The Caldwell Objects

the two jets of radio-wave-emitting plasma
streaming out of Centaurus A pointed right to
our next target — the nearly edge-on barred
spiral galaxy NGC 4945 (Caldwell 83). Although
NGC 4945 is about 7½° due southwest of
Centaurus A on the sky, the two belong to the
Centaurus Group of galaxies and lie at similar
distances from Earth. Does the radio jet connect
the two galaxies? Maybe, maybe not. "Most
astronomers dismissed Arp's diagrams and his
computer-processed photographs as
coincidences," Overbye writes. But the notion of
"intergalactic pipelines" is not totally absurd,
especially since HST has imaged a dark string of
material flowing between two battered galaxies
that bumped into each other about 100 million
years ago. That pipeline begins in NGC 1410 and
wraps around NGC 1409 like a ribbon around a
package. Astronomers used HST to confirm that
the 20,000-light-year-long pipeline is a
continuous string of material linking these
compact galaxies, and it probably was created
during a tussle between them. In photographs
NGC 4945 is a similarly
83
chaotic brew of stellar clumps and dusty patches
seen through a scrim of foreground stars. The
galaxy is quite large, spanning 77,000 light-years,
and is receding from us at a speed of 560 km per
second. It has the luminosity of 28 billion Suns
and is the third-brightest galaxy in the IRAS
Point Source Catalog (a list of objects whose
brightnesses were measured at far-infrared
wavelengths by the Infrared Astronomical
Satellite). Most of the galaxy's infrared radiation
comes from a compact region around the nucleus,
which is surrounded by a dense, roughly 3 lightyear-wide
torus of dust. Seyfert galaxies, like
NGC 4945, are characterized by small, bright
nuclei that emit intense and variable X-ray,
infrared, and radio radiation while looking
comparatively normal at visible wavelengths.
Unlike the spectra of most spiral galaxies, which
display a continuum and a few absorption lines,
those of Seyferts are dominated by emission lines.
These lines probably come from interstellar gas
heated by X-rays streaming out of the galaxy's
nucleus.
There are two types of Seyfert galaxies. Type
2 Seyferts (like NGC 4945) have narrower
emission lines than Type 1 Seyferts, indicating
gas velocities of a few hundred km per second
(or less) as opposed to thousands of km per second.
The difference between the two Seyfert
types is a consequence of the galaxy's apparent
tilt. Type 1 emissions are associated with face-on
galaxies, which allow us a clear view of the
rapidly moving gas near the core. Since the cores
of edge-on galaxies, like NGC 4945, are obscured
by dust, we get to see only the narrower emission
lines coming from slow-moving gas farther away
from the center.
Next to NGC 4151 in Canes Venatici, NGC
4945 is the brightest Type 2 Seyfert galaxy at high
X-ray energies, leading astronomers to believe
that its nuclear activity may be powered by a
supermassive black hole. In a 2000
The Caldwell Objects 333