Alan Dressler
Astronomers study variations in galaxy properties with environment
because they seek to understand the process of galaxy formation,
which, for the vast majority of galaxies, occurred long ago. The
dependence of galaxy type on environment, first noted by Edwin Hubble
and Milton Humason in 1931, provides an opportunity to learn about
events that took place in a largely unobservable past.
Galaxies cluster; although a small fraction live in splendid
isolation, the average galaxy is found in a low-density, marginally
bound group. The typical density of such a group is only 1 galaxy for
a volume of the order of 20 cubic megaparsecs (a sphere with diameter
~ 10 million light-years); therefore, encounters between galaxies are
relatively rare. However, the sky is punctuated by rich clusters of
several hundred galaxies in regions 100-1000 times as dense as
this. These rich clusters, which contain only ~ 5-10% of all galaxies,
are surrounded by extensive plateaus of modestly enhanced galaxy
density, called superclusters, that reach like vast, lacy bridges to
neighboring clusters.
The existence of groups and clusters is itself an important clue to
the conditions of galaxy formation. Gravity, which unlike
electromagnetism is a one-pole force, relentlessly gathers matter
together. After the universe had cooled sufficiently from the Big Bang
for the first segments of galaxies to form, fluctuations in density,
perhaps arising from quite specific physical processes, were
continually amplified by gravity. Only energy released from star
formation could have temporarily resisted this process of
``hierarchical clustering'' by which smaller lumps aggregate into larger
and larger ones. Models of stellar structure and energy generation
show that a star more than 100 times as massive as the Sun would blow
itself apart, so it is understood why hierarchical clustering ended
with billions of stars per galaxy mass rather than a few supermassive
stars. What physics prevented the coalescence of cluster galaxies into
a single supergalaxy?
The answer appears to lie in the play-off between the time scale for
collapse of the protogalaxy and the cooling time of its largely
gaseous mass. Galaxies of the size we see today are the largest
objects that could have radiated away significant amounts of their
binding energies by dissipation in the gaseous component. As also
happens in the birth of a star, dissipation enables a galaxy to
contract to a higher density than specified by its initial size and
temperature. The densities and temperatures associated with
protoclusters, on the other hand, result in cooling times much longer
than their dynamical times. Consequently, galaxies retain their
individual identities and the effect of clustering is to collect
galaxies into high-density regions rather than to build massive
supergalaxies. Thus, the existence of galaxy groups and clusters is
indicative of the role of gas dynamics and hierarchical clustering in
the process of galaxy formation.
The most fundamental property of a galaxy is its mass; however,
astronomers usually substitute absolute luminosity because its
determination requires only a single photometric measurement and a
redshift (as an estimate of the galaxy's distance). (Studies of the
internal dynamics of galaxies indicate a range in mass-to-light ratio
of about a factor of 5 compared to a range in luminosity of the order
of 1000, moderately justifying this substitution.) The number
distribution of galaxy luminosities (the luminosity function) is thus
used in place of the more fundamental mass distribution.
How does the luminosity function vary with environment? In
cataloging thousands of rich clusters, George Abell found little or no
variation from cluster to cluster; subsequent investigations
determined a similar luminosity function for ``field galaxies,'' those
in low-density regions. That galaxies in what are today strikingly
different environments could have formed with approximately the same
distribution in luminosity (roughly equated to mass) suggests that the
process of galaxy formation took place at an early epoch, when
environmental differences were considerably smaller. This is an
argument that galaxies formed before groups and clusters were well
developed.
The second major property of a galaxy is its morphology. Averaged
over all environments, spiral and irregular galaxies account for ~ 70%
of all galaxies, the balance being elliptical and SO galaxies. In the
typical environment (that of loose groups), elliptical and SO galaxies
account for only 10-20% of the population. Hubble and Humason, and
later (1936) Fritz Zwicky, noted the startling difference that rich
clusters are mainly composed of elliptical and SO galaxies. In
addition to their unusual population, rich clusters are also notable
for their unusual environment: Cluster galaxies exist in a relatively
crowded space 1000 times more dense than the locale of a typical
galaxy. Moreover, the core volume of a rich cluster is usually
permeated by extremely hot plasma with a pressure much higher than
that of a galaxy's interstellar gas. It is reasonable to speculate
that some property of this extreme cluster environment favored the
formation or evolution of less-common galaxy types - a clue to
discovering the processes that produced morphological variation.
