GALAXIES, SPIRAL, STRUCTURE
Bruce G. Elmegreen and Debra Meloy Elmegreen
Spiral galaxies are flattened stellar systems that rotate in a nearly
circular fashion. They generally contain stars with a wide range of
ages. The gas and youngest stars are in a flattened disk, older stars
occupy a slightly thicker disk, and the oldest stars tend to reside in
a more three-dimensional distribution, which may include a central
bulge, a system of globular clusters, and a low-luminosity halo. This
division of stars into distinct populations was first recognized by
Walter Baade in 1944.
Spirals form when compressional waves propagate through the disk,
growing in length and amplitude because of self-gravitational forces.
The waves appear as spirals because the angular rotation rate of the
disk varies approximately inversely with distance from the center. In
many galaxies, spiral waves become so strong that they drive shock
fronts into the gas, trigger new stars to form, and impart large
random motions to the older disk stars at critical resonance
points. Then the spiral structure can dominate the appearance,
internal dynamics, and evolution of a galaxy.
The optical appearance of spiral galaxies is extremely diverse. A
classification system was developed in the 1940s by Edwin Hubble and
expanded in the 1960s by Allan Sandage, and by Gerard and Antoinette
de Vaucouleurs. Along the Hubble sequence from type Sa to Sb, Sc and
Sd, the bulge gets smaller and the spiral arms become more open. Two
parallel sequences, SBa to SBd and SABa to SABd, describe galaxies
with central bars and ovals, respectively. Because Hubble originally
envisioned an evolutionary sequence, the Sa and Sb galaxies are termed
early type and the Sc and Sd galaxies are termed late
type. Early type galaxies contain proportionately more old stars and
less gas and star-forming activity than late type galaxies; presumably
the early type galaxies formed stars more rapidly in the past.
The spiral structure itself varies from galaxy to galaxy, even
within a Hubble type. A classification system based on the regularity
of the spiral arms was introduced in the 1960s by Sidney van den Bergh
and revised in the 1980s by Debra and Bruce Elmegreen. Approximately
10% of spiral galaxies contain only a single, bisymmetric spiral that
extends in a grand design from the edge of the galactic bulge to the
outer limit of the perceptible disk. Most galaxies have a multiple-arm
spiral structure, or even a highly chaotic or flocculent spiral-like
structure. Multiple arm galaxies represent 60% of the early and
intermediate Hubble types with bars and intermediate types without
bars. Flocculent spirals represent 60% of early Hubble types without
bars.
An example of a grand design galaxy, Messier 81, is shown in
Fig. 1,
rectified by computer to a face-on orientation and enhanced in detail
by subtraction of the average radial light profile. The spiral pattern
could be a quasistationary wave mode, as discussed by Chia-Chiao Lin,
Giuseppe Bertin, and collaborators. Such a mode comprises both inward
and outward propagating spiral wave packets. Interference between
these wave packets may produce the gentle oscillation of the arm
amplitudes seen in the figure. Wave modes are thought to grow by the
transfer of energy and angular momentum from the inner to the outer
region at the radius where the spiral pattern corotates with the
disk. The trailing spiral components propagate away from this radius,
outward and inward, until they either lose their energy by resonant
interactions with the peculiar (noncircular) motions of stars or they
reflect or refract off regions where a high stellar velocity
dispersion makes propagation impossible. The energy-losing resonances
are called Lindblad resonances, after Bertil Lindblad; they occur
where the period of a star's random excursion around its circular
orbit equals the time spent between the main spiral arms. The
reflection point occurs in the inner region, near the bulge. After
reflection, the wave returns to the corotation radius as either a
leading wave packet or an open trailing wave packet; there it
stimulates more of the tightly wound trailing components to reinforce
the original trailing waves. Theory predicts that an initially small
spiral perturbation should double in amplitude approximately once in
each orbit time. Eventually the growth of the spiral wave mode is
limited by energy dissipation in shocked gas and by stars scattering
at the Lindblad resonances.
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Figure 1. Blue band image of M81, rectified
to a face-on orientation and with the average radial light profile
subtracted. The calibration bar on the left represents 100 pixels; the
bar on the right has a length equal to the isophotal radius at 25
magnitudes per square arc second. (From Elmegreen, Elmegreen, and
Seiden, 1989. Reprinted with permission from The Astrophysical Journal.)
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Highly symmetric spirals like those in Messier 81 can also form when
a galaxy is strongly perturbed by a close encounter with another
galaxy. This mechanism was illustrated in an atlas of peculiar
galaxies by Halton Arp (1966) and it was modeled theoretically by Alar
and Juri Toomre (1972). Tidal forces during such an encounter can
perturb the stars so much that their previously near-circular orbits
become elongated, especially in the outer parts. This elongation may
be initially along a common line of apses, but under the influence of
galactic shear, the elongations at different radii rotate at different
rates, causing the stars to bunch together in a symmetric spiral
pattern. Self-gravity then amplifies this pattern, and it propagates
into the inner disk, possibly reflecting off the bulge. The outer
tidal arms may also stimulate the growth of an inner spiral wave mode,
which then persists for many more orbits than the initial tidal wave.
An example of a multiple-arm galaxy is Messier 74, shown with
computer enhancement in Fig. 2. The symmetric
spiral in the inner
region is surrounded by numerous, apparently independent, long-arm
spirals in the outer region. Theory suggests that the inner symmetric
spiral could be a single wave mode or a constructive superposition of
several wave modes with different angular speeds, or it could be a
rapidly evolving wave packet. The outer spirals could be the
constructively interfering parts of several overlapping wave modes, or
they could be random and independent wave packets triggered by local
gravitational instabilities in the gas and stars.
