Axel wrote:The red spiral arms (said to contain stellar nurseries) seem to be evenly spaced. There are three and they seem roughly 1/3rd of a circle apart. Is this so?
I meant three main red arms. I know it's not a perfectly neat picture, but the trend seems clear enough. And besides whether it's true for M106, I wonder if this is often observed.
I'm not exactly sure what you mean by three red arms.
Please note that the red color of hydrogen alpha emission has been enhanced in today's beautiful image of M106. 900 minutes of H-alpha exposure went into the making of the image, whereas the normal Red, Green and Blue exposures only got 740 minutes put together
. Therefore the H-alpha emitting red nebulae look very bright in this image, and they seem to dominate much of the inner part of the galaxy. If there had been no H-alpha exposure at all, the arms would only have displayed small patches of red nebulae, and the hydrogen alpha jets emanating from the center of the galaxy would have been barely visible.
What this means is that M106 doesn't have three red arms. Personally I would describe it as a galaxy with two starforming rings, an inner, incomplete ring of star formation and an outer, more complete ring. The galaxy also has two jets of ionized red hydrogen gas emanating from its center. These jets don't contain any star formation.
I would compare M106 with M82, which is violently emitting ionized red hydrogen gas from its center:
M82 is expelling much more hydrogen from its center than M106 is doing, so the galaxies are not fully comparable. Also the causes of the outflows are probably different, since the jets of M106 appear to be driven by the galaxy's central black hole, whereas in M82's case the outflows appear to be caused by a central starburst. In both cases, however, the red gas streaming out of the galactic centers doesn't contain any star formation.
As for galaxies having three starforming arms, the only example I can think of off hand is Messier 99. Here is an image which was made without any special H-alpha exposure, and the emission nebulae look quite pale: http://www.robgendlerastropics.com/M99JM.jpg
Please note, however, that M99 contains much more star formation and many more and larger emission nebulae than M106.