With a new moon approaching and two weeks of clear skies forecast, I was scratching my head trying to find a broadband target to take advantage of the (relatively) dark skies when I stumbled across the Fireworks galaxy, and figured it would make a great subject with NGC 6939 in the same frame. With only 2-3 hours of dark per night, this took 6 nights to gather just over 13 hours and I kept the best 95% as conditions were pretty decent each night. I hadn't intended on gathering enough integration time to try and bring out the IFN, but when I saw it in the stack I knew I had to at least try. I hope I didn't overdo it!
NGC 6946, known as the
Fireworks Galaxy for its numerous observed supernovae (10 in the last 100 years, compared to the 1-2 on average for our own Milky Way galaxy) is about 25 million light years away. While many of the finer details of NGC 6946 aren't resolved at 504mm FL (given my seeing) you can clearly make out the blue circular bulge known as "Hodge's Complex", which is a fascinating object in its own right:
Initially thought to be a supergiant cluster, this area of NGC 6946 is a puzzler, not least because some of what we're seeing appears to be foreground gas from our own Milky Way galaxy (known as extraplanar diffuse ionized gas or eDIG), but it bears a striking resemblance to dwarf galaxy
NGC 1705. The morphology of Hodge's Complex and its similarity to NGC 1705 has led researchers into positing that it may be a dwarf galaxy superimposed and interacting with NGC 6946. Larger images show what appears to be an arc or bow shape formed on the edge of the object in the direction it is believed to be moving through NGC 6946, where it may be ramming into gas from the larger galaxy. Neat!
NGC 6939 is an open star cluster within the much less distant confines of the Milky Way galaxy, and close to the galactic plane. It is estimated to be approximately 1.6 billion years old. It has a number of fanciful names including the Ghost Bush Cluster, the Flying Geese Cluster, and the Silk Fan Cluster.
Integrated Flux Nebulae or
IFN are regions of dust which can be found in the outer regions of our galaxy, and are similar in concept to reflection nebulae in that they are illuminated by visible light rather than releasing light via ionization as in emission nebulae, but the light they reflect is that of the combined glow of the Milky Way galaxy. They're faint, I've not (to my knowledge) ever captured them before, and it was a real treat to allow them to shine, albeit dimly, in this image!
COLLABORATIVE REVISION:Jim Raskett's
excellent image of the same two subjects caught my attention, and he very kindly agreed to share his master stack with me, which I combined with mine and reprocessed. Jim's Explore Scientific ED102 has a slightly longer FL (please view Jim's
original image for full details of his equipment & data aquisition) but we use the same camera, and I drizzle my broadband images so matching his pixel scale was very manageable. Jim's star colour was a big improvement on the more yellow stars (even after SPCC) that my L-Pro captured, and our combined data revealed extra detail within NGC 6946. Many thanks to Jim!
RESOURCES:Efremov, Yu. N.. "Unusual Objects in the Spiral Galaxy NGC 6946"
Open Astronomy, vol. 25, no. 4, 2016, pp. 369-376.
https://doi.org/10.1515/astro-2017-0255Joanne M. Rosvick and David Balam. "CCD Observations of the Open Cluster NGC 6939"
The Astronomical Journal,124:2093–2099, 2002.
https://iopscience.iop.org/article/10.1086/342449/meta
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