A brief historical review of the discovery and exploration of elliptical galaxies in the past two centuries is presented.

Systematic study of galaxy cores has become possible through improved understanding of seeing and the reduction of photometric errors by the use of CCDs. Many cores are well resolved in a photometry program with the Canada-France-Hawaii Telescope (median stellar FWHM = 0″.80). Core profile shape correlates with galaxy luminosity L: the brightest galaxies have isothermal profiles; fainter ellipticals and bulges have profiles that do not completely flatten inward into a core. This may be due to velocity anisotropies. Core parameters are correlated: more luminous galaxies have larger core radii and fainter central surface brightnesses. Large deviations suggest special events: Fornax A has too small and bright a core for its luminosity; it may be the remnant of a merger with a smaller galaxy. Core mass-to-light ratio M/L ∝ L0.2, as expected from the metallicity-luminosity relation. A kinematic search shows strong evidence for a central black hole in M31 and weaker evidence in M32 and NGC 3115. The nucleus of M31 rotates very rapidly but has an outer dispersion of only 107 km s−1. This implies that it is a disk; it may have formed from gas falling into the center. Rapid rotation and a central velocity dispersion > 241 ± 7 km s−1 imply a central M/Lv ≳ 20 − 35. Velocity anisotropies are not a major uncertainty because of the rapid rotation. Therefore there is strong evidence for a nuclear point mass of ~ 2 × 107M⊙.

The shapes of the isophotes of elliptical galaxies are discussed. Ellipticity and position angle variations with distance from the centres of galaxies are described, as well as deviations from perfectly elliptical shape. The factors contributing to these deviations, in particular edge-on disks and boxiness, are described and portrayed.

The data base of photometric properties of galaxies has grown enormously in the last few years. Surveys in various colors, detailed studies of selected objects, and a swarm of papers with underlying photometric information have invaded the market, fostered by advances in the technology of observations and by improvements and wider availability of reduction facilities. A bibliographical compilation, of great use in the jungle of old and modern references, has been published by Davoust and Pence (1982), and is currently updated (Pence and Davoust 1985).

The kinematic properties of elliptical galaxies are summarized. New developments are discussed in four areas: (i) the Faber-Jackson relation and the role of second parameters (ii) the luminosity-rotation relation (iii) the figures of elliptical galaxies and (iv) the mass-to-light ratio as a function of radius.

Global properties of elliptical galaxies, such as the luminosity, radius, projected velocity dispersion, projected luminosity density, etc., form a two-dimensional family. This “fundamental plane” of elliptical galaxies can be defined by the velocity dispersion and mean surface brightness, and its thickness is presently given by the measurement error-bars only. This is indicative of a strong regularity in the process of galaxy formation. However, all morphological parameters which describe the shape of the distribution of light, and reflect dynamical anisotropies of stars, are completely independent from each other, and independent of the fundamental plane. The M/L ratios show only a small intrinsic scatter in a luminosity range spanning some four orders of magnitude; this suggests a constant fraction of the dark matter contribution in elliptical galaxies.

cD galaxies are the most luminous galaxies in the universe. They are characterized by a surface brightness profile that falls off more slowly with radius than most elliptical galaxies. In most respects D galaxies are a continuous extrapolation from other ellipticals: their M/L and their colors are comparable to other ellipticals, their inner parts are fitted by an r1/4 law, and they follow the same relation between L and σ. On the other hand, their luminosity is too bright to be consistent with the luminosity function of other ellipticals and they are always found at the center of a cluster of other galaxies. Being at the center of a cluster of galaxies often endows D galaxies with a very faint, very extended halo of luminosity and multiple nuclei, but these are more properly associated with the cluster than the D galaxy itself. The connection between the formation of cD galaxies and the formation of clusters remains a mystery. It is still unresolved whether cDs are a byproduct of cluster evolution, whether they formed in parallel with clusters, or whether primeval D are galaxies the seed around which clusters accreted.

We summarize our present knowledge on low-mass high-surface brightness elliptical objects near massive galaxies that are often called M32-type or compact elliptical galaxies. The origin of the low mass of these objects is a controversial matter: is it intrinsic to their formation or produced, as classically believed, by tidal stripping from the massive neighbor? We present new observational data allowing to define better the characteristics of these objects and a simple theoretical model whose consequences support the idea that the precursors of compact ellipticals are related to the low-mass end of the luminosity function of elliptical galaxies.

Progress in the 50 years since the discovery of ionized gas in NGC 1052 is reviewed. As discovery has proceeded from H II to H I and recently to 107 K gas, the known amount of gas in ellipticals has increased dramatically: from 103–6M⊙ for H II, to 105–9M⊙ for H I in some 15% of E's, to 109–10M⊙ for the X-ray emitting gas. Although a few ellipticals with dust have long been known, recent CCD surveys have revealed dust lanes in nearly half of all E's. The cooler gas — as traced by H I, warm H II, and dust — is generally distributed in the form of a disk that often shows an outer warp. To deduce the true shapes of ellipticals from the geometry and kinematics of such disks has proven a challenging task. There is mounting evidence that many disks consist of gas accreted by mass transfers and mergers, and that some of them may not yet have reached dynamical equilibrium. Some of the major unsolved problems concern the atmospheric balance between sources and sinks of gas, and the importance of cooling flows and galactic winds.

