The Caldwell Objects

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the knowledge of the 19th-century observer was,
in the words of French author Georges Simenon,
"a little island in a great ocean of nonknowledge."
Today we know that NGC 4449 is an active
star-forming dwarf galaxy seen at a 43° angle
from face on. It has a bar running through it,
much like the one we see in the Large Magellanic
Cloud. The galaxy is surrounded by an enigmatic
halo of diffuse hydrogen gas some 10 times larger
than the galaxy's visible disk. At its estimated
distance of 10 million light-years NGC 4449 has a
modest linear diameter of 16,000 light-years (not
including the invisible hydrogen halo) and an
equally modest luminosity of 1.8 billion Suns.
The galaxy is receding from us at a speed of 200
km per second. NGC 4449 has many ΗII regions
concentrated primarily at its northern end; in
photographs they appear as knots. Seeing these
features helps us understand why William
Herschel believed that, "with difficulty" he
resolved this object, seeing specifically "3 or 4
bright compressed spots."
The Hubble Space Telescope has resolved
hundreds of extended objects centered on the
galaxy; they may be populous star clusters
similar to those in the Large Magellanic Cloud.
One bright central object may be a super star
cluster only 6 to 10 million years old. Hubble also
resolved intense regions of starburst activity. The
galaxy's high star-formation rate is believed to
have been caused by a merger or interaction with
a companion galaxy. Radio observations have
shown NGC 4449's hydrogen halo to have a
disklike feature and a lopsided arm structure. In
a series of computer simulations, Christian Theis
and Sven Kohle (University of Kiel, Germany)
have demonstrated that these features were
created by a gravitational interaction between
NGC 4449 and another dwarf galaxy called DDO
125. The
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two galaxies have followed nearly parabolic
orbits around one another, write the scientists,
and were nearest to one another about 4 to 6 billion
years ago, when they may have gotten as
close as 80,000 light-years — nearly the diameter
of our Milky Way. The origin of the hydrogen
halo remains a mystery; it may have been drawn
out from NGC 4449 by an earlier interaction.
NGC 4449 can be found just 3° northnorthwest
of 4th-magnitude Beta (β) Canum
Venaticorum, midway between two 8th-magnitude
stars (one roughly 1° to the east and the
other a similar distance to the west). The galaxy
shines with the light of an unfocused 9.6magnitude
star and has a high surface brightness,
making it an easy find. In fact, NGC 4449 is
visible in 7x35 binoculars as a tiny diffuse glow,
though dark skies, patience, and averted vision
are required to see it. The view through the 4inch
at 23x is quite remarkable. The galaxy
appears as a bright 5'-long oval adorned with a
brightly dappled center. A dif-
Deep-Sky Companions: The Caldwell Objects