Irregular Galaxies
Approximately 3 percent of galaxies observed cannot be classified as either
ellipsoidal or spirals.
These galaxies have little symmetry in their structure and are termed
irregular galaxies (abbreviated Irr).
Properties of Irregular Galaxies
Irregular galaxies contain 108-1010 solar masses,
have diameters from 1-10 kpc,
and absolute blue
magnitudes -13 to -20. Thus, they are often faint and hard to see relative to the
brighter spirals and elliptical galaxies.
Irregular galaxies contain both young and old stars, and substantial gas and dust.
The fainter irregular galaxies are sometimes called dwarf irregulars.
Although dwarf irregulars and dwarf elliptical galaxies are very difficult to see
because they are so faint, we have reason to believe
that they may be the most common types of
galaxies in the Universe.
Example: The Magellanic Clouds
Two of the
best known irregular galaxies
are the Large and Small Magellanic Clouds (often designated as the LMC and SMC,
respectively), which are visible
to the naked eye as small bright clouds
in the Southern Hemisphere sky. The LMC and SMC are shown in the
top right figure. They get their names because
they were observed by Magellan in 1521 while on his voyage around the world.
The Large Magellanic Cloud and Small Magellanic Cloud
are satellites of our own galaxy, lying only 50-60 kpc away. In fact, as we
note in the right panel, they are sufficiently nearby that they are
interacting tidally with the Milky Way. They are much smaller than the Milky Way, with
diameters of only several kpc
and masses about 100 times smaller for the LMC and 1,000 times smaller for the SMC.
Example: Sextans A
Another example of an irregular galaxy is
Sextans A, shown in the image on the left. This irregular galaxy is also
a
neighbor of our own galaxy, at a distance of only about 10 million light years.
The blue regions in the image
are clusters of
young stars. The brighter stars are in the foreground, much closer
to us than Sextans A. They are members of our own Milky Way galaxy.
Peculiar Galaxies
Hubble's original classification lumped all galaxies that are not spirals or
ellipticals into the irregular category. It is common today to make
further distinction between more "normal" irregular galaxies and peculiar
galaxies, which are galaxies that look unusual in some respect.
For example, some peculiar galaxies are
objects that have been tidally distorted by interaction with another
galaxy.
Others are
active
galaxies
with some evidence of violent
internal processes taking place.
Still others may be more normal galaxies,
but given an unusual appearance because of obscuring dust.
We shall discuss these special kinds of
irregular galaxies separately later.