoothed to different resolutions), allowing direct measurements of the equivalent widths of particular absorption features. We show that Hγ's strong age-disentangling power is due to a compensating effect: at a specified age, Hγ strengthens with metallicity owing to an adjacent metallic absorption, but on the other hand, the adopted pseudocontinua are depressed by the effects of strong neighboring Fe I lines on both sides of Hγ. Despite the fact that this effect depends strongly on the adopted resolution and galaxy velocity dispersion σ, we propose a system of indicators that are completely insensitive to metallicity and stable against resolution, allowing the study of galaxies up to σ ~ 300 km s-1. An extensive analysis of the characteristics of these indices indicates that observational spectra of very high signal-to-noise ratio and relatively high dispersion are required to gain this unprecedented age-discriminating power. Once such spectra are obtained, accurate and reliable estimates for the luminosity-weighted average stellar ages of these galaxies will become possible for the first time, without assessing their metallicities. We measured this index for two globular clusters, a number of low-luminosity elliptical galaxies, and a standard S0 galaxy. We find a large spread in the average stellar ages of a sample of low-luminosity ellipticals. In particular, these indices yield 4 Gyr for M32. This value is in excellent agreement with the age provided by the extraordinary fit to the full spectrum of this galaxy that we achieve in this paper.

The Survival of the Core Fundamental Plane against Galactic Mergers

The basic dimensional properties of the centers of elliptical galaxies, such as length scale, luminosity, and velocity dispersion, lie on a fundamental plane similar to that of elliptical galaxies as a whole. The orientation of this plane, and the distribution of core parameters within it, point to a strong correlation of core density with either core or total luminosity, and indicate that low-luminosity ellipticals are much denser than high-luminosity galaxies (Hubble Space Telescope data suggest that this relationship may be as steep as ρc∝L−2). In addition, low-luminosity ellipticals have a much smaller length scale than their high-luminosity counterparts. Since we think that small galaxies are occasionally accreted by big ones, the high density of these galaxies and their likely durability against the time-varying tidal field of the bigger ones suggests that they will survive substantially intact in the cores of larger galaxies and would be easily visible. Their presence would destroy the observed correlation. Motivated by this apparent inconsistency between an observed fact and a simple physical argument, we have developed an effective simulation method and applied it to the problem of the accretion of very dense secondary companions by tenuous primaries. We have studied the accretion of objects of varying luminosity ratios, with sizes and densities drawn from the fundamental plane under the assumption that the mass distribution in the central parts of the galaxies follows the light. The results indicate that in mergers with mass ratios greater than 10, chosen with an appropriate central density dependence on luminosity, the smaller object is only stripped down to the highest density encountered in the primary during the accretion process. Thus, the form of the core fundamental plane suggests that the mass distribution in galaxy centers is different from the light distribution, or that an understanding of secondary survival requires more than the dynamics of visible stars.

On the physical connections between galaxies of different types

Galaxies can be classified in two broad sequences which are likely to reflect their formation mechanism. The `main sequence', consisting of spirals, irregulars and all dwarf galaxies, is probably produced by gas settling within dark matter haloes. We show that the sizes and surface densities along this sequence are primarily determined by the distributions of the angular momentum and formation time of dark haloes. They are well reproduced by current cosmogonies provided that galaxies form late, at $z \la 2$. In this scenario, dwarf ellipticals were small `disks' at $z\sim 1$ and become `ellipticals' after they fall into cluster environments. The strong clustering of dwarf ellipticals is then a natural by-product of the merging and transformation process. The number of dwarf galaxies predicted in a cluster such as Virgo is in good agreement with the observed number. On the other hand, the `giant branch', consisting of giant ellipticals and bulges, is probably produced by the merging of disk galaxies. Based on the observed phase-space densities of galaxies, we show that the main bodies of all giant ellipticals can be produced by dissipationless mergers of high-redshift disks. However, high-redshift disks, although denser than present-day ones, are still not compact enough to produce the high central phase space density of some low-luminosity ellipticals. Dissipation must have occurred in the central parts of these galaxies during the merger which formed them.

