Regular solar observer & correspondent Harry Roberts reports on his observations of the galaxy NGC 55, also known as Dunlop 507.
“NGC 55 – a long spindle – edge on. Denser (stars) at right-hand (west) end. Suggestion of a dark lane on the north side”. Thus was NGC 55 logged with the C8 in the Blue Mountains west of Sydney on my first view of this unusual galaxy; a rough sketch was made and thunderstorms over the distant coast were noted: it was a great night some 34 years ago!
Yet NGC 55 remains something of an enigma: a big edge-on spiral apparently, part of the Sculptor galaxy cluster (it was said) and, as galaxies go, is bright and detailed.
The presence of galaxy NGC 253 nearby prompts debate over which is the bigger and brighter of the two. Recently, with a 32plossl eyepiece, it was possible to pan from one galaxy to the other and resolve this debate – at least to my satisfaction. N55 is larger (just) while N253 is about a magnitude brighter: both are southern showpieces. Finally, a detailed sketch was made over a couple of nights (Fig).
James Dunlop, its discoverer (in 1826) described NGC 55, or Dunlop 507, as “A beautiful long nebula, about 25″ in length; position north preceding, and south following, a little brighter towards the middle, but extremely faint and diluted at 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”. It is Fig 21 in his 1828 paper and was viewed 4 times. That Dunlop could detect colour in this object with a 9in speculum is unlikely; yet it is a blue object. His figure shows a long narrow bar, brighter at one end – a fair impression.
Photographs show the galaxy as an elongated cigar of bluish star fields with, at the western end, the galaxy’s yellowish central core shining through them– surprisingly as Dunlop described it.
NGC 55. In the eyepiece this is a strange object: it doesn’t look like a typical edge-on spiral, with a central core and arms on both sides. Instead, we see a large ‘arm’ on the east side only, a bright cigar-shaped core and just a hint of an arm on the west side. This apparent ‘asymmetry’ may suggest that NGC 55 is a classic barred-spiral, type SBb, viewed edge-on at such an angle that only the nearer arm is visible, superimposed over the yellow core, with the farther arm hidden in the background.
Other sources, however, suggest NGC 55 is a barred irregular galaxy – rather like the Large Magellanic Cloud – and liken it to northern galaxy NGC 4631. While usually said to be part of the Sculptor Cluster, the latest data suggests NGC 55 and 300 are foreground objects that may be interacting.
NGC 55 is a large object, about 30 arcmin in length, i.e. its apparent length equals the Moon’s diameter! Yet it is hard to see in small binoculars while N253 is easily seen. What do we make of Dunlop’s ‘25″’ length? 25 seconds of sidereal time equals 6 arcmin, so perhaps it’s the N-S minor diameter, which is 6.25 arcmin. Or, as some suggest, was the double prime symbol ″ used in error for 25 arcmin?
In a ten-inch ‘scope (250/1270) there is detail on show (Fig). Numerous stars are seen; some have their reference magnitudes added – in case we see a supernova! The brighter western ‘cigar’ shows two or three bright knots of likely blue OB star clusters that, photos show, have HII emission nebulae involved. Some darker lanes separate these knots. One is noted that resolved into what seemed like a stellar galaxy nucleus.
To the east is a ‘streak’ of what, at first, resembles a comet’s tail; it seems wider to the east and is fainter than the ‘cigar’ yet still has some ‘knots’ of unresolved stars. This, it seems, is a spiral arm of blue star-forming activity that wraps around the central core of yellow stars to the right. The dark ‘waist’ that seems to separate the two components may be a contrast effect as the eastern arm is much fainter than the ‘cigar’.
NGC 55 is great object, with some of its detail visible in a four-inch ‘scope, and is fairly easy to locate near star Alpha-Phoenicis. It has other galaxies nearby, but its 40degS declination puts it out of reach for many northern viewers. As Dunlop said, “This is a beautiful object”. He was not wrong!
Harry Roberts is a Sun and Moon observer, a regular contributor to the Sydney Observatory blog and a member of the Sydney City Skywatchers.