William A. Baum
Ellipticals are galaxies that appear ellipsoidal in form, have no
disks, and are devoid of features such as spiral arms, bars, or dust
lanes. Because such features would be associated with recent or
ongoing star formation, their absence indicates that nearly all of the
stars in elliptical galaxies must be somewhat old.
The majority of bright galaxies in large clusters are ellipticals, but
fewer than 15% of galaxies in the general field are ellipticals.
Classical ellipticals (E galaxies), with their bright compact nuclei
and steep brightness gradients, range from absolute visual magnitude
MV ~ -23 down to MV ~ -16 mag. The
very brightest of them, called cD
galaxies, are found at the centers of clusters, and are among the most
luminous galaxies in the Universe.
Dwarf ellipticals (dE galaxies) include an assortment of
morphological types and range from MV ~ -19 down at
least to MV ~ -12
mag, where their numbers are rising steeply and where surveys become
seriously incomplete. They probably extend to (and may overlap) the
range of globular star clusters, which commences at MV
~ -10
mag. Dwarfs differ greatly from one another in compactness.
Seen on the plane of the sky, some E galaxies are quite round, and
others are elongated. Although various classification schemes have
been devised, the degree of elongation is commonly designated by Edwin
P. Hubble's subclass, 10(1-b/a), where a and b refer to major and
minor axes. Thus, an EO galaxy is round, and an E7 (the most elongated
subclass) has a projected axis ratio b/a ~ 0.3. In the elongated
galaxies, the ellipticity is typically a function of the isophotal
level. In some, the position angles of the major axes of the isophotes
are also a function of the isophotal level; that is, such galaxies
possess an isophotal twist. Moreover, isophotes sometimes depart from
pure ellipses in the sense of being slightly rectangular (``boxy'').
The three-dimensional shape of a galaxy has to do with the
distribution of stellar velocities within it. In disk galaxies
(spirals and their featureless S0 cousins), the angular momentum due
to Keplerian rotation dominates over random motions, and the resulting
galaxy is an oblate spheroid. But in E galaxies the angular momentum
is not dominant, and the three-dimensional shape of the system is
maintained mainly by the dispersion of stellar velocities within
it. In principle, the velocity dispersion can be anisotropic, so that
an E galaxy can be a prolate ellipsoid or even a triaxial
one. Unfortunately, the three-dimensional shapes cannot be directly
observed.
Since E galaxies are not primarily supported by Keplerian rotation,
it is not possible to calculate individual masses from their rotation
curves in the manner used for tilted spirals. Internal velocity
dispersions give only limited information. If one is willing to assume
that clusters of galaxies (which often consist mainly of ellipticals)
are in gravitational equilibrium, the mass of a whole cluster of
galaxies can be estimated by application of the virial theorem to the
observed dispersion of galactic velocities within the cluster, but the
inferred mass tends to be puzzlingly large. The mass-luminosity ratio
of the stellar population in giant ellipticals therefore remains quite
uncertain. It might even be different in a cluster environment than in
the general field.
Various formulas have been used to fit the observed radial
distribution of brightness of E galaxies in the plane of the sky, but
the one most often used today is the de Vaucouleurs law (after
Gerard de Vaucouleurs). It says that the logarithm of the surface
brightness (usually expressed on a stellar magnitude scale) is an
approximately linear function of the 1/4-power of the radial distance
from the nucleus. This is a purely empirical finding that has no
direct theoretical basis. Also, the goodness of fit varies somewhat
from object to object.
At the very center of a large cluster of galaxies there is typically
an unusually luminous elliptical surrounded by a faint but extensive
envelope pervading the heart of the cluster. These cD galaxies are
suspected of having grown by mergers and by the cannibalization of
nearby dwarfs.
In the 1940s, elliptical galaxies were assigned by Walter Baade to his
Population II, by which he meant specifically that the color-magnitude
diagram of their stellar population should be essentially the same as
that for globular star clusters of the Milky Way. Owing to their high
random velocities and their nearly spherical distribution in the Milky
Way, globular star clusters were known to have formed during a very
short interval before the Milky Way had settled into a disk and before
the interstellar medium had become enriched with heavy elements
(``metals''). It was therefore assumed that elliptical galaxies had
similarly formed during an early burst of star formation.
But integrated photoelectric photometry soon showed (and
spectroscopy later confirmed) that giant elliptical galaxies differ
greatly in spectral energy distribution from globular star
clusters. Therefore, the dominant stellar population of giant
ellipticals cannot be similar to that of globular clusters. There was
thus already good evidence in the 1950s that star formation in giant
ellipticals must have continued long enough for a high degree of metal
enrichment to permeate the stellar population. It is only with the
accumulation of further evidence in recent years, however, that the
concept of temporally distributed star formation in ellipticals has
finally gained general acceptance.