Constraints on the merger models of elliptical galaxies from their globular cluster systems

The discovery of protoglobular cluster candidates in many present-day mergers allows us to understand better the possible effects of a merger event on the globular cluster system of a galaxy, and to foresee the properties of the end-product. By comparing these expectations with the properties of globular cluster systems of elliptical galaxies at the present time we can constrain merger models. The observational data indicate that (i) every gaseous merger induces the formation of new star clusters, and (ii) the number of new clusters formed in such a merger increases with the gas content of the progenitor galaxies. Low-luminosity (about MV > −21), discy ellipticals are generally thought to be the result of a gaseous merger. As such, new globular clusters are expected to form but have not been detected to date. We investigate various reasons for the non-detection of subpopulations in low-luminosity ellipticals, i.e. absence of an old population, absence of a new population, destruction of one of the populations and, finally, an age–metallicity conspiracy that allows old and new globular clusters to appear indistinguishable at the present epoch. All of these possibilities lead us to a similar conclusion, namely that low-luminosity ellipticals did not form recently (z < 1) in a gas-rich merger, and might not have formed in a major merger of stellar systems at all. High-luminosity ellipticals do reveal globular cluster subpopulations. However, it is difficult to account for the two populations in terms of mergers alone and, in particular, we can rule out scenarios in which the second subpopulation is the product of a recent, gas-poor merger.

ITAL]ROSAT[/ITAL] Temperatures and Abundances for a Complete Sample of Elliptical Galaxies

We determine X-ray temperatures and abundances for elliptical galaxies drawn from a complete, optically selected sample. The optical magnitude-limited sample consists of 43 galaxies, complete to a (corrected) B magnitude of m_B^o=11.36. Of these, 30 have enough X-ray spectral counts to allow temperature determinations. We find that the temperatures of the X-ray emitting gas in these ellipticals are correlated with the central stellar velocity dispersions: T ~ sigma^1.45; this is a shallower trend than the simple thermal relation T ~ sigma^2. The diffuse gas is substantially hotter than the kinetic temperature of the luminous stars, since kT > mu m_p sigma^2 for all galaxies with measurable temperatures. This strongly indicates that dark matter halos are characterized by velocity dispersions which exceed those of the luminous stars. We see no evidence of emission from X-ray binaries becoming progressively more dominant in lower luminosity ellipticals. We find that so-called "supersoft" sources adhere to the observed kT-sigma relation, so they are no softer than expected for their velocity dispersions. We also find that ISM temperatures and abundances are correlated, with the gas in hotter systems being more enriched than in cooler galaxies. However, no correlation is found between gaseous abundances and stellar abundances, as inferred from Mg_2 indices.

A Proposed Revision of the Hubble Sequence for Elliptical Galaxies

The Hubble classification scheme has a well-known drawback for elliptical galaxies: the sequence E0-E6 correlates primarily with inclination and not with fundamental properties. In contrast, later-type galaxies are ordered by fundamental physical parameters. We propose to revise the Hubble sequence so that it orders ellipticals by isophote shape. Specifically, we suggest that the Im-spiral-S0 sequence be connected to disky ellipticals and thence to boxy ellipticals. The sequence is continuous from S0's to disky E's. However, boxy E's may be unrelated to other ellipticals: global and core properties both show signs of a dichotomy between (1) normal- and low-luminosity ellipticals that rotate rapidly, that are nearly isotropic and oblate-spheroidal, that are coreless, and that have disky-distorted isophotes, and (2) giant ellipticals that are essentially nonrotating, that are anisotropic and moderately triaxial, that have cuspy cores, and that are boxy-distorted. In the classification, we use isophote shape as an implicit indicator of velocity anisotropy. Two observations support this interpretation: (1) ellipticals with disky isophote distortions rotate rapidly while boxy ellipticals tend to rotate slowly; (2) disky ellipticals show little minor-axis rotation while boxy ellipticals often have large amounts of minor-axis rotation. An approximate classification by velocity anisotropy extends the Im-spiral-S0 sequence through the ellipticals in a physically reasonable way, and it is diagnostic of formation mechanisms. Care is required to recognize unfundamental complications, such as the building by accretion of disky substructure inside a boxy elliptical. However, we believe that the proposed scheme distills the essential physics that should be embodied in a relevant morphological classification.

An HST Survey of Cores of Early-Type Galaxies

Photometry of the central parts of bulges and elliptical galaxies with the Hubble Space Telescope (HST) confirms and extends ground-based results. Most giant ellipticals have cuspy cores: at the “break radius” rb (formerly the core radius rc), the steep outer surface brightness profile turns down to a shallow inner power law I(r) ∝ r–γ, 0 ≤ γ ≲ 0.25. The corresponding slope of the deprojected profile is derived; the flattest cores allow box orbits to survive. Cores continue to satisfy fundamental plane parameter correlations like those found from the ground. In particular, HST confirms that the luminosity sequence of elliptical galaxies (from cDs to M 32) is physically unrelated to spheroidal galaxies like Fornax. The latter are closely related to late-type dwarfs. Low-luminosity ellipticals do not show cores: 0.5 ≲ γ ≲ 1.3. The most important new result is that global and core properties both show signs of a dichotomy between (i) low-luminosity ellipticals that rotate rapidly, that are nearly isotropic and oblate-spheroidal, that have disky-distorted isophotes, and that are coreless and (ii) giant ellipticals that are essentially nonrotating, anisotropic, and moderately triaxial, that are boxy-distorted, and that have cuspy cores.

