by Ann » Sun May 01, 2011 6:32 am
I love Adam Block's image of Arp 178. What a quartet, no, what a quintet of galaxies and what a fantastic "centerpiece"!
The large galaxy, NGC 5614, is incredible. Note how many different populations are visible in it! The bulge is strongly yellow. It reminds me of those big elliptical galaxies that were recently found to contain tremendous numbers of small red dwarfs. That's the impression I get when I look at the bulge of NGC 5614, that it is jam-packed with small M-type dwarfs. But I also think that the bulge of this galaxy is full of stars with a really high metallicity, which is also typical of large elliptical galaxies (or so I think anyway).
Then there is a fascinating blue ring surrounding the very yellow nucleus. But I don't think that the blue ring is active or that it contains any recent star formation. Look how smooth the blue ring is, not at all clumpy. Ongoing active star formation typically looks very clumpy. Besides, in James D Wray's Color Atlas of Galaxies, which shows galaxies in UBV, the ring surrounding the bulge of NGC 5614 is all green, even yellow-green, and contains no hints of blue at all. That suggests that the ring of NGC 5614 contains many bluish stars of spectral class A and F, but probably no stars of spectral class O and B. This in turn means that the ring is the fossil remnant of a bright burst of ring-shaped star formation in the past. Note that we can even see that the lower part of the ring appears to be slightly younger than the upper part of it!
Outside the blue ring is a dusty disk, strongly dominated by the brown color of dust and mostly, but not completely, devoid of star formation. Note the pink emission nebulae in the part of the disk that is "above" the bulge.
Outside the dusty disk is a mostly dust-free, smooth population, yellow close to the dusty disk and slightly bluer further out. Typically, stars outside the bright disk of a galaxy are often relatively young.
Now take a look at the squarish yellow shape with a bright band through it at 5 o'clock. What is that thing? Why, it's a small galaxy, and the bright band must be a bar! The galaxy is NGC 5615, and it reminds me of M32, the closest of the satellite galaxies of Andromeda, in that NGC 5615 is also very bright, very small, very yellow and very close to the large galaxy that it is orbiting. The main difference is that NGC 5615 is not as compact as M32 but more extended because of its bar. NGC 5615 is only three magnitudes fainter than NGC 5614, but M32 is almost five magnitudes fainter than M31.
Note how a fantastic blue "plume" of stars appears to emerge from NGC 5615. The plume is bent in a general "direction of rotation" around NGC 5614, except I'm not sure I managed to express that quite correctly. But with that fantastic long bent "tail" behind it, generally pointing away from NGC 5614, the entire small galaxy of NGC 5615 reminds me of a comet about to plunge into the Sun - and the "Sun" in this case would be galaxy NGC 5614, of course!
There are three other galaxies in the picture. NGC 5613 is the galaxy to the lower left of NGC 5614. NGC 5613 has an oval bulge which is yellow but not not as orange-yellow as the bulge of NGC 5614. Smaller galaxies are typically more metal-poor than large galaxies, and their yellow old population is not as strongly yellow in color as the old population of large galaxies. NGC 5613 also has a ring structure around its bulge, but this ring is more neutral-colored than the ring of NGC 5614. This is an older structure. The ring is brighter to the right and to the left of the bulge than it is above and below the bulge. When a ring looks like this, there is usually a bar across the bulge that ends at the bright parts of the ring, which are called bar-end enhancements. But in this case the bright parts of the ring are located perpendicularly to the oval, bar-like bulge. NGC 5613 "all but ends" outside the bright ring, but there are two large, extended, wide-ranging, rather faint blue spiral arms outside it. These arms may originate at the two brightest points of the ring.
In the lower left of the picture is another spiral galaxy, which may well be part of the NGV 5613-5614-5615 action. But my software has nothing to say about this galaxy more than that it is designated NGC 5609, that it is a spiral galaxy and that it is "most extremely faint". So I won't say more about it, either. In the far right of the picture is a fifth galaxy, which looks like a dwarf spheroidal to me, but my software can't even find it!
What an extremely fascinating image, Adam!
Ann
I love Adam Block's image of Arp 178. What a quartet, no, what a quintet of galaxies and what a fantastic "centerpiece"!
The large galaxy, NGC 5614, is incredible. Note how many different populations are visible in it! The bulge is strongly yellow. It reminds me of those big elliptical galaxies that were recently found to contain tremendous numbers of small red dwarfs. That's the impression I get when I look at the bulge of NGC 5614, that it is jam-packed with small M-type dwarfs. But I also think that the bulge of this galaxy is full of stars with a really high metallicity, which is also typical of large elliptical galaxies (or so I think anyway).
Then there is a fascinating blue ring surrounding the very yellow nucleus. But I don't think that the blue ring is active or that it contains any recent star formation. Look how smooth the blue ring is, not at all clumpy. Ongoing active star formation typically looks very clumpy. Besides, in James D Wray's Color Atlas of Galaxies, which shows galaxies in UBV, the ring surrounding the bulge of NGC 5614 is all green, even yellow-green, and contains no hints of blue at all. That suggests that the ring of NGC 5614 contains many bluish stars of spectral class A and F, but probably no stars of spectral class O and B. This in turn means that the ring is the fossil remnant of a bright burst of ring-shaped star formation in the past. Note that we can even see that the lower part of the ring appears to be slightly younger than the upper part of it!
Outside the blue ring is a dusty disk, strongly dominated by the brown color of dust and mostly, but not completely, devoid of star formation. Note the pink emission nebulae in the part of the disk that is "above" the bulge.
Outside the dusty disk is a mostly dust-free, smooth population, yellow close to the dusty disk and slightly bluer further out. Typically, stars outside the bright disk of a galaxy are often relatively young.
Now take a look at the squarish yellow shape with a bright band through it at 5 o'clock. What is that thing? Why, it's a small galaxy, and the bright band must be a bar! The galaxy is NGC 5615, and it reminds me of M32, the closest of the satellite galaxies of Andromeda, in that NGC 5615 is also very bright, very small, very yellow and very close to the large galaxy that it is orbiting. The main difference is that NGC 5615 is not as compact as M32 but more extended because of its bar. NGC 5615 is only three magnitudes fainter than NGC 5614, but M32 is almost five magnitudes fainter than M31.
Note how a fantastic blue "plume" of stars appears to emerge from NGC 5615. The plume is bent in a general "direction of rotation" around NGC 5614, except I'm not sure I managed to express that quite correctly. But with that fantastic long bent "tail" behind it, generally pointing away from NGC 5614, the entire small galaxy of NGC 5615 reminds me of a comet about to plunge into the Sun - and the "Sun" in this case would be galaxy NGC 5614, of course!
There are three other galaxies in the picture. NGC 5613 is the galaxy to the lower left of NGC 5614. NGC 5613 has an oval bulge which is yellow but not not as orange-yellow as the bulge of NGC 5614. Smaller galaxies are typically more metal-poor than large galaxies, and their yellow old population is not as strongly yellow in color as the old population of large galaxies. NGC 5613 also has a ring structure around its bulge, but this ring is more neutral-colored than the ring of NGC 5614. This is an older structure. The ring is brighter to the right and to the left of the bulge than it is above and below the bulge. When a ring looks like this, there is usually a bar across the bulge that ends at the bright parts of the ring, which are called bar-end enhancements. But in this case the bright parts of the ring are located perpendicularly to the oval, bar-like bulge. NGC 5613 "all but ends" outside the bright ring, but there are two large, extended, wide-ranging, rather faint blue spiral arms outside it. These arms may originate at the two brightest points of the ring.
In the lower left of the picture is another spiral galaxy, which may well be part of the NGV 5613-5614-5615 action. But my software has nothing to say about this galaxy more than that it is designated NGC 5609, that it is a spiral galaxy and that it is "most extremely faint". So I won't say more about it, either. In the far right of the picture is a fifth galaxy, which looks like a dwarf spheroidal to me, but my software can't even find it!
What an extremely fascinating image, Adam!
Ann