Spiral Galaxies (and other disks)

Click icon to see a spiral galaxy of Messier’s catalog

The icon shows M51, the Whirlpool Galaxy.

Among the galaxies, there are apparently three main categories,

according to their appearance:

the disk galaxies (`cosmic frisbies‘ according to P. Murdin, D. Allen,

and D. Malin), consisting of a huge disk of stars and interstellar matter,

which may form interesting patterns,

the elliptical galaxies (`cosmic footballs‘)

which are uniformly looking, ellipsoidal agglomerations of stars, and the

irregular galaxies (`cosmic misfits‘) which cannot

be integrated in this scheme.

Physically, it is not necessary so clear (at least in the opinion of the present

author) if this classification is real, because there exist intermediate types

even between ellipticals and spirals, i.e. spiral galaxies often have an

ellipsoidally formed “bulge” which may be very luminous (as in case of the

Sombrero galaxy M104) or rather inconspicuous;

some spirals seem to lack this component at all. A heavy bulge is often connected

with the presence of a big ellipsoidal core. On the other hand, at least some

ellipticals seem to house a disk component also; ths most conspicuous example of

such a galaxy is probably Centaurus A (NGC 5128), a prominent galaxy in the

Southern hemisphere which is not a Messier object because of its southern

declination, but forms a group with the beautiful spiral

M83. Centaurus A is regarded as peculiar.

One may speculate that e.g. the disk around the center of

M87, which is often regarded as the accretion disk

around the supermassive object in that galaxy’s nucleus, may be a smaller

manifestation of the same disk phenomenon.

Focussing on the disk (or disk dominated) galaxies, these often show beautiful

and conspicuous patterns in the form of spiral arms and/or luminous bars. These

structures have been a mystery for a long time, it was thought that there may

be physically different classes of disks (e.g., “normal” and “barred” disks),

but now it seems as if they are

all the consequences of gravitational interactions with neighboring galaxies.

Encounters with neighbors cause inhomogenities and un-symmetries in the

gravitational field within the disk, which tends to compress the gas in some

regions. If the density of the gas in these regions exceeds a certain critical

value (which depends on parameters as the temperature), star formation can take

place, resulting in the formation of red emission nebulae and blue clusters of

hot young stars, which slowly change their color to the yellow when they come

to age, and their hottest stars have disappeared (i.e., exploded as supernovae).

The star forming regions tend to be aligned along spiral arms, as the denser

regions in the interstellar matter apparently prefer to form such patterns. When

getting older, they sometimes stay conspicuous as yellowish “fossil arms”, which

can be traced in several galaxies.

Galaxies are classified, according to their appearance, in the so-called

Hubble scheme (after its inventor, Edwin Powell Hubble;

see e.g. our illustration of the

Hubble Scheme with Messier galaxies).

This scheme defines the classes listed above, i.e. spiral, elliptical and

irregular galaxies, and is especially interesting for spirals:

Those with pronounced bar structures are called “barred spirals” and classified

“SB”, while normal spirals are simply called “S” or sometimes “SA”; some authors

take “SAB” or “S(B)” for mixed types. Spiral galaxies, “normal” and barred, with

conspicuous bulges (especially near their center) are classified “Sa” or “SBa”,

those which have prominent bulges and pronounced arms are clssified “Sb” or

“SBb”, and those which are dominated by the arms are “Sc” or “SBc”.

If the core and bulge seems to be lacking, a galaxy is classified “Sd” or “SBd”,

and those which have no pronounced core and show irregularities are classified

as “Sm”; these represent a type between disk and irregular galaxies.

Some of the galaxies, mostly those who had no closer encounters for a longer

period of time, and those who have lost most of their interstellar matter for

some reason, do not show any conspicuous pattern within their disks; these are

often called “S0” or “lenticular” galaxies.

Although they are disks, they can

often hardly be distinguished from ellipticals from their appearance, and have

often been misclassified in the past. This misclassification happened to all the

four Messier lenticulars in the past, and to many other galaxies of this type.

When undergoing a heavy interaction, or collision, with a massive neighbor,

disk galaxies may be distorted very peculiarly, and then are often classified

as irregular; this is the case for the only Messier irregular,

M82.

All disk galaxies have a very different appearance, depending from what

direction they are seen, or under which angle toward the line of sight

(to us) their disk is inclined. According to this situation, they are

either seen from their edge (or “edge-on”), or from near their equatorial

plane, as thin, flat, linear and elongated patches, often with dusty

structures along their equators, or almost from their poles so that we can

see their disks “face-on”.

Tom Polakis has

featured some

edge-on galaxies.

Our Milky Way is one of the big and more massive

spiral galaxies, and is of Hubble type Sbc, or perhaps SBbc if it should have

a bar.

Spiral galaxies in Messier’s catalog:

M31,

M33,

M51,

M58,

M61,

M63,

M64,

M65,

M66,

M74,

M77,

M81,

M83,

M88,

M90,

M91,

M94,

M95,

M96,

M98,

M99,

Other early known spiral galaxy:

Milky Way.

(Probable) S0 galaxies in Messier’s catalog:

M84,

M85,

M86,

NGC 5866 (M102 ?).

The irregular galaxy

M82 is also a distorted disk galaxy.

Last Modification: 25 Jan 1998, 16:26 MET