Can we observe a galaxy rotating?

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ddale51
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Can we observe a galaxy rotating?

Post by ddale51 » Sat Jun 11, 2011 5:34 pm

I was looking at the recent, side-by-side APOD pictures of the M51 Spiral Galaxy that were taken six years apart, revealing two supernova explosions. Looking at the two, they of course looked identical except for the two supernovas. I began wondering: How long would it take to actually observe the rotation of a galaxy? Our Milky Way galaxy rotates once every million years, I've read; let's say M51 takes a similar length of time. So if we place a circle around M51 and divide it into 360 degrees, each degree would represent the distance traveled by the galaxy's outer stars every 2,778 years. But if we look at a hundredth of a degree, the time involved is only about 28 years. Since M51 has been photographed for at least 80 years (I'm guessing) mightn't it be possible to observe a tiny bit of rotation in the galaxy? 80 years would equal a little under 3/100th of a degree. This is a tiny arc, but could it be seen in superimposed photos today? Of course I realize photography 80-some years ago did not have near the resolution power of today's astro-photos. So maybe 100 years from now, when the galaxy has rotated, let's say, 5/100ths or 1/20th of a degree since the earliest clear photo, astronomers could note a definite rotational shift between that earlier photo and one taken in their time? Or is this still too tiny of an arc to be observed?

In a similar vein: Since stars in our own galaxy are moving at different velocities and orbital paths, