Quote:
Originally Posted by Paddy
NGC 2442 GX in Volans
175x 7’ wide p-f bar with bright nucleus. A 4’ arm sweeps from p end and heads Sf. 3’ arm from f end heads Np and is slightly fainter
10’ f is the 2’x1’ p-f glow of ES9-11 (GX) and 20’ N is the 2’ distinct round glow of GX NGC 2434
|
Hi Paddy,
You were doing well to see the fainter of the two spiral arms as clearly as you did see it, because it is structured very differently from the brighter of the two arms;
the fainter spiral arm can look more like a short stub, at times, and it is "smeared and broadened" in its appearance;
I recall a paper by Mihos & Bothun which explains the unusual arm structure and the unusual bar-like structure in this galaxy, by reference to a model in which NGC 2442 encounters a smaller galaxy.
(the longer and brighter of the two spiral arms is well-behaved, pointy, and quite narrow)
You say in your observing notes that one of the spiral arms is
slightly fainter than the other one. This is arguably true, though I myself reported a substantially greater difference between the brightness of the two arms, in my own visual observations. In general, my own observing notes (from several visual observing sessions) tend to support a much bigger difference between
the length of the two spiral arms than you report in your own observations.
There is indeed a "bar-like" main body in this galaxy, but if you compare this bar structure to what is found in virtually every other barred spiral galaxy, the bar structure is not at all what you typically find in the majority of other barred-spiral galaxies.
The apparent (two-dimensional)(as viewed on the image plane) morphology of N2442 is in fact very interesting and noticeably unusual, in terms of the unusual bar structure and the unusual arm structure.......so it comes as no surprise that I have been puzzling about this galaxy for many years!
I am interested in your observations of the "nucleus" or "centralmost compact brightening" in this galaxy; because these high-surface-brightness regions at the very centres of galaxies can actually be easier to see in visual observations than in photographs.
Q. Paddy, how star-like is the "apparent nucleus" that is seen in your observations, or how diffuse is it? Is it really stellar in appearance, or does it have a certain extent?
I note that it is more accurate not to refer to the visually-observed central brightening in a barred-spiral galaxy as being an actual "nucleus", because this term is reserved in professional astronomy for the tiny & virtually stellar "point-like source" of light that is often seen at the very centre of many galaxies.
What you are seeing as a "nucleus" in NGC 2442 could/might be some mixture of several different superposed three-dimensional structures:
- a central star-like source of light (e.g. this could be, for instance, a Nuclear Star Cluster plus a Seyfert Nucleus)
- a star-forming region (which is often called a Starburst Region) that surrounds the stellar-like nucleus. This star-forming region may have a disky or flat shape (when it is seen in three-dimensional space).
- a small but significantly-extended Bulge or Spheroidal structure (with mainly old stars, and shaped like a prolate or oblate spheroid)
[[ These three central Morphologically & Kinematically distinct Stellar Population components are commonly found at the centers of barred spiral galaxies; though not every individual barred-spiral galaxy contains all three of these components.]]
Best Regards,
Robert
Oh, and one more strange thing about this galaxy.....
in NGC 2442, the plane (or planes) in which the spiral arms exist, arguably has a different orientation in 3-D space to the plane in which some of the more inner structures exist. This galaxy is one of the many "exceptions" in the bright galaxy population, where a galaxy is not occupying a single plane in 3-dimensional space!
P.S.
Some of the above information is derived from my own analysis of the two-dimensional morphology of NGC 2442 as imaged at various wavelengths. I have yet to read this recent paper analyzing the structure of this galaxy:
http://iopscience.iop.org/0004-637X/723/1/530