At such densities, interactions with the plasma and even direct
encounters with other cluster members are likely to be important
events in the life of a galaxy. In 1951, Walter Baade and Lyman
Spitzer suggested that SO galaxies might be formed when spiral
galaxies cross in the dense environment of a cluster, stripping each
other of their Interstellar gas and losing their ability to form new
stars. To be effective, this process requires a relatively rapid
encounter (as would be common in a rich cluster) because a low-speed
collision would probably result in the merging or disruption of the
original spirals. A related idea, published by James E. Gunn and
Richard Gott in 1972, held that a rapidly moving spiral would be
stripped of its gas by ram pressure from the hot intracluster plasma.
In the mid-1970s Alar Toomre suggested a model for the formation of
ellipticals that, like these SO production models, imagined a
metamorphosis of spiral galaxies in later life, this time by
disruptive, largely nondissipative mergers with other
galaxies. However, the prevalence of ellipticals in rich, dense
clusters (where the encounter velocities are ~ 1000 km s-1)
is somewhat
problematical in this model because at these speeds, much greater than
the orbital speeds of stars in the galaxies, mergers are less likely
than more elastic encounters. This suggests that if mergers were
responsible for forming cluster ellipticals, they occurred relatively
early, when the rich cluster was a collection of poorer, less-dense
groups, each with a lower characteristic speed of member
galaxies. Furthermore, the fact that the stellar density at the center
of an elliptical galaxy is typically much higher than in a spiral
suggests that a significant amount of dissipation must have occurred,
indicating that the process occurred at an earlier time when galaxies
were more gaseous. Both of these arguments suggest that if mergers
were important in forming elliptical and perhaps SO galaxies, they
might have occurred so early as to be considered part of the formation
process.
Elliptical galaxies are very similar to the bulges of disk galaxies.
It is therefore natural to extend the merger picture to suggest that
the spheroidal components of disk galaxies also formed by
mergers. However, a counterargument holds that the chaotic addition of
even a small amount of mass would disrupt the thin disk found in most
spiral galaxies. Although a possible aid in explaining the evolution
of SO galaxies, the fact that many spirals with relatively large
bulges still retain thin disks might be hard to understand unless
these dish themselves formed after the bulges. If disk disruption is a
serious problem for the model of building bulges by accretion, the
importance of mergers in the formation of ellipticals is brought into
question as well.
Perhaps the best evidence for an environmentally induced merger
process is the unique presence of giant galaxies in clusters. William
W. Morgan identified a class that he called cD galaxies which are
surrounded by extensive stellar envelopes. Many of these have
extraordinary luminosities and masses, well above those found for
galaxies in low-density environments. The view is commonly held (and
supported by some admittedly disputed evidence) that cD galaxies have
cannibalized neighboring galaxies, accreting by nondissipative mergers
unfortunate companions that stray into their domain. Here again,
however, both observation and theory suggest that this process was
more important in the past when clusters were coalescing from smaller
associations of galaxies with lower relative speeds.
The remarkably tight dependence of galaxy type on local density shown
in Fig. 1 can be interpreted as evidence that
local processes, perhaps
early in the lifetime of the galaxy, played an important role in
determining galaxy type. Over four orders of magnitude of increasing
local density, the fraction of spirals decreases steadily as the
fraction of SOs and ellipticals increases. This means that most
elliptical and SO galaxies exist outside of rich clusters, even though
they are more prevalent in rich clusters. A mechanism like
ram-pressure stripping by a hot plasma does not then offer a complete
model for what makes a galaxy a dormant SO instead of a star-forming
spiral, because it accounts for only a small fraction of the
population. Low-velocity encounters between spirals, as would be
typical in the lower-density regions where most SOs are found, would
be more likely to result in a merger than the mere removal of
interstellar gas. However, a better description of the relation
between the luminous baryonic component of a galaxy and its massive,
nonluminous halo is needed to evaluate fully such models.
