by Ann » Wed Aug 03, 2011 8:30 pm
nobby wrote:NGC 3628 seems to be about twice as big as the other two, which to a novice like me would mean that it was either twice as big or much closer. Can someone please explain why they are judged to be in the same physical area?
We can be sure that three galaxies are in the same physical area, because they show quite similar redshifts, generally between 700 and 800 kilometers per second. That is to say, all three galaxies are moving away from us at a velocity of between 700 and 800 kilometers per second. Because space is expanding and carrying its galaxies with it, and because expansion-induced velocities of galaxies correspond to given distances from us (more or less), we can be quite confident that the Leo Triplet galaxies are at similar distances from us. They are about 30 million light-years away, as the caption of today's APOD pointed out.
But the galaxies also show physical signs of interaction. Look at this image of the Leo Triplet, which you could see if you followed one of the links of today's APOD:
A tidal tail like the one belonging to NGC 3628 is an almost sure sign of interaction. So NGC 3628 is interacting with one or more other galaxies, but which one or which ones? Well, take a look at the neighbourhood of NGC 3628. What other galaxies can you see nearby? That's right, you can see M66 and M65. M65, admittedly, looks quiet and undisturbed, but M66 looks rather warped. There can be no doubt that M66 is interacting with NGC 3628, and there can't be much doubt that M65 is part of the action, too.
nobby, you pointed out that NGC 3628 looks much bigger than M65 and M66. Assuming that it really is bigger, would that be surprising or unnatural? Is there, in other words, a "standard size" for galaxies? No, absolutely not. Space is full of large galaxies and small galaxies, and sometimes the small and the large ones interact with one another. Take a look at interacting galaxies NGC5394 and NGC5395, one big and one small:
The picture is by Alex Cherney, amateur astrophotographer, and Noel Carboni, astroimage processing expert from ProDigital Software. Surely Noel Carboni must be "our" Noel C? Congratulations, Noel, on helping Alex Cherney produce such a fine picture of interacting galaxies!
But to summarize: The galaxies of the Leo Triplet are at a similar "depth" from us, are close together in the two other spatial dimensions, too ("up" and "down" and "right" and "left"), and show morphological signs of interaction. They are most certainly a physical group in the same physical area.
Ann
[quote="nobby"]NGC 3628 seems to be about twice as big as the other two, which to a novice like me would mean that it was either twice as big or much closer. Can someone please explain why they are judged to be in the same physical area?[/quote]
We can be sure that three galaxies are in the same physical area, because they show quite similar redshifts, generally between 700 and 800 kilometers per second. That is to say, all three galaxies are moving away from us at a velocity of between 700 and 800 kilometers per second. Because space is expanding and carrying its galaxies with it, and because expansion-induced velocities of galaxies correspond to given distances from us (more or less), we can be quite confident that the Leo Triplet galaxies are at similar distances from us. They are about 30 million light-years away, as the caption of today's APOD pointed out.
[float=right][img2]http://3dastronomer.com/news/wp-content/uploads/2010/05/the_tidal.jpg[/img2][c]Photo: Steve Mandel[/c][/float]But the galaxies also show physical signs of interaction. Look at this image of the Leo Triplet, which you could see if you followed one of the links of today's APOD: :arrow:
A tidal tail like the one belonging to NGC 3628 is an almost sure sign of interaction. So NGC 3628 is interacting with one or more other galaxies, but which one or which ones? Well, take a look at the neighbourhood of NGC 3628. What other galaxies can you see nearby? That's right, you can see M66 and M65. M65, admittedly, looks quiet and undisturbed, but M66 looks rather warped. There can be no doubt that M66 is interacting with NGC 3628, and there can't be much doubt that M65 is part of the action, too.
[float=left][img2]http://physicsworld.com/blog/assets_c/2011/07/NGC5394%20and%20NGC5395-thumb-500x546-2041.jpg[/img2][/float]
nobby, you pointed out that NGC 3628 looks much bigger than M65 and M66. Assuming that it really is bigger, would that be surprising or unnatural? Is there, in other words, a "standard size" for galaxies? No, absolutely not. Space is full of large galaxies and small galaxies, and sometimes the small and the large ones interact with one another. Take a look at interacting galaxies NGC5394 and NGC5395, one big and one small:
The picture is by Alex Cherney, amateur astrophotographer, and Noel Carboni, astroimage processing expert from ProDigital Software. Surely Noel Carboni must be "our" Noel C? Congratulations, Noel, on helping Alex Cherney produce such a fine picture of interacting galaxies! :D
But to summarize: The galaxies of the Leo Triplet are at a similar "depth" from us, are close together in the two other spatial dimensions, too ("up" and "down" and "right" and "left"), and show morphological signs of interaction. They are most certainly a physical group in the same physical area.
Ann