The Luminosity Function of Elliptical Galaxies

We present 292 new redshifts of elliptical galaxies selected from the APM Bright Galaxy Survey. We use these data together with published redshifts and b<SUB>J</SUB> apparent magnitudes from the APM Bright Galaxy Survey in order to estimate the shape of the Luminosity Function of a total sample of 535 ellipticals. We use the Maximum Likelihood method for the determination of the best fitting parameters α and M<SUP>*</SUP> of a Schechter Function. In order to provide tests for systematics and suitable error estimates we apply Monte Carlo techniques which are also used to deal with incompleteness effects in the data. In agreement with other authors our results indicate a relative lack of low luminosity ellipticals compared to other morphological types. The best fitting shape parameters derived are α=0.20±0.2S and M<SUB>BJ</SUB><SUP>*</SUP>=-20.0±0.3. We do not find evidence for a dependence of these parameters on environment

Dark matter in X-ray-emitting elliptical galaxies

view Abstract Citations (16) References (55) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Dark Matter in X-Ray--emitting Elliptical Galaxies de Paolis, F. ; Ingrosso, G. ; Strafella, F. Abstract To investigate the dynamical mass distribution in elliptical galaxies, we study the gravitational equilibrium of spherically symmetric three-component models constituted by stars, hot diffuse gas, and dark matter. Assuming a constant mass-to-light ratio for the stars, described with an isotropic distribution function in phase space (King), an emission in the X-ray band from the hot gas, and a dark component in the form of massive fermions, we derive constraints on the dark matter distribution by using optical and X-ray brightness profiles. We find that, while optical profiles are not useful to discriminate between models with and without dark matter, fits to X-ray observations of high- luminosity ellipticals require the presence of a third dark component, mainly distributed in the outer radial regions, beyond the half- luminosity radius. The total amount of dark matter is, however, poorly determined because X-ray brightness profiles can be fitted by models with values of dark mass M_d_ in the range 0.4-9 times the stellar mass M_*_ inside R_25_. In any case, the lower limit for M_d_M_*_ can be weakened for low-luminosity ellipticals, for which X-ray observations can be fitted by models with a negligible amount of dark matter. A better determination of the ratio M_d_/M_*_ is obtained if stellar kinematical (velocity dispersion) data are also considered. Limits on the mass of fermions composing the dark matter are also discussed, and they are found to be in agreement with previous particle mass determinations based on the analysis of rotation curves in spiral galaxies. The possibility of fitting all the available observational data by using two-component (stars and hot gas) models with the stellar component described by an anisotropic distribution function in the phase space is also explored. For high-luminosity ellipticals also these models cannot fit X-ray observations because the corresponding gas temperatures are systematically higher than the observed values. Publication: The Astrophysical Journal Pub Date: January 1995 DOI: 10.1086/175056 Bibcode: 1995ApJ...438...83D Keywords: Astronomical Models; Dark Matter; Elliptical Galaxies; Mass Distribution; X Rays; Gravitation; High Temperature Gases; Luminosity; Mass To Light Ratios; Stars; Velocity Distribution; Astrophysics; COSMOLOGY: DARK MATTER; GALAXIES: ELLIPTICAL AND LENTICULAR; CD; GALAXIES: STRUCTURE; X-RAYS: GALAXIES full text sources ADS | data products SIMBAD (2)

Dynamically hot galaxies. II - Global stellar populations

The global relationship between the stellar populations and the structural properties of dynamically hot galaxies (DHGs) is investigated using the same sample as was analyzed by Bender et al. (1992), which includes giant ellipticals, low-luminosity ellipticals, compact ellipticals, diffuse dwarf ellipticals, dwarf spheroidals, and bulges. It was found that all DHGs follow a single relationship between global stellar population (represented by Mg2 index or B-V color) and central velocity dispersion sigma(0), and that the Mg2-sigma(0) relation is significantly tighter than the relation between the Mg2 index and absolute luminosity. The relation between central Mg2 index and bulk B-V color was also found to be tight.