The Caldwell Objects

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but a single massive object — a link between
elliptical and spiral galaxies. But recent Hubble
Space Telescope (HST) images have helped
astronomers zero in on a new solution. We now
know that NGC 5128 is truly a giant, containing
more than a trillion solar masses. R. Brent Tully
determined that the galaxy lies about 16 million
light-years distant and spans 80,000 light-years,
though a NASA press release places the galaxy
merely 10 million light-years off, implying a
reduced diameter of 50,000 light-years. The HST
data support the galactic cannibalism scenario,
though with a twist. Apparently a massive black
hole lies hidden at the center of this Seyfert 2
galaxy, which is voraciously consuming a smaller
spiral galaxy that it collided with between 160
and 500 million years ago. The spiral will be fully
consumed in a few hundred million years.
Whenever one galaxy plows into another,
the interaction triggers a wave of star formation.
Indeed, in NGC 5128 HST has found 21
candidate globular clusters, half of which are old,
like those in our Milky Way, while the other half
are young, implying they might have formed
during the recent merger. HST also resolved
open clusters of hot, blue stellar newborns along
the fringes of the galaxy's dust lanes, which
astronomers penetrated with HST's infrared
camera. Behind that ring of dust lies a twisted
disk of hot gas swept up into what appears to be
a gravitational vortex surrounding a black hole.
The evidence suggests that the black hole
contains the mass of perhaps a billion stars
squeezed into a region of space about the size of
our solar system. Radio jets have also been
detected streaming out of the black hole. These
jets reveal the orientation of the black hole's spin
axis. They can be seen forming about 16,000 lightyears
from the galaxy's nucleus, then expanding
into plumes that extend 800,000 light-years into
intergalactic space. Interestingly, the maelstrom
of gas surrounding the black hole is not
perpendicular to the jets. Astronomers conjecture
that the gas disk might be so young that it hasn't
yet become aligned to the black hole's spin axis.
Or it may simply have been influenced more by
the galaxy's gravitational tug than by the black
hole's. The hot gas disk is perpendicular to the
galaxy's outer dust belt, while the black hole's
spin axis is tilted approximately diagonally to
these axes. The black hole's origin remains a
mystery. Was it always a part of the giant
elliptical? The small spiral? Or was it created
during the merger?
You won't solve that puzzle when you
visually explore this orb of cosmic wonder with
your telescope, but you can contemplate it
nevertheless. To find this celestial masterpiece,
all you have to do is look 4¼° essentially due
north of another significant astronomical
treasure — the great naked-eye globular cluster
Omega (ω) Centauri (Caldwell 80). You could
also spot it by sweeping a similar distance west
of 3rd-magnitude Mu (μ) Centauri. In 7x35
binoculars NGC 5128 is a large diffuse glow,
nearly 60 percent the size of the full Moon. From
dark skies there's no mistaking this remarkably
bright (magnitude-6.6) glow, which can be seen
with the unaided eye. Indeed, NGC 5128 is 0.3
magnitude (32 percent) brighter than M81, 0.9
magnitude (2.3 times) brighter than M83, and 0.5
magnitude (58 percent) brighter than NGC 253
(Caldwell 65) — all of which have also been
dimly detected with the naked eye. That brings
to six the number of galaxies that I know to be
within the reach of human vision. I believe there
could be more. The standard of achievable
naked-eye limits in amateur astronomy has
progressed well beyond the rudimentary
boundaries of M31 and M33. W have e now
journeyed beyond a safe 2.3 million light-years
to the razor's edge
308 Deep-Sky Companions: The Caldwell Objects