An elliptical galaxy is a type of galaxy having an approximately ellipsoidal shape and a smooth,
nearly featureless brightness profile. Unlike flat spiral galaxies with organization and
structure, they are more three-dimensional, without much structure, and their stars are
in somewhat random orbits around the center. They are one of the three main classes of
galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae,
along with spiral and lenticular galaxies. They range in shape from nearly spherical
to highly flat and in size from tens of millions to over one trillion stars. Originally, Edwin
Hubble thought that elliptical galaxies may evolve into spiral galaxies, which later turned
out to be false. Stars found inside of elliptical galaxies are very much older than stars found
in spiral galaxies. Most elliptical galaxies are composed of older,
low-mass stars, with a sparse interstellar medium and minimal star formation activity,
and they tend to be surrounded by large numbers of globular clusters. Elliptical galaxies
are believed to make up approximately 10–15% of galaxies in the Virgo Supercluster, and
they are not the dominant type of galaxy in the universe overall. They are preferentially
found close to the centers of galaxy clusters. Elliptical galaxies are also called "early-type"
galaxies, due to their location in the Hubble sequence, and are found to be less common
in the early Universe.
General characteristics
Elliptical galaxies are characterized by several properties that make them distinct from other
classes of galaxy. They are spherical or ovoid masses of stars, starved of star-making gases.
The smallest known elliptical galaxy is about one-tenth the size of the Milky Way. The motion
of stars in elliptical galaxies is predominantly radial, unlike the disks of spiral galaxies,
which are dominated by rotation. Furthermore, there is very little interstellar matter,
which results in low rates of star formation, few open star clusters, and few young stars;
rather elliptical galaxies are dominated by old stellar populations, giving them red colors.
Large elliptical galaxies typically have an extensive system of globular clusters.
The dynamical properties of elliptical galaxies and the bulges of disk galaxies are similar,
suggesting that they are formed by the same physical processes, although this remains
controversial. The luminosity profiles of both elliptical galaxies and bulges are well
fit by Sersic's law. Elliptical galaxies are preferentially found
in galaxy clusters and in compact groups of galaxies.
Star formation The traditional portrait of elliptical galaxies
paints them as galaxies where star formation finished after an initial burst at high-redshift,
leaving them to shine with only their aging stars. Elliptical galaxies typically appear
yellow-red, which is in contrast to the distinct blue tinge of most spiral galaxies. In spirals,
this blue color emanates largely from the young, hot stars in their spiral arms. Very
little star formation is thought to occur in elliptical galaxies, because of their lack
of gas compared to spiral or irregular galaxies. However, in recent years, evidence has shown
that a reasonable proportion of these galaxies have residual gas reservoirs and low level
star-formation. Researchers with the Herschel Space Observatory have speculated that the
central black holes in elliptical keep the gas from cooling enough for star formation.
Sizes and shapes
Elliptical galaxies vary greatly in both size and mass, from as little as a tenth of a kiloparsec
to over 100 kiloparsecs, and from 107 to nearly 1013 solar masses. This range is much broader
for this galaxy type than for any other. The smallest, the Dwarf elliptical galaxies, may
be no larger than a typical globular cluster, but contain a considerable amount of dark
matter not present in clusters. Most of these small galaxies may not be related to other
ellipticals. The Hubble classification of elliptical galaxies
contains an integer that describes how elongated the galaxy image is. The classification is
determined by the ratio of the major to the minor axes of the galaxy's isophotes:
Thus for a spherical galaxy with a equal to b, the number is 0, and the Hubble type is
E0. The limit is about E7, which is believed to be due to a bending instability that causes
flatter galaxies to puff up. The most common shape is close to E3. Hubble recognized that
his shape classification depends both on the intrinsic shape of the galaxy, as well as
the angle with which the galaxy is observed. Hence, some galaxies with Hubble type E0 are
actually elongated. There are two physical types of ellipticals;
the "boxy" giant ellipticals, whose shapes result from random motion which is greater
in some directions than in others, and the "disky" normal and low luminosity ellipticals,
which have nearly isotropic random velocities but are flattened due to rotation.
Dwarf elliptical galaxies have properties that are intermediate between those of regular
elliptical galaxies and globular clusters. Dwarf spheroidal galaxies appear to be a distinct
class: their properties are more similar to those of irregulars and late spiral-type galaxies.
At the large end of the elliptical spectrum, there is further division, beyond Hubble classification.
Beyond gE giant ellipticals, lies D-galaxies and cD-galaxies. These are similar to their
smaller brethren, but more diffuse, with larger haloes. Some even appear more akin to lenticular
galaxies. Evolution
Current thinking is that most if not all elliptical galaxies may be the result of a long process
where two or more galaxies of comparable mass, of any type, collide and merge.
Such major galactic mergers are thought to have been common at early times, but may carry
on more infrequently today. Minor galactic mergers involve two galaxies of very different
masses, and are not limited to giant ellipticals. For example, our own Milky Way galaxy is known
to be consuming a couple of small galaxies right now. The Milky Way galaxy is also, depending
upon an unknown tangential component, on a collision course in 4–5 billion years with
the Andromeda Galaxy. It has been theorized that an elliptical galaxy will result from
a merger of the two spirals. Every massive elliptical galaxy is believed
to contain a supermassive black hole at its center. The mass of the black hole is tightly
correlated with the mass of the galaxy, via the M–sigma relation. It is believed that
black holes may play an important role in limiting the growth of elliptical galaxies
in the early universe by inhibiting star formation. Examples
M32 M49
M59 M60
M87 M89
M105 IC 1101, one of the largest galaxies in the
observable universe. Maffei 1, the closest giant elliptical galaxy.
Centaurus A, a radio galaxy, elliptical/lenticular disputed.
See also Firehose instability
Galaxy color–magnitude diagram Galaxy morphological classification
Hubble sequence Lenticular galaxy
M–sigma relation Osipkov–Merritt model
Sersic profile Further reading
Ho, Houjun; van den Bosch, Frank; White, Simon, Galaxy Formation and Evolution, Cambridge
University Press, ISBN 978-0521857932 References
External links Elliptical Galaxies, SEDS Messier pages
Elliptical Galaxies