As in the case of the luminosity function, the very slow change in
the morphological mix over orders of magnitude in density suggests
that differentiation took place at an early epoch before the density
contrast was very large. The morphology-density relation could even be
evidence for a process that formed galaxies of different types ab
initio, with little or no alteration later in the galaxy's life. In
this picture, the formation of high-density spheroids, such as the
bulges of disk and elliptical galaxies, is the result of more
efficient star formation in denser environments. Crosstalk between the
early fluctuations that formed galaxies and the much longer wave
perturbations that formed clusters would then account for the fact
that denser galaxies are more commonly found in denser
environments. However, an elliptical or a disk galaxy with a dominant
spheroid could form even in a globally low-density region if there had
been a small-scale fluctuation of unusually high amplitude. Here
again, the nonluminous halo and variations in its properties (e.g.,
density, angular momentum) with environment could play the key role.
Recent work suggests that the luminosity functions for different
galaxy types are different but that each may show little or no
variation with environment. If so, it is even more curious that these
luminosity functions play off against the morphology-density relation
in such a way as to make the summed luminosity function nearly
universal. However, it is important to remember that the mass-to-light
ratio for spirals is significantly lower than that of elliptical or SO
galaxies. Thus, the universality of the luminosity function may be
little more than an accident masking a more fundamental dependence of
the mass function on environment.
In the 198Os astronomers began observational studies of the dependence
of galaxian properties on environment as a function of cosmic time.
clusters of galaxies can be recognized at great distances
corresponding to look-back times of 5-10 billion years, and the
identification of these clusters as the ancestors of present-day
clusters allows for a statistical comparison of their populations. It
has not yet been possible to determine morphological types for these
distant galaxies (due to limitations in image detail imposed by the
atmosphere on observations with Earth-based telescopes) and
observations with the Hubble Space Telescope are eagerly awaited to
correct this deficiency. In the meantime, however, it has been
possible through spectrophotometry to detect in distant galaxies the
level of star formation, closely related to galaxy type at the present
epoch.
The surprising result is that, in contrast to the relatively dormant
elliptical and SO galaxies found in today's clusters, rich clusters at
z ~ 0.5 contained a much higher fraction of galaxies with vigorous star
formation. This implies that the ancestors of some elliptical and SO
galaxies (or the component parts that preceded them) were much more
active in star formation only a few billion years ago and/or that
strong bursts of star formation were once common in the spiral
galaxies of these clusters. It is not yet understood to what extent
this is related to the cluster environment rather than just a
reflection of galaxy evolution with cosmic time. It is certain,
however, that adding the dimension of time to the observational data
base will help distinguish between alternate models for the evolution
of different galaxy types as a function of environment.
GALAXIES, PROPERTIES IN RELATION TO ENVIRONMENT
THE ENVIROMENT
VARIATIONS OF PROPERTIES WITH ENVIROMENT:
IMPLICATIONS FOR GALAXY FORMATION AND EVOLUTION
STUDYING ENVIRONMENTAL INFLUENCES IN THE PAST
Binggeli, B., Sandage, A., and Tammann, G.A. (1988). The luminosity
function of galaxies. Ann. Rev. Astron. Ap. 26 509.
Dressler, A. (1984). The evolution of galaxies in clusters.
Ann. Rev. Astron. Ap. 22 185.
Giovanelli, R. and Haynes, M.P. (1986). Morphological segregation
in the Perseus-Pisces supercluster. Ap. J. 300 77.
Gott, J.R. (1977). Recent theories of galaxy formation.
Ann. Rev. Astron. Ap. 15 235.
Schweizer, F. (1986). Colliding and merging galaxies.
Science 231 227.
Strom, S.E. and Strom, K.M. (1979). The evolution of disk
galaxies. Scientific American 240 (No. 4) 72.
See also Clusters of Galaxies, Component Galaxy
Characteristics; Galaxies, Disk, Evolution; Galaxies, Elliptical, Origin
and Evolution; Galaxies, Formation; Intracluster Medium.