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Dan Good is doing alot of damage at CAV! His M63 image particularly intrigues me! I have been curious about that “appendage” of stars sitting out separate from the main galaxy for years. Your image rekindled my interest and I did some research. That appendage is labelled “PGC 46093” in SkyTools 2 (see image above) and is cross-referenced as “UGCA 342” in NED (see image above). UGCA is the Uppsala General Catalog Appendix (1974). As you can see from the attached Sky Tools 2 chart, PGC 46093 is considered a separate galaxy from M63 at a red shift of 0.21 at a distance of 2.4 billion light years!!! The UGCA chart puts it at a very small red shift (essentially equal to M63) and, therefore, at about the same distance as M63 (about 23 million ly). NED calls it an “Im” galaxy (highly irregular) probably physically related to M63, like the LMC is related to the Milky Way.
I find this all interesting because I think a more plausible explanation (IMHO) for that structure they label PGC 46093 / UGCA 342 is really a portion of extincted spiral arm coming off of M63 rather than a separate galaxy, irregular or distant. You can trace an arm going all the way from M63 to the condensation. My image, I think, illustrates just how much dust (and extinction) there is over this galaxy probably hiding most of the spiral arms with only a few traces (like UGCA 342) shining through. No wonder the red shift is the same as M63; it is part of M63! Could this be an irregular galaxy off of M63 like our LMC? Yes – but why even postulate that when you can optically see extensions tracing out to it directly from M63 peeking through a thick dust lane?? Perhaps “flocculent” galaxies like M63 (also N3521, N4414) are really spirals with the arms heavily masked by dust rather than having no or truncated arms and are not truly a separate class of galaxies! See Tony Hallas’ M63 APOD showing hiden spiral structure (link below).
I guess it also becomes a matter of definition what an “irregular galaxy” constitutes. I was, for example, very surprised to read that some think Omega Centauri is actually a dwarf Galaxy (see link below)! So perhaps I need to expand my understanding of the different types of galaxies. I was shocked to read about Hobbit Galaxies and Green Pea Galaxies on Wikipedia! Below are some links about Green Pea Galaxies and the “Peas Corps” of non-scientist civilians evaluating Sloan Telescope (New Mexico) data for almost 10 years (see Galaxy Zoo pdf below)!
http://www.robgendlerastropics.com/M63text.html – M63 Gendler exposition
http://nedwww.ipac.caltech.edu/cgi-bin/nph-ex_refcode?refcode=1974UGCA..C…0000N —UGCA description
http://en.wikipedia.org/wiki/Galaxy_morphological_classification – Wikipedia on types of galaxies.
http://arxiv.org/PS_cache/arxiv/pdf/0907/0907.4155v1.pdf – Galaxy Zoo Green Peas pdf – check out page 7
http://en.wikipedia.org/wiki/Omega_Centauri – Omega Centauri a dwarf galaxy???? – scroll down and read!
http://apod.nasa.gov/apod/ap080417.html – Tony’s very deep M63 APOD
http://www.galaxyzoo.org/ – AMAZING data reduction website