Judged from integrated colors and spectra, some dwarf ellipticals
must be at least partly similar in stellar content to giant
ellipticals, while other dwarfs consist more of stars like those in
globular star clusters. There are also a few dwarf ellipticals near
enough for telescopic resolution of the brightest stars, but the
resolved stars do not necessarily belong to the population that
dominates the integrated light or the mass. Such is clearly the case
for M32, the compact metal-rich companion of the
Andromeda galaxy. On
the other hand, several of the very tenuous metal-poor galaxies
classed as dwarf spheroidals are near enough that the color-magnitude
diagrams of their stellar populations can be definitively identified.
Owing to the fact that no giant elliptical happens to lie near us,
none has ever been resolved well enough to permit photometry of
individual stars. Incipient resolution of the nearest ellipticals can
barely be achieved under the very best Earth-based observing
conditions. Some improvement can be expected with telescopes in
space. Using a form of noise analysis on such images, one can infer
the nature of the brightest stars, the number of them Per unit surface
brightness, and the relative distance of nearby ellipticals. Noise
parameters, taken together with known properties of the integrated
light, should enhance our knowledge of the population mix.
It has been known for a long time that giant ellipticals tend to be
slightly redder in their inner regions than in their outskirts,
suggesting a gradient in the metallicity of the stellar population.
Gradients have now been measured spectroscopically for several
elements, and the gradient for the prominent magnesium feature around
5175 Å has been extended to faint levels in the outskirts by
narrow-band CCD photometry. On reasonable assumptions, those data
indicate the inner regions to be more metal rich than stars in the
solar neighborhood, while the halos are only a little less so. In
other words, the halo population of ellipticals is much more metal
rich than that of the Milky Way. On the other hand, globular star
clusters that have been detected in the halos of nearby ellipticals do
not seem (from their colors) to be so metal rich; so the globulars
must have formed at an earlier time than many of the individual stars
that now populate the same halo regions.
It should be noted that the strength of an absorption feature such
as the 5175-Å magnesium band is dependent upon stellar surface gravity
as well as metallicity. However, the temperature of the giant branch
of the H-R diagram (or color-magnitude diagram) is largely controlled
by metallicity, and it is the giant branch that would be expected to
dominate the integrated light. For subgiants just above the
main-sequence turnoff, surface gravity differences (which are
correlated with age differences) play a role, but the turnoff stars
probably do not contribute strongly to the integrated light.
No existing theoretical model for the formation and evolution of giant
elliptical galaxies is able to explain all of their observed
properties, but some form of inhomogeneous dissipative collapse
appears to be indicated, and star formation (though not active today)
was evidently not limited to a single early burst. It presumably
required a long time to build up the high observed
metallicity. Mergers may also have played a role. Any successful
theory of giant ellipticals must take account of the following
observed properties: (1) They have high metallicities but low
metallicity gradients, resulting in relatively metal-rich halos. (2)
They are inferred to have triaxial figures supported by anisotropic
velocity dispersion, rather than by rotation. (3) Globular clusters
in giant ellipticals have lower metallicities than halo stars in the
same regions. (4) The distribution of globular clusters in giant
ellipticals is less centrally concentrated than is the main body of
stars.
GALAXIES, ELLIPTICAL
BASIC PROPERTIES
STELLAR POPULATIONS
MODELS
Burstein, D. (1985). Observational constraints on the ages and
abundances of old stellar populations.
Publ. Astron. Soc. Pac. 97 89.
de Vaucouleurs, G. (1987). General historical introduction. In
Structure and Dynamics of Elliptical Galaxies (Proceedings of
IAO Symposium 127), T. de Zeeuw, ed. Reidel, Dordrecht, p. 3.
Dressler, A. (1984). The evolution of galaxies in clusters.
Ann. Rev. Astron. Ap. 22 185.
O'Connell, R.W. (1986). Analysis of stellar populations at large
lookbacks. In Spectral Evolution of Galaxies, C. Chiosi and
A. Renzini, eds. Reidel, Dordrecht, p. 321.
Pickles, A. (1987). Population synthesis of composite systems. In
Structure and Dynamics of Elliptical Galaxies (Proceedings of
IAO Symposium 127), T. de Zeeuw, ed. Reidel, Dordrecht,
p. 203.
See also Galaxies, Dwarf, Spheroidal; Galaxies, Elliptical,
Dynamics; Galaxies, Elliptical, Origin and Evolution.