There's an article in the June 1 2000 issue of Astrophysical Journal (Vol
535: 748-758) titled "A Young Globular Cluster in the Galaxy NGC 6946" --
http://www.journals.uchicago.edu/ApJ/journal/issues/ApJ/v535n2/51146/51146.html.
The introduction states "during a recent search for massive young clusters in
the nearby spiral galaxy NGC 6946, Larsen & Richtler (1999) found a circular
bubble containing numerous small clusters and a bright compact cluster that
resembled a young globular cluster. Some of the small clusters in the bubble
are organized into arclike shapes." I'm assuming this is the same object you
observed?
According to the article the "bubble" is located at the end of a short spiral
arm, downstream from a dust lane, with a circular bubble outlined by an arc of
clusters in the west and a dark region, suggesting dust, just to the right.
There's a nice color image of the galaxy and bubble in the article, so I
looked up my observing notes for N6946 --
17.5": bright, very large, 6' diameter to main body, elongated 3:2 ~E-W.
Three arms are visible. A long bright arm is attached at the north side of
the core and trails to the east. This eastern arm splits; a short fainter
branch bends south following the core and a long curving bright arm
terminates with a very faint, very small HII knot. On the west side a
fainter arm shoots sharply to the north from the core. These outer arms
significantly increase the diameter of the main body. The galaxy has a very
large brighter middle but the core is just a very small brighter region close
SW of the geometric center. A very faint stellar nucleus was seen with
direct vision. Observation from 12,000' in the White Mountains.
Based on my notes and the image, the "very small HII knot" probably refers to
the "bubble", although the globular itself is embedded in the feature. Were
you able to resolve both features?
Steve
In a message dated 9/29/03, David Kingsley writes:
> We first started talking about 6946 at dinner Saturday night when Jim
>mentioned that the new issue of Scientific American has a good article on
>globular cluster formation, including the fact that some globular clusters
>are much newer than the ancient clusters of the Milky Way. I haven't
>seen the article, but had learned the same thing over the last year or so
>of the galactic and extragalactic globular cluster observing project I have
>been doing. I had observed the 6946 cluster as part of my extragalactic
>observing project last year at CalStar, and remembered the object as an
>observable example of a young cluster. Fortunately it is well illustrated
>in the image of 6946 found in that Concise Catalog of Deep Sky Objects from
>Springer that we were looking at with you over dinner. Jim and I both
>tracked it down again this year using the diagram in that book, which
>specifically highlights the region as a young globular cluster.
> For a primary literature reference, see:
> Larsen, S. S. , Brodie, J. P., Elmegreen, B. G., Efremov, Yu.
>N., Hodge, P. W. and Richtler, T. 2001: ``Structure and mass of a
>young globular cluster in NGC 6946'',
> ApJ 556 , 801-812 (astro-ph/0104133)
>
> The entire paper is available online at:
>http://adsabs.harvard.edu/bib_abs.html
> The brief synopsis is that this region appears to be an example of
>supermassive star cluster that has formed in the last 15 million years, and
>whose structure, properties and mass suggest that it will remain
>gravitationally bound as a globular cluster. A young globular to be sure,
>but that is part of why it is an interesting object.
> David Kingsley
>
>