The Caldwell Objects

83
Astronomy and Astrophysics
article,
Alessandro Marconi and
his colleagues discuss
how they used the
Hubble Space Telescope
to detect a roughly 300light-year-wide
starburst ring as well as
a conical "super bubble"
blown by supernova
explosions. Yet not even
HST could penetrate
NGC 4945's opaque
central torus to reveal
the active nucleus. With
the currently available
data it is still not
possible to establish
whether the intense
energy streaming from
the galaxy's
heart is powered by the active nucleus or by the
starburst. If the active nucleus dominates, it must
be fully obscured.
You'll find this extragalactic treasure tucked
away in a dense nub of Milky Way 4° southwest
of Omega Centauri, where background galaxies
are dimmed by as much as 0.8 magnitude (a
factor of 2) by our galaxy's intervening dust.
Were it not for that dust, NGC 4945 would shine
at 8th magnitude. And had NGC 4945's disk been
tipped a little more our way, it would shine
brighter still. To locate this enigmatic galaxy, look
halfway between Omega Centauri and 2ndmagnitude
Gamma (γ) Centauri, where you'll see
a l½°-long triangle (oriented north-south) of 4th-
and 5th-magni-tude stars: Xi 1 (ξ 1 ), Xi 2 (ξ 2 ), and f
Centauri. (Confirm that you can find these stars
with your binoculars.) NGC 4945 lies inside that
triangle 17' due east of Xi 1 Centauri, the westernmost
member of the triangle. The galaxy is a
334
9th-magnitude, 19'-long needle of light oriented
northeast-southwest. From Hawaii I find it very
challenging to see in 7x35 binoculars. But with
time its silvery haze glints in and out of view.
Seeing NGC 4945 this way requires keen averted
vision and patience. Look for a fine thread, sharp
yet fuzzy, like a dew-laden strand of cobweb
reflecting the first glimmers of dawn.
In the 4-inch at 23x NGC 4945 is very
apparent as a slender and diffuse shaft of light,
with some patchiness and a hint of a broad
central region. The view begs for more power. At
72x the galaxy begins to crumble into tiny details,
each requiring time and averted vision to
decipher. Most prominent is a bright nub at the
galaxy's core, which has a faint skirt of light
extending on either side of the major axis. On
both ends the galaxy's disk looks gutted, leaving
the northeastern and southwestern rims sharply
defined and bright, like the prongs of a
Deep-Sky Companions: The Caldwell Objects