More than half of all nearby elliptical galaxies contain modest amounts (103 − 105 M⊙) of ionized gas. In bright elliptical galaxies this gas appears to lie in a rotating, kiloparsec-scale central disk, with a spectrum characteristic of non-thermal ionization. Low-luminosity ellipticals have a clumpy gas distribution and the gas in these galaxies is photoionized by young stars.

NGC 5128 (Cen A) has been known for many years as a peculiar elliptical, but only recently has it been recognized as the prototype of a new class of object (Bertola and Galletta 1978). This class of galaxy is characterized by an elliptical–like stellar body crossed along the minor axis by a dust lane, and was comprised initially of five objects. The class was extended in a remarkable way by Hawarden et al. (1981), who drew attention also to the existence of ellipticals with the dust lane along the major, and along intermediate axes. Ninety objects are now listed as dust lane ellipticals in the compilation by Ebneter and Balick (1985) and its updated version (Ebneter 1986).

About 10% of nearby elliptical galaxies contain HI, with typical values of M(HI) ~ 5 × 108 M⊙ and M(HI)/LB ~ 0.03 M⊙/L⊙. The HI content is unrelated to the stellar content, (unlike the situation in spiral galaxies) suggesting that the HI in early-type galaxies has an external origin and is not produced by mass loss. This conclusion is strengthened by the distribution and kinematics of the HI structures, which lie outside the main optical body of the galaxies, have much larger specific angular momentum than do the stars, and are often highly inclined to the kinematic and distribution axes of the optical bodies.

The HI and stellar kinematics show that the rotation curves of E and SO galaxies are approximately flat from small (a few hundred pc) to large (10-20 kpc) radii, as is the case for spirals. Likewise, large mass-to-light ratios are found for some systems. Comparison with mass models derived from X-ray emission suggests that these may in some cases overestimate the mass.

The presence of HI is shown to enhance the likelihood that an E/SO galaxy has a nuclear radio continuum source, in agreement with models which suggest that the central engine is fuelled by cold gas. Current data suggest that the gas-to-dust ratio for the cold interstellar medium in ellipticals has a value similar to that found in the solar neighborhood.

X-ray observations have shown that early-type galaxies contain a hot interstellar medium. This implies that the galaxies have a) a low supernova rate; b) high total gravitational binding masses and c) continuous star formation. Much of the gas in isolated galaxies is probably due to stellar mass-loss. The details of its behaviour are complex.

Star formation, probably with an abnormal initial mass function, represents the most plausible sink for the large amounts of material being accreted by cD galaxies from cooling flows. There are three prominent cases (NGC 1275, PKS 0745-191, and Abell 1795) where cooling flows have apparently induced unusual stellar populations. Recent studies show that about 50% of other accreting cD's have significant ultraviolet excesses. It therefore appears that detectable accretion populations are frequently associated with cooling flows. The questions of the form of the IMF, the fraction of the flow forming stars, and the lifetime of the flow remain open.

Recently, x-ray observations have shown that elliptical galaxies generally contain large quantities of hot gas. Central dominant cluster ellipticals have even more gas, which they have accreted from the surrounding clusters. The mass distributions in these galaxies can be derived from the condition of hydrostatic equilibrium. M87, the best studied central dominant galaxy, has a massive, dark halo with a total mass of about 4 × 1012 M⊙ within a radius of 300 kpc. The total mass-to-light ratio within this radius is at least 150 M⊙/L⊙. The x-ray observations of normal ellipticals also strongly suggest that they have heavy haloes, although the distribution of the mass is much less certain than in M87.

We have measured line-strength gradients in a sample of 15 early-type galaxies. The line-strength measures include the Mg2 index and the equivalent widths of Hβ and two iron blends at 5270Å and 5335Å. In most of the galaxies we find gradients in the metallic line-strengths. However, the gradients vary markedly from object to object and do not correlate strongly with other parameters such as total luminosity, rotation, etc. A comparison of the line-strengths in the outer parts of these galaxies with galactic globular clusters suggests relatively modest abundance gradients in early-type galaxies.

Metallicity and age dispersion among the stars comprising stellar composite systems are discussed, with emphasis on the present and future implications for the technique of population synthesis.

Distribution functions for the intrinsic shapes of elliptical galaxies are discussed, starting with the simplest and proceeding to the more complex. A variety of competing “proxy” observables, which can in principle be used to recover at least some of information lost in the projection of a galaxy onto the plane of the sky, are considered.