Hoag's object

[Martin Meredith]

This is something I spotted on the Deep Sky Forum 'Object of the Week' series for 2013 (pp 92-94; the doc can be found towards the base of this page) and decided to have a go at last week.

Hoag's object (PGC 54559) in Serpens looks like a planetary nebula with a very bright central star and a dim ring, but is thought to be a mag 15.1(v) ring galaxy at a distance of 619 MLY. Discovered by Hoag in 1950, an initial hypothesis was that the ring was a result of gravitational lensing, but this explanation is now out of favour; instead, it is thought to be the result of a merger. A recent study (Brosch et al., 2013) suggests that any such event must have happened at least 1-2 billion years ago.

Here's the SDSS image

This object can be found at 15h 17.2', +21deg 35', not far from Hickson compact group 75 (if you're in the area, Hickson 72, 73 and 74 are not far away). 

Here's my attempt using 30s subs. The ring is exceedingly dim (I'm afraid this is another one for a darkened room). I could begin to detect its presence after a stack of 4 x 30s shots, with a gradual improvement after that. You'll see from the sequence below (the number of subs are shown) that I wasn't really able to set the background to a reasonable dark level without losing the ring until I reached the 13th sub.

Here's the full image for the 13 x 30 case. There are a couple of tiny edge-on galaxies (mag 17.9 and 18.0) WSW of centre too. As pointed out by one of the visual observers in the OOTW guide, the 'fish hook' asterism to the right of the galaxy can help in finding this object.

BTW This was before I started experimenting with fast stacking. This would make a great test object for that approach.

==

As an aside, we sometimes speculate on how big an increase in equivalent aperture we get from using a camera. The question isn't really well-posed, as variables such as exposure time are not specified. But is is interesting from a purely practical perspective to compare these captures with the observations and sketches in the document I link to above. For instance, there are several reports of not being able to see the ring (as opposed to a diffuse glow) with a 28" under SQM 21+ skies (with one positive report at this aperture under 21.63 skies). Yet with my 8" under SQM 18 skies and a 70% moon the ring is apparent with a clear separation from the central nucleus. I say this simply to point out that electronically-assisted viewing really is a wonderful tool to see very challenging, unusual and astrophysically interesting objects in modest scopes under far from ideal conditions on practically any clear night of the year.

Thanks for looking

Martin

[laudropb]

Thanks for sharing such an unusual object

[shirva]

great stuff martin

[michael.h.f.wilkinson]

Very interesting. Unusual object indeed

[RobertI]

Thanks Martin, another fascinating and inspiring post.

[Paul81]

Very interesting object - from the picture you would have put a bet on it being a planetary nebula!

[arkosg]

Nicely done Martin!

- Greg A

[Martin Meredith]

Thanks everyone! I love the fact that there are so many unusual objects within reach these days. Talking of planetaries with rings, here's a real one captured the same night in Ophiuchus (NGC 6369; mag 11.5) which I hadn't seen before. Looks like a smaller version of M57. Central star possibly visible -- not yet sure of magnitude nor if it is the cause of the nebula.

Martin

[nytecam]

Nice one Martin and the ring, even on Sloan DSS [=Wikisky] is faint so very well done      You may like to try some heavy processing [ignore the purists !] on this one to bring out the ring better eg from the image create a negative and positive image and subtract one from the other either way.   Results are  a bit odd but for me the out halo around M57 just begins to show in 5m exp and IC 1296 is wow!    

I need a challenge and will give PGC 54559 a go eg east of Arcturus - in exchange you may like to try NGC 5906 on Draco/Bootes borders that I spied on an old e-finder shot.   It's bright, long and very thin gx.