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Figure 2. Blue band image of M74, as in
Fig. 1. [By permission from
G. B. Elmegreen, in Galactic Models, New York Academy of
Sciences, New York (1990).]
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A different theory, developed by Philip E. Seiden and Humberto
Gerola in 1978, suggests that some of the outer structure in
multiple-arm galaxies could result entirely from star
formation. Gaseous instabilities first trigger star formation, and
then supernova explosions and shell-like compressions following star
formation trigger more star formation, leading to chain reactions that
are swept back into spirals arcs by shear.
Flocculent galaxies contain many more, and much shorter, spiral arms
than multiple arm galaxies. Such short arms could form by the same
mechanisms as the longer arms in multiple arm galaxies, with the
larger number of arms causing the chaotic appearance, or they could
differ because of a lack of global wave stimulation, or an inability
to amplify global waves in flocculent galaxies. An example is
Messier 63 (not shown).
Bars and oval distortions may drive spiral waves in some galaxies.
Among galaxies with early Hubble types, those with bars are twice as
likely to have symmetric spirals in their outer disks as those without
bars. Among intermediate and late-type galaxies, however, those with
bars have approximately the same proportions of grand design, multiple
arm, and flocculent spirals as those without bars. This dependence of
the bar-spiral correlation on Hubble type may result from a variation
of bar length and strength with Hubble type, that is, the bars in
early-type galaxies tend to be larger and stronger than the bars in
late-type galaxies. Perhaps early-type bars end near the corotation
resonance, where they can drive outward-moving trailing waves that
fill the outer disk. The bars in late-type galaxies may end far inside
this resonance, possibly at an inner Lindblad resonance.
An important quantity to determine is the angular rate of the spiral
pattern, which equals the angular rate of the stars at the corotation
resonance, in the modal theory. Stars move faster than this pattern
rate inside corotation and slower outside corotation. Recent work
suggests that the angular pattern speed can sometimes be determined
from optical tracers of wave-orbit resonances. In the most symmetric
spirals, the far outer extent of the spiral should be the outer
Lindblad resonance, where outward moving wave packets convert their
energy into random stellar motions. The inner extent may be an inner
Lindblad resonance if there is a bar or oval there, because such ovals
can reflect an inward-moving wave. If the inner galaxy is symmetric,
or if there is a strong bulge, then the inner extent of the spiral
need not be a resonance but merely a reflection point. The corotation
resonance occurs at the radius where gaseous shocks and wave-triggered
star formation end, and in some cases, where large patches of interarm
star formation occur. The 4:1 resonance, where the time spent between
the main spiral arms equals twice the period of the peculiar stellar
motions (rather than one times this period, as for a Lindblad
resonance), may contain small spiral-like spurs midway between the
main arms. Once any one resonance location is identified in a galaxy,
all of the others follow from the radial distribution of circular
velocities.
Considerable progress has been made in understanding galactic
spirals, but neither the theoretical framework nor the observations of
galaxies are sufficiently detailed at the present time to determine
the origin and evolution of spirals on a case-by-case basis. Part of
the problem is that the basic state of a galaxy is uncertain, that is,
the spatial and temporal distributions of the gas and stars, and of
their velocity dispersions, are poorly known, as are the magnitudes
and histories of environmental perturbations. Progress requires
detailed observations at optical, infrared, and radio wavelengths of
all types of spirals in a variety of environments. Fully nonlinear
theories, including gas and stars, with or without external
perturbations, are also needed.
Additional Reading
Arp, H. (1966). Atlas of peculiar galaxies.
Ap. J. Suppl. 14 1.
Athanassoula, E. (1984). The spiral structure of galaxies.
Phys. Rep. 114 321.
Baade, W. (1944). The resolution of Messier 32, NGC 205, and the
central region of the Andromeda nebula. Ap. J.
100 137.
Bertin, G. (1980). On the density wave theory of normal spiral
galaxies. Phys. Rep. 61 1.
de Vaucouleurs, G., de Vaucouleurs, A., and Corwin, H. G., Jr.
(1976). Second Reference Catalog of Bright Galaxies.
University of Texas Press. Austin.
Elmegreen, B.G. (1989). Grand design, multiple arm and flocculent
spiral galaxies. In Galactic Models, J.R. Buchler, S.T.
Gottesman, and J.H. Hunter, eds.
Academy of Science New York.
Elmegreen, B.G., Elmegreen, D.M., and Seiden, P.E. (1989).
Spiral arm amplitude variations and pattern speeds in the grand
design galaxies M51, M81, and M100. Ap. J. 343 602.
Elmegreen, D.M. and Elmegreen, B.G. (1987). Arm classes for
spiral galaxies. Ap. J. 314 3.
Gerola, H. and Seiden, P.E. (1978). Stochastic star formation and
the structure of galaxies. Ap. J. 223 129.
Hubble, E.P. (1926). Extra-galactic nebulae. Ap. J.
26 321.
Lin, C.C. and Shu, F.H. (1964). On the spiral structure of disk
galaxies. Ap. J. 140 646.
Lindblad, B. (1958). Stockholm Observatory Annals 20 No. 6.
Sandage, A. (1961). The Hubble atlas of galaxies. Publication 618,
Carnegie Institute of Washington.
Toomre, A. (1977). Theories of spiral structure.
Ann. Rev. Astron. Ap. 15 437.
Toomre, A. and Toomre, J. (1972). Galactic bridges and tails.
Ap. J. 178 623.
van den Bergh, S. (1960). A preliminary luminosity classification of
late type galaxies. Ap. J. 131 215.
See also Galactic Structure, Large Scale; Galactic Structure,
Spiral, Observations; Galaxies, Spiral, Nature of Spiral Arms.