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An Argleton in the sky


acey

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Google Maps lists the town of Argleton, Lancashire, which turns out to be an empty field. No one knows how the spurious entry crept in. The NGC has its own share of non-existent objects, one of which I encountered last night.

My target was the compact galaxy group Hickson 93 (Arp 99) in Pegasus. Transparency at my dark site was poor, only around 5.5, but with the 12-inch at 75x I easily found the group’s dominant galaxy, NGC 7550, having identified the field precisely using Uranometria Chart 64. The galaxy sits on one edge of an equilateral triangle of stars and its brightness profile could be seen to increase considerably towards the centre; it’s classed as a giant elliptical, distance 270 million light years, about an arcminute across on photographs. Its reported magnitude is a healthy 12.2, so an 8-inch should manage it, though surprisingly it’s not listed in Luginbuhl and Skiff’s Observing Handbook.

Having easily found NGC 7550 I then began the task of picking up the other group members. Even a red light interferes with dark adaptation so I had to wait for my eyes to re-adjust properly before I could begin to see faint stars popping into view in my 250x eyepiece. Two more galaxies emerged, and once I knew where they were I could hold them steady. From the sketch I made, I was able to identify these as NGC 7547 and -49. I found them roughly equal in visibility, both featureless, though 7549 was noticeably larger and elongated. Photographs show it to be a mag 13.0 barred spiral with unusually extended arms; I was probably just seeing the bar. NGC 7547 is listed as mag 13.7. Arp lists only these three galaxies in his catalogue of anomalous groupings.

Hickson, however, lists five, and the remaining two proved a real challenge. One possible candidate turned out on closer inspection to be a pair of faint stars, but eventually I did mark down two objects which I considered non-stellar, one definite and the other tentative. My tentative one matches the position of NGC 7558, a tiny, almost stellar galaxy of listed magnitude 14.9, size 0.4 arcminute, and distance 467 million light years. More definite was the object that I guessed must be NGC7553, the only other group member plotted in Uranometria; so I had my five, and I moved on to try for Abell 2572, a cluster on whose edge the Hickson group lies – just as clouds rolled in to spoil the fun.

Back home, I looked up images of Hickson 93 on the internet – and learned that I had come upon an Argleton. NGC 7553 was recorded by astronomers using Lord Rosse’s 72-inch reflector, who in 1877 made a sketch showing four galaxies in their correct positions, but with NGC 7553 in a place where no galaxy is situated. There is however a mag 14.7 galaxy in the vicinity, now catalogued as CGCG 454-015, and I believe this is what I saw. Or else I made the same mistake as the Rosse observers and mis-recorded a faint double star.

Either way, the spurious NGC7553 is still plotted on Uranometria. I spent a lot of time hunting for it – and learned a little piece of history. Those folks at Rosse’s observatory were doing exactly what I was doing, stretching their equipment and observing skills to the limit, even making the same sort of errors, because we’re all human, not silicon. That’s the joy and wonder of visual astronomy.

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