i don't quite get ann's comment on star formation, since the dust lane is so broad and dense. dust is just as much part of star formation as elemental hydrogen and helium, and the dust lane clearly obscures everything that is going on inside the galactic disk, which is evidently highly disordered. it might be useful to review other galaxy photographs, tilted just enough to show internal structure, where the relationship between circumferential dust and interior star forming regions can be generalized. and mergers may have disrupted the spiral shock waves enough so that the process of sweeping up gas into clouds is temporarily inefficient.
I think astronomers have a pretty good idea of when star formation can happen and when it can't, but I actually believe that a few mysteries remain for them, too.
So I'm going to post a few more-or-less edge-on galaxies here as links. Some of them show a lot of star formation, while others seem to be completely devoid of if.
Iconic edge-on galaxy M82
has a violent ongoing starburst near its nucleus, but no obvious star formation regions are visible in its disk.
Nearby edge-on galaxy NGC 253
is also undergoing a central starburst. The pink star formation regions in its disk are relatively few and small, but they can definitely be seen in a good RGB image, like this one by R. Jay Gabany.
Some edge-on galaxies with a dust lane appear to be totally devoid of star formation. This RGB Hubble image of the Sombrero Galaxy, M104
, shows no sign of either pink emission nebulae or young blue clusters.
Another iconic edge-on galaxy that seems to be incredibly poor in young stars is NGC 3190
. However, this Hubble image, processed by Robert Gendler, does show, at highest magnification
, small clusters and associations of blue stars. Star-like objects of other colors are also visible, and it is possible that we are seeing individual red supergiants. In the upper right part of the dust lane, we are seeing what just might be a dust-enshrouded region of ongoing star formation. However, it is also possible that we are seeing a young-to-middle-aged bluish cluster which is deeply buried in the dust and therefore strongly reddened. No pink emission nebulae are visible. A possible reason for that may be that the Hubble image is perhaps not an RGB image, but perhaps an IGB (infrared-green-blue) image instead. These filters will not be good at detecting emission nebulae, although the green light of the HB emission should be visible.
You may also want to check out this image of the NGC 3190 Group
by Adam Block. At top, you can see elliptical galaxy NGC 3193. NGC 3190 is below it, and NGC 3187 is to the right of NGC 3190. Note the blue color of NGC 3187 and the many pink star formation regions in the dust lane. Near the bottom of the image you can find NGC 3185. This galaxy has a dusty bluish ring surrounding an inner yellow disk or bulge. It is not obvious, however, that there are any pink emission nebulae in the dusty ring.
Edge-on galaxy M98
is not very rich in star formation. However, this rather old image (from January 2004) by Adam Block, taken with equipment that is not top class by today's standards, nevertheless shows an abundance of small pink emission nebulae in the galaxy's disk and in its dust lane.
I couldn't find a good image of M108
. This was the best image I found. It is pretty obvious that there are many pink emission nebulae here. Interestingly, the same photographer (Jon K. Olson) has taken an image of the Leo Triplet
, of which NGC 3628, the galaxy portrayed in today's APOD, is a member. You can see from this image that no pink spots or young blue clusters are visible. The pink nebulae are definitely there, because today's APOD doesn't lie, but they are very faint.
One edge-on galaxy which hides its star formation regions pretty well is NGC 4565
. This image by Bob Franke, which was the APOD of October 15, 2011, is very beautiful. My impression is that the image, like today's APOD, "enhances the blue signal" in order to bring out the younger population in the galaxy's disk. It is very hard to detect any pink emission nebulae in the disk or the dust lane, however.
One of the most perfectly edge-on galaxies that I know of is NGC 891
. This RGB image by R. Jay GaBany shows how the dust lane of NGC 891 is full of pink nebulae. Note the "dust chimneys", columns of dust rising vertically from the dust lane. They may be formed by exploding supernovae.
Finally, let's look at the "mother of all starforming edge-on galaxies", NGC 4631
. This image by R. Jay GaBany was an APOD. But no matter how you look at this galaxy, its star formation is overwhelming. Look at this
. Or this
. This is a combined Hubble and Chandra image of NGC 4631. What you can see in red is the churning, star forming disk of NGC 4631, and the blue "cloud" surrounding the galaxy is an X-ray halo produced by large numbers of hot young stars and supernovae in this galaxy.
So my point remains: The star formation rates of different galaxies vary. NGC 3628 is probably poor in star formation.