The Caldwell Objects

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1613 is one of the few galaxies known to be
approaching our Milky Way. And though its
approach speed of 234 km per second is comparable
to that of NGC 147, it is not part of the
Andromeda Galaxy subsystem. To understand
why, we have to travel in our minds above the
plane of our Local Group. Imagine a circle whose
radius equals 3 million light-years. If we place the
Milky Way at the center of this circle, we see the
Local Group dominated by two groupings of
galaxies. One huddles around the Milky Way at
the center; the other lies at the 9:30 position, a
little more than two-thirds of the way out from
the center. (The latter group comprises the
Andromeda Galaxy, its four brightest dwarf
attendants, and M33, the Great Spiral in
Triangulum.) IC 1613 would be in the same
concentric circle as the Andromeda Galaxy
grouping, but 40° away from it, in the 7:30
position; the spatial separation between the
Andromeda Galaxy and IC 1613 — nearly 2
million light-years — is far too great for the
objects to be considered closely associated. In
fact, IC 1613 is but one of dozens of dwarf
galaxies that populate the Local Group. Barnard's
Galaxy (Caldwell 57) is another, and astronomers
believe that many more of these diminutive
galaxies have yet to be discovered. One of the
problems, of course, is that if a loosely structured
dwarf galaxy is seen against the plane of the
Milky Way, its stars are hard to differentiate
between those of our own galaxy.
Fortunately, IC 1613 lies well away from the
stellar madness of the Milky Way. We see it
against a relatively star-poor backdrop in Cetus.
In photographs taken with large telescopes, IC
1613 looks, ironically, like a close-up shot of the
galactic center taken with a 100-mm lens, but
with one major difference: while the system has
an oval appearance overall, its interior looks
stretched, fragmented, shorn, and tattered. In
other words, IC 1613
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looks like extragalactic road kill. This analogy is
not far from the truth. Throughout their histories
irregular galaxies have taken severe gravitational
beatings from passing galaxies or larger
neighboring ones. Left alone, the irregulars
would rearrange themselves into more
symmetrical systems, which is what IC 1613 is
trying to do. That's why I call IC 1613 the
Scarecrow. In a remote way it reminds me of the
scarecrow in the "Wizard of Oz"; after the witch's
winged monkeys tore the straw man apart, he
started to piece himself back together.
Indeed, IC 1613 may be a highly flattened
spiral system seen tilted 27° from face on. But its
spiral structure, if any, is certainly different than
that of classical spirals like the Pinwheel Galaxy
(M33) or the Whirlpool Galaxy (M51). IC 1613's
features are more like those of the Large
Magellanic Cloud, for both systems display a
prominent central mass, slight spiral structure,
and a well-resolved axial bar. Actually
astronomers find that IC 1613 most closely
resembles the transition-type dwarf galaxy DDO
216 in Pegasus as well as Barnard's Galaxy. So
irregular galaxies may not be so disorganized as
first impressions imply.
IC 1613's brightest stars are concentrated in
the galaxy's northeastern quadrant, which is
separated from its core by an oval swath of dust
that is rimmed by what appears to be a spiral
arm. This quadrant also contains a huge
association of giant blue stars and may be a
cradle of star formation. Indeed, the galaxy is
riddled with star-forming regions. Spectroscopic
observations conducted with the Canada-France-
Hawaii Telescope atop Mauna Kea have revealed
20 stellar associations in the galaxy's core and its
northeastern and northwestern quadrants.
Studies of the northeastern quadrant have shown
that its youngest stars are between 5 and 20
million years old, while its oldest stars have ages
up to 10 billion
Deep-Sky Companions: The Caldwell Objects