by Ann » Mon Nov 21, 2016 3:39 pm
NGC 4789A. Credit: ESA/Hubble & NASA
Acknowledgement: Judy Schmidt (geckzilla)
NGC 3274. Credit: ESA/Hubble & NASA, D. Calzetti
How absolutely fascinating!!!
Just last week, bystander posted the picture at left of dwarf irregular galaxy NGC 4789A, and I was quite unimpressed with the looks of it. The galaxy looks just "faint all over", with some scattered little sprinklings of blue across the face of it. There are no large clusters there, no bright nebulas, nothing. Yet the U-B and B-V colors of NGC 4789A are
extraordinarily blue, -0.270 and 0.300, respectively.
Now bystander has just posted a picture of another small galaxy, NGC 3274, which compared with NGC 4789A has a much higher surface brightness. We can tell that from the fact that the brightness of NGC 3274 peaks in the middle, whereas NGC 4789A is just faint everywhere.
And doesn't NGC 3274 look
blue, compared with NGC 4789A? Look at all those bright clusters and obvious nebulas. It's much bluer than NGC 4789A, isn't it?
Well, looks are deceiving. The U-B and B-V color indexes of NGC 3274 are "only" -0.140 and 0.390, clearly less blue than the colors of NGC 4789A! How is that possible?
Well, we must bear in mind that the two images are not directly comparable, since different filters have been used for the two pictures. But an important part of the answer is that NGC 3274, with its much higher surface brightness, clearly has a much more substantial old yellow population than NGC 4789A. I think that is the major reason why NGC 4789A is bluer than NGC 3274, namely, that the old yellow population of the former galaxy is so weak.
There is one more reason why NGC 3274 is redder than NGC 4789A, and that is dust. To me, NGC 4789A looks all but dust-free, whereas NGC 3274 contains several visible clouds of dust. NGC 3274 is slightly brighter in the far infrared than in the B magnitude, which implies healthy amounts of dust and a good source of raw material for new star formation. But this dust has probably also reddened the light that reaches us from NGC 3274.
To me this is just so fascinating!!!
Ann
[float=left][img2]http://cde.laprensa.e3.pe/ima/0/0/1/5/1/151896.jpg[/img2][c][size=85]NGC 4789A. Credit: ESA/Hubble & NASA
Acknowledgement: Judy Schmidt (geckzilla)[/size][/c][/float] [float=right][img2]https://cdn.spacetelescope.org/archives/images/screen/potw1647a.jpg[/img2][c][size=80]NGC 3274. Credit: ESA/Hubble & NASA, D. Calzetti[/size][/c][/float]
How absolutely fascinating!!! :shock:
Just last week, bystander posted the picture at left of dwarf irregular galaxy NGC 4789A, and I was quite unimpressed with the looks of it. The galaxy looks just "faint all over", with some scattered little sprinklings of blue across the face of it. There are no large clusters there, no bright nebulas, nothing. Yet the U-B and B-V colors of NGC 4789A are [i]extraordinarily[/i] blue, -0.270 and 0.300, respectively.
Now bystander has just posted a picture of another small galaxy, NGC 3274, which compared with NGC 4789A has a much higher surface brightness. We can tell that from the fact that the brightness of NGC 3274 peaks in the middle, whereas NGC 4789A is just faint everywhere.
And doesn't NGC 3274 look [i][b][color=#0040FF]blue[/color][/b][/i], compared with NGC 4789A? Look at all those bright clusters and obvious nebulas. It's much bluer than NGC 4789A, isn't it?
Well, looks are deceiving. The U-B and B-V color indexes of NGC 3274 are "only" -0.140 and 0.390, clearly less blue than the colors of NGC 4789A! How is that possible?
Well, we must bear in mind that the two images are not directly comparable, since different filters have been used for the two pictures. But an important part of the answer is that NGC 3274, with its much higher surface brightness, clearly has a much more substantial old yellow population than NGC 4789A. I think that is the major reason why NGC 4789A is bluer than NGC 3274, namely, that the old yellow population of the former galaxy is so weak.
There is one more reason why NGC 3274 is redder than NGC 4789A, and that is dust. To me, NGC 4789A looks all but dust-free, whereas NGC 3274 contains several visible clouds of dust. NGC 3274 is slightly brighter in the far infrared than in the B magnitude, which implies healthy amounts of dust and a good source of raw material for new star formation. But this dust has probably also reddened the light that reaches us from NGC 3274.
To me this is just so fascinating!!!
Ann