--- Joe Bauman <bau@desnews.com> wrote: <snip>

I am wondering if it [a cluster] is interacting with another galaxy to cause the arms to be in such a peculiar orientation. One almost seems to double back the wrong way. Or maybe that's part of the galaxy it's interacting with. I'd like to know more about this one.

I'm not sure about the cluster you mentioned, but here's some info on NGC6946. NGC6946 (Caldwell 12, Arp 29, Herschel400 #76-4, PGC065001) is classified by Arp as an interacting galaxy "with one heavy-arm." I suspect Arp's description is based on the heavy-arm being the "second galaxy." The heavy-arm is apparent in your photo. Here's Arp's photo: http://nedwww.ipac.caltech.edu/level5/Arp/Figures/big_arp29.gif in http://nedwww.ipac.caltech.edu/level5/Arp/frames.html As you mentioned, what is unique about NGC6946 - which is oriented at about a 42 deg tilt in our view - is the galaxy is in an extreme state of starburst. Going to Simbad ( http://simbad.harvard.edu/sim-fid.pl ) and entering NGC6946 use the Aladin Java Applet viewer to see all labelled objects in this galaxy. It's full of bright clusters indicative of starburst. NGC6946 hoasts one of the largest number of known supernova for any one galaxy.

From Sternberg's 2004 catalogue of extragalactic nebula at -

http://cdsweb.u-strasbg.fr/viz-bin/VizieR?-source=II/256 - here's a list of the known supernova for NGC6946. SN# Mag Type 1917A < 14.60 II 1939C 13.20 I 1948B 14.70 IIP 1968D < 13.50 II 1969P < 13.90 1980K 11.60 IIL 1992U < 18.50 II 2002hh 15.0 II 2004et 12.2 > II Pictures of some of these SNs in NGC6946 can be found at the Supernova network archives: http://www.rochesterastronomy.org/sn2004/sn2004et.html http://www.rochesterastronomy.org/sn2002/sn2002hh.html Starburst is characteristic of an interacting galaxy and of galaxies with active galactic nuclei. E.g. The Antennae Galaxies (NGC4038-NGC4039). http://hubblesite.org/gallery/album/galaxy_collection/pr1997034d/ You can see the starburst in NGC6946 in your image via the very bright, condensed core, and the unusual number of high luminosity HII nebula regions. As O'Meara notes in his Caldwell List 12 entry, the galaxy has a galactic longitude of 11 degrees; hence the relatively large number of foreground stars seen in your image. You'd almost be tempted to think that the core of this Hubble Scd type spiral is a chance optical alignment with a foreground star, but that really is the core of the galaxy shining that bright. More detail in the bright core is seen in the Hubble image of NGC6946. http://hubblesite.org/gallery/album/entire_collection/pr1999010e/ NGC6946 is classified as a Hubble late-type Scd - a barred spiral with few defined arms. A characteristic of these types of galaxies is their distinct distributions of H2 molecularized gas and ionized HI gas. Normally earlier-type Hubble spiral galaxies have a distribution of molecular hydrogen gas (H2) and HI ionized hydrogren that has a central depression of low density nearer the galaxy's core. M31 is an example of a Hubble Sb earlier type galaxy with this "normal" distribution of H2 and HI gas, as is our own galaxy, the Milky Way. In NGC6946, H2 molecular gas is concentrated around the core with no central depression and does not extend beyond the visible arms - that are about 24 kpc in radius or 48 kpc in diameter. HI ionized hydrogren extends out to a radius of 30 kpc and has only a slight reduction in density near the core. NGC6946 is able to maintain this halo distribution of ionized hydrogren because it is in a relatively less dense region of galaxies. The galaxy's production of HI can accure without being stripped off by interaction with neighboring galaxies. In contrast, in galaxies near the core of the Virgo cluster, few galaxies have extensive HI ionized hydrogren gas clouds surrounding them. This is because near the "Great Attractor", the galaxies are packed so close together that they strip such gas off. That stripping process of HI gas be seen in our own Milky Way with respect to the Large and Small Magellanic Clouds. Both clouds have faint streamers of HI one gas (and stars) being stripped off them by their interaction with the Milky Way. In contrast, NGC6946 has an extensive HI halo. The distribution of HI gas in this galaxy is significant in studying dark matter. Estimating the dark matter mass of galaxies generally is based on the rotation curves of HI gas. Knowing how the distribution of such gases varies by galaxy type is important to determining the ratio of "warm" dark matter in the universe. Another odd feature of NGC6946, that is probably related to its starburst state, is its non- optical magnetic field - also discussed by O'Meara. In a spiral galaxy like our own, the lines of the galaxy's magnetic field are perpendicular to gravity density waves that form the arms. The arms are associated with the bright matter of the arms. Where's there more matter, galactic magnetism is higher. In NGC6946, this situation is reversed. The regions of high galactic magnetism are out-of-phase with the optical arms. Less bright matter, less magnetism; more dark matter, a stronger magnetic field. Now that's pretty weird. NGC6946 is an exception to general models of how galaxies form magnetic dynamos. Because there is so much going on in NGC6946, because it's close and favorably oriented, there have about 200 published articles after 1980 listed in NASA/ADS. http://adsabs.harvard.edu/abstract_service.html Since the starburst phase of a galaxy is relatively short-lived, we are fortunate to be positioned to watch a relatively close one at a favorable "face-on" angle. - Canopus56 Refs: Binney. 1998. Galactic Astronomy. p. 498-500 O'Meara. 2002. The Caldwell Objects. (Caldwell List 12) Beck, R. 1991A&A...251...15B __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com