Starburst Galaxies (4) ...
The following figure
is a composite of two images in blue and
near-infrared light taken with the
Wide-Field and
Planetary Camera (WFPC2) of the
Hubble Space Telescope. It
suggests that two galaxies have actually
collided with each other, with a violent wave of star-forming material
having been ejected
as a result.
For obvious reasons, the galaxy shown in the preceding figure
is called the Cartwheel Galaxy. It
is a member of a group of galaxies about 500 million light years away in
the constellation Sculptor. The remarkable ring-like structure seen in the right side of the
figure is about 150,000 light years across, which is 50 percent larger than
the Milky Way Galaxy.
This ring of star formation is a wave
traveling outward at about 300,000 kilometers per
hour. It
results from energy deposited in the center of the galaxy by the passage of another galaxy of the
group through it. As the wave
passes outward it compresses and heats the matter
that it passes through, triggering star formation.
Secondary Star Formation
The top left inset shows
a magnified portion of the ring. The bright blue regions are clusters of new star formation and the loops
and bubbles result from supernovae produced by massive star formation. The loop
extending up and to the left
near the center of
this inset appears to be a wave of new secondary star formation.
It lies on the leading edge of an expanding
bubble that is probably a supernova remnant. The wave
of star formation in the full ring is estimated to
represent two billion new stars!
The lower left inset shows the nucleus and inner part
of the galaxy, which is just beginning to recover its spiral
structure. The center contains large amounts of dust and clusters of new star formation (the bright
points). We may guess that this galaxy was a spiral much like our own Milky Way before this collision
took place.
Trail of the Intruder?
Initially it was thought that one of the two galaxies just to the right of the Cartwheel
Galaxy was the likely instigator of the Cartwheel starburst. This is supported by the
observation that the lower one of
these galaxies is partially disrupted, indicating a recent
collision (caught with smoking gun in hand, so to speak). However, the
adjacent figure tells a different story!
This figure is a
neutral hydrogen map of the region near the Cartwheel Galaxy constructed from radio
telescope data,
with the contours indicating neutral
hydrogen concentration.
The trail of neutral hydrogen
leading from the Cartwheel Galaxy to the galaxy in the upper right
suggests rather strongly that this galaxy, not one of the two that appear to be closer,
may have passed through the Cartwheel and initiated the vigorous
activity found there now.
(Note that the two galaxies nearer the Cartwheel in the image
may not actually be closer in distance, since this image does not tell us how far in
front or behind these galaxies are relative to the Cartwheel.),
