Ann's galactic favorites, part 1 and 2: IC 5332 and NGC 4631
Posted: Sat Nov 06, 2010 6:14 am
I've been a bit unlucky with the APODs presented recently, or it could be that my tastes in astronomic imagery is different from most people's. 
So I thought I'd rectify the situation by posting pictures that I myself like to see and write captions that I find interesting.
I already posted a picture of IC 5332, but I didn't write anything about it. So I'll just post it here again and say a little something about it:
As you can see, this is an extremely delicate spiral, extremely regular. It is most regular near the center, but it breaks up into long "streamers" at the outer part of the visible disk.
The colors of this galaxy are extremely subtle. There is no bright yellow component here, which clearly suggests that there is no large population of old stars. Instead the color of the inner part of the galaxy is "intermediate", suggesting an intermediate population, probably dominated by stars between, perhaps, one and five billion years old. In the outer part of the disk blue patches of young stars, at most perhaps two hundred million years old, start appearing. The blue patches don't trace the spiral structure too well, but seem relatively randomly scattered.
We can summarize IC 5332 like this: It is a low-surface brightness galaxy, extremely regular, with very little star formation near the center but more further out. There have been few bursts of star formation here, particularly at the center, and the center has "settled down" into this beautiful regular spiral structure. We may assume that there are few or no nearby galaxies that can disturb the lovley pattern here.
Well, now let's look at NGC 4631, which is another kind of beast altogether:
Unlike IC 5332, we see NGC 4631 edge-on. Interaction with the dwarf galaxy NGC 4627 has triggered an enormous amount of activity in NGC 4631. The entire galaxy is sort of undulating, almost as if it was a horse trying to throw off an insolent person trying to ride on it. But NGC 4631 is usually compared with another animal species than a horse, as it is called "the Whale galaxy". You can see a large star formation region hanging down from the "body" of the galaxy like a "fin".
NGC 4631 is positively boiling and simmering with star formation almost all over. Its "body" is "bloated" and "puffed up" with bright young stars blowing hot air in all directions! All this activity means that NGC 4631 is wonderfully blue and pink: it is blue because of all new young stars, and it is pink because of all emission nebulae. But NGC 4631 has a bright yellow center, too. This galaxy has been around for quite a while. It has undergone many episodes of star formation. More and more old yellow stars have gathered at the center of it. This is no puny galaxy, no lightweight contender. And the galaxy doesn't just have old stars at its center, but old stars are scattered all over its disk, too. Many of these old stars now form a halo around the bright disk. This halo shows up quite well in the photograph above.
Okay, but now look at the dwarf companion galaxy, NGC 4627. This galaxy has no bright yellow component. It has no pink emission nebulae. It has no large irregular patches of young blue stars. Mostly the galaxy appears to be made up of two components: a halo probably made up of old stars, and a bright inner aqua-colored" bulge. To me the aqua color suggests that NGC 4627 underwent a tremendous burst of star formation perhaps a billion years ago. Perhaps two billion years ago. Since then, however, no new stars have formed in this galaxy. The violent stellar winds and supernovae explosions blew away the hydrogen gas, the star forming material, from this galaxy, because the galaxy's own gravity was not enough to hold on to it. Ah, but NGC 4631, the big Whale Galaxy, holds on to its gas and recycles it in tremendous bursts of star formation.
It is not just NGC 4627 that affects NGC 4631 and drives its star formation. Further away, but still nearby, is spiral galaxy NGC 4656. Its color properties are quite interesting compared with both NGC 4631 and NGC 4627:
Here you can see NGC 4631 and NGC 4627 at the lower right, and NGC 4656 at the upper left. The color balance of the picture is rather yellow, so the middle part of NGC 4631 looks very yellow and NGC 4627 looks all yellow. But you can see the bright pinkish-purple emission nebulae dotting the disk of NGC 4631.
Now look at NGC 4656. Its color properties are quite different. There is barely a hint of yellow at the center of it. One half of its disk is faint, relatively smooth and blue. The other half of its disk is curiously bent at the edge (NGC 4656 is called the Hockey Stick Galaxy) and it is patchy and blue, with perhaps a hint of purple in it. But there is nothing even resembling the audacious display of purple in NGC 4631.
This is my interpretation of NGC 4656. This galaxy has had very few episodes of star formation, perhaps no more than three. The first episode created the faint, faint old yellowish population in the center of the galaxy, a couple of billion years ago. The second episode created the mostly smooth and faint blue population of one half of the disk, perhaps, say, 500 million years ago. The third episode of star formation was a regular burst and happened a hundred or two hundred million years ago. The burst happened because of interaction with NGC 4631, and it was violent enough that it mostly depleted NGC 4656 of star forming material. (Admittedly I've also come across a hypothesis that the "blade" of the Hockey Stick galaxy may have been a separate galaxy which collided with the main body of NGC 4565 and triggered star formation, but I'll leave it to you to decide how likely that is.) Now the hottest and brightest stars of NGC 4656 have already gone out in blasts of glory, leaving the galaxy with no bright emission nebulae and few opportunities to create new brilliant stars of spectral classes O and B. NGC 4656 is, or so I think, a galaxy with very little remaining gas and a dark future!
