The Caldwell Objects

Dunlop discovered NGC 55 with his 9-inch f/12
reflecting telescope and cataloged it as his 507th
object. " beautiful A long nebula," he wrote of it,
"about 25 ['] in length; position north preceding
[northwest], and south following [southeast], a
little brighter towards the middle, but extremely
faint and diluted to the extremities. I see several
minute points or stars in it, as it were through the
nebula: the nebulous matter of the south
extremity is extremely rare, and of a delicate
bluish hue. This is a beautiful object." John
Herschel was equally impressed with the galaxy's
appearance, seeing it on the night of October 4,
1836, as a 'Very long irregular crooked ray with 3
nuclei, the second of which appears to consist of
stars." In photographs the galaxy appears most
unusual. A dense, dusty lens of light dominates
the western end of its ½°-long spindle. A lagoon
of dark material separates this mass from its
faint, splotchy eastern extension, which has a
separate IC catalog number. Apparently this dark
lagoon caused some problems years ago, as
Harold G. Corwin (Infrared Processing and
Analysis Center) explains on the NGC/IC Project
Web page:
IC 1537 is the east-southeast[ern] arm of NGC
55. It was first seen, described, and sketched by
James Dunlop in the 1820s. [John Herschel]
provided a more detailed description and
sketch a decade later. Both clearly noted that
the south-following [east-southeastern] end of
the nebula was much fainter than the north-
preceding [north-northwestern], and their
estimated sizes (note the typo in Dunlop's
paper: in place of 25 arcsec, read 25 arcmin)
include the whole galaxy, not just the brighter
portion. Furthermore, the fainter following
[eastern] part is clearly shown in both pub-
lished sketches.
In spite of these published observations,
[Lewis] Swift claimed this part of [NGC] 55
as his own discovery: "As Sir John Herschel
does not mark it [NGC 55] with a sign as
being a remarkable object, lends [sic] plau-
sibility to the idea that it [IC 1537] was not
seen even by him." And this after implying
that Dunlop had certainly not seen the fainter
eastern end.
70 & 72
As with NGC 300, deep infrared images have
confirmed that NGC 55' disk contains s an aged
stellar population. Shell structures have also been
observed, and their velocities support the idea
that diffuse gas is being pushed into the galaxy's
halo by supernova explosions and stellar winds.
The galaxy is replete with ionized gas features
like giant Η II complexes, supergiant filaments,
and very faint diffuse emission nebulae. Many of
these features protrude well above the galaxy's
plane, as does one very faint, fragmented shell
extending 8,500 light-years above the disk. All of
the protruding features are believed to be
"chimneys" that allow hot gas from star-forming
regions in the disk to funnel into the halo. Several
chimneys are "capped" with clumps of ionized
gas, one of which appears to be the site of recent
star formation. None of this, of course, can be
seen with the eye, but this kind of information
does help fire our imaginations when we look at
this remarkable object.
As noted above, NGC 55 is visible in 7x35
binoculars. But it is not visible to the naked eye
(at least not to mine; I encourage others to try).
As with NGC 300,1 question its published magnitude.
Lists often refer to its photographic blue
magnitude (7.9), which also matches a 1991
photometric visual-band (V-band) estimate by
the late Gerard de Vaucouleurs and his
collaborators. My binocular estimate places the
galaxy's visual magnitude a bit higher at 7.5. In
binoculars the galaxy's spindle is oriented in
more or less in the same direction as a 2°-long
line of three roughly 7th-magnitude stars, and
The Caldwell Objects 281