Starburst Galaxies
Some galaxies undergo a riotous wave of star formation
that is much more rapid than
expected, even for
gas-rich spirals.
These are called starburst galaxies.
We already saw one example for the interacting galaxies
Arp 273. Starburst activity tends to be concentrated within the central
kiloparsec of a starburst galaxy, but some starbursts span the entire disk of a galaxy.
A mature spiral galaxy like the Milky Way probably makes on average
2-3 new stars every year, but in a starburst the star production
rates may be a hundred times higher. What would cause a mature galaxy to
suddenly begin manufacturing stars a hundred times faster than normal?
As we now discuss, the cause of such starbursts often lies in
interactions between galaxies.
Example: M82
The nearest example of a starburst galaxy is the irregular galaxy M82 (also called
NGC 3034), which lies about 3.4 Mpc away in the constellation
Ursa Major. The top right figure shows
M82 at visual wavelengths.
The appearance of M82
has long been a mystery. At one time, it was thought by some
astronomers to be "exploding". We now understand the strange appearance at optical
wavelengths to result from strong starburst activity in the central region of the galaxy
coupled with obscuring dust. This starburst activity appears in turn to be associated
with interactions among M82 and its neighboring galaxies.
Interactions in the M81 Group
The galaxy M82 is part of a group of 12
galaxies called the M81 group, which at 3.4 Mpc is one of the
nearest groups of galaxies to our Local Group. The most prominent galaxies in this
group are M82 and the Sc spirals M81 and NGC 2403.
Unlike our local group, several of the
galaxies in the M81 group appear to be interacting strongly with each other.
This can be seen most clearly in radio telescope observations, which show that there are
bridges and mutual envelopes of neutral hydrogen (which is invisible at optical
wavelengths) between various galaxies in this group.
The following figure
shows the galaxies M81 and M82 on the right in infrared, and a larger region
centered on M81 and M82 at radio frequency wavelengths on the left.
The RF
image illustrates the strong interaction among three of the larger galaxies, M81, M82,
and the E2 elliptical galaxy NGC 3077. (Also seen as small red islands to the left of
M81 are two of the dwarf galaxies in the group.) Notice in particular
the bridges of neutral hydrogen
connecting these galaxies--click the "Show Labels" button to mark this clearly.