Ann
So I thought I'd rectify the situation by posting pictures that I myself like to see and write captions that I find interesting.I already posted a picture of IC 5332, but I didn't write anything about it. So I'll just post it here again and say a little something about it:
As you can see, this is an extremely delicate spiral, extremely regular. It is most regular near the center, but it breaks up into long "streamers" at the outer part of the visible disk.
The colors of this galaxy are extremely subtle. There is no bright yellow component here, which clearly suggests that there is no large population of old stars. Instead the color of the inner part of the galaxy is "intermediate", suggesting an intermediate population, probably dominated by stars between, perhaps, one and five billion years old. In the outer part of the disk blue patches of young stars, at most perhaps two hundred million years old, start appearing. The blue patches don't trace the spiral structure too well, but seem relatively randomly scattered.
We can summarize IC 5332 like this: It is a low-surface brightness galaxy, extremely regular, with very little star formation near the center but more further out. There have been few bursts of star formation here, particularly at the center, and the center has "settled down" into this beautiful regular spiral structure. We may assume that there are few or no nearby galaxies that can disturb the lovley pattern here.
Well, now let's look at NGC 4631, which is another kind of beast altogether:
Unlike IC 5332, we see NGC 4631 edge-on. Interaction with the dwarf galaxy NGC 4627 has triggered an enormous amount of activity in NGC 4631. The entire galaxy is sort of undulating, almost as if it was a horse trying to throw off an insolent person trying to ride on it. But NGC 4631 is usually compared with another animal species than a horse, as it is called "the Whale galaxy". You can see a large star formation region hanging down from the "body" of the galaxy like a "fin".
NGC 4631 is positively boiling and simmering with star formation almost all over. Its "body" is "bloated" and "puffed up" with bright young stars blowing hot air in all directions! All this activity means that NGC 4631 is wonderfully blue and pink: it is blue because of all new young stars, and it is pink because of all emission nebulae. But NGC 4631 has a bright yellow center, too. This galaxy has been around for quite a while. It has undergone many episodes of star formation. More and more old yellow stars have gathered at the center of it. This is no puny galaxy, no lightweight contender. And the galaxy doesn't just have old stars at its center, but old stars are scattered all over its disk, too. Many of these old stars now form a halo around the bright disk. This halo shows up quite well in the photograph above.
Okay, but now look at the dwarf companion galaxy, NGC 4627. This galaxy has no bright yellow component. It has no pink emission nebulae. It has no large irregular patches of young blue stars. Mostly the galaxy appears to be made up of two components: a halo probably made up of old stars, and a bright inner aqua-colored" bulge. To me the aqua color suggests that NGC 4627 underwent a tremendous burst of star formation perhaps a billion years ago. Perhaps two billion years ago. Since then, however, no new stars have formed in this galaxy. The violent stellar winds and supernovae explosions blew away the hydrogen gas, the star forming material, from this galaxy, because the galaxy's own gravity was not enough to hold on to it. Ah, but NGC 4631, the big Whale Galaxy, holds on to its gas and recycles it in tremendous bursts of star formation.
It is not just NGC 4627 that affects NGC 4631 and drives its star formation. Further away, but still nearby, is spiral galaxy NGC 4656. Its color properties are quite interesting compared with both NGC 4631 and NGC 4627:
Here you can see NGC 4631 and NGC 4627 at the lower right, and NGC 4656 at the upper left. The color balance of the picture is rather yellow, so the middle part of NGC 4631 looks very yellow and NGC 4627 looks all yellow. But you can see the bright pinkish-purple emission nebulae dotting the disk of NGC 4631.
Now look at NGC 4656. Its color properties are quite different. There is barely a hint of yellow at the center of it. One half of its disk is faint, relatively smooth and blue. The other half of its disk is curiously bent at the edge (NGC 4656 is called the Hockey Stick Galaxy) and it is patchy and blue, with perhaps a hint of purple in it. But there is nothing even resembling the audacious display of purple in NGC 4631.
This is my interpretation of NGC 4656. This galaxy has had very few episodes of star formation, perhaps no more than three. The first episode created the faint, faint old yellowish population in the center of the galaxy, a couple of billion years ago. The second episode created the mostly smooth and faint blue population of one half of the disk, perhaps, say, 500 million years ago. The third episode of star formation was a regular burst and happened a hundred or two hundred million years ago. The burst happened because of interaction with NGC 4631, and it was violent enough that it mostly depleted NGC 4656 of star forming material. (Admittedly I've also come across a hypothesis that the "blade" of the Hockey Stick galaxy may have been a separate galaxy which collided with the main body of NGC 4565 and triggered star formation, but I'll leave it to you to decide how likely that is.) Now the hottest and brightest stars of NGC 4656 have already gone out in blasts of glory, leaving the galaxy with no bright emission nebulae and few opportunities to create new brilliant stars of spectral classes O and B. NGC 4656 is, or so I think, a galaxy with very little remaining gas and a dark future!
Ann