by Ann » Tue Dec 28, 2010 7:35 am
I'm horribly late, because commenting is such an uphill battle for me these days, but I have to say something about this galaxy pair, Adam.
First of all, the two galaxies look extremely interesting together. One is an edge-on disk galaxy with a prominent dust lane and some blue stars, and the other one is a more or less spherical but clearly distorted monocolored elliptical galaxy.
The disk galaxy, NGC 678, is not a Milky Way twin, however. Several clues tell us that this is an evolved galaxy with little star formation and a lot of old stars. The bulge is large and prominent. Young blue stars are seen only at the outermost edges of the disk. It is likely that bluish stars are found all around the outermost visible edge of the disk of of NGC 678, but this blue "circle" of stars is so faint that it can only be seen at the far edges of the disk, where the blue circle is seen "superimposed" on itself and not overwhelmed by a bright yellow disk behind it.
The blue "outermost circle" is generally rather smooth and not clumpy, which is the opposite of what we would expect if there was a lot of star formation going on, since ongoing star formation always looks like a peppering of small bright knots:
Well-known starforming galaxy liberally sprinkled with bright bluish knots on the right, yellow elliptical galaxy with no bright knots at all on the left. Obviously NGC 678 is nothing like famous M 51. The blue areas of NGC 678 probably contain moderate class A stars like Vega and Sirius rather than massive O and B stars like Rigel and the stars of Orion's Belt.
The dust lane which crosses the bulge of NGC 678 contains a bright "ribbon" on the left side. My guess is that this is the remnant of a past burst of star formation. Once this whitish ribbon was blazing with hot blue supergiants and a multitude of pink emission nebulae. Now the nebulae are gone and the blue stars have all burnt out and disappeared. It is quite possible that the whitish ribbon in the dust lane contains no hotter stars than those of spectral class F, like Procyon, and G, like the Sun. On the right side of the bulge, a broad faint ever so slightly bluish structure seems to emerge from the dust lane and disappear behind the bulge. That is probably the middle-aged F and G stars of the dust lane whose color is bluer than the redder K and M stars of the bulge and inner disk.
Anyway, like I said, the overall structure and color distribution of NGC 678 suggests that it is a galaxy dominated by yellow and reddish stars. And indeed that is the case. The B-V index of NGC 678 is 1.05, which is actually quite remarkably red for a galaxy. Its U-B index is 0.42, which is also red.
I don't know anything about the color indexes of NGC 680, but it is hugely interesting to note that the overall color of this galaxy is definitely whiter and far less yellow than the overall color of NGC 680. Why is that? Aren't elliptical galaxies yellower than disk galaxies? Well, not always. NGC 678 obviously contains a lot more dust than NGC 680, and the color of NGC 678 is clearly affected by dust reddening. But it is interesting to consider the possibility that NGC 678, for some reason, contains redder stars than NGC 680. But if that is the case, I have no idea why that would be so.
Finally, we may note that NGC 680 is clearly distorted, and this distortion must certainly be caused by interaction with NGC 678.
What an interesting pair, Adam! Thank you for photographing them!
Ann
[quote="ngc1535"][size=120][b]NGC 678 (and NGC 680)[/b][/size]
http://www.caelumobservatory.com/gallery/n678.shtml
Adam Block/Mount Lemmon SkyCenter/University of Arizona
[img2]http://www.caelumobservatory.com/mlsc/n678s.jpg[/img2][/quote]
I'm horribly late, because commenting is such an uphill battle for me these days, but I have to say something about this galaxy pair, Adam.
First of all, the two galaxies look extremely interesting together. One is an edge-on disk galaxy with a prominent dust lane and some blue stars, and the other one is a more or less spherical but clearly distorted monocolored elliptical galaxy.
The disk galaxy, NGC 678, is not a Milky Way twin, however. Several clues tell us that this is an evolved galaxy with little star formation and a lot of old stars. The bulge is large and prominent. Young blue stars are seen only at the outermost edges of the disk. It is likely that bluish stars are found all around the outermost visible edge of the disk of of NGC 678, but this blue "circle" of stars is so faint that it can only be seen at the far edges of the disk, where the blue circle is seen "superimposed" on itself and not overwhelmed by a bright yellow disk behind it.
The blue "outermost circle" is generally rather smooth and not clumpy, which is the opposite of what we would expect if there was a lot of star formation going on, since ongoing star formation always looks like a peppering of small bright knots:
[img]http://s3.amazonaws.com/readers/2009/07/09/m51-whirpool-galaxy_1.jpg[/img]
Well-known starforming galaxy liberally sprinkled with bright bluish knots on the right, yellow elliptical galaxy with no bright knots at all on the left. Obviously NGC 678 is nothing like famous M 51. The blue areas of NGC 678 probably contain moderate class A stars like Vega and Sirius rather than massive O and B stars like Rigel and the stars of Orion's Belt.
The dust lane which crosses the bulge of NGC 678 contains a bright "ribbon" on the left side. My guess is that this is the remnant of a past burst of star formation. Once this whitish ribbon was blazing with hot blue supergiants and a multitude of pink emission nebulae. Now the nebulae are gone and the blue stars have all burnt out and disappeared. It is quite possible that the whitish ribbon in the dust lane contains no hotter stars than those of spectral class F, like Procyon, and G, like the Sun. On the right side of the bulge, a broad faint ever so slightly bluish structure seems to emerge from the dust lane and disappear behind the bulge. That is probably the middle-aged F and G stars of the dust lane whose color is bluer than the redder K and M stars of the bulge and inner disk.
Anyway, like I said, the overall structure and color distribution of NGC 678 suggests that it is a galaxy dominated by yellow and reddish stars. And indeed that is the case. The B-V index of NGC 678 is 1.05, which is actually quite remarkably red for a galaxy. Its U-B index is 0.42, which is also red.
I don't know anything about the color indexes of NGC 680, but it is hugely interesting to note that the overall color of this galaxy is definitely whiter and far less yellow than the overall color of NGC 680. Why is that? Aren't elliptical galaxies yellower than disk galaxies? Well, not always. NGC 678 obviously contains a lot more dust than NGC 680, and the color of NGC 678 is clearly affected by dust reddening. But it is interesting to consider the possibility that NGC 678, for some reason, contains redder stars than NGC 680. But if that is the case, I have no idea why that would be so.
Finally, we may note that NGC 680 is clearly distorted, and this distortion must certainly be caused by interaction with NGC 678.
What an interesting pair, Adam! Thank you for photographing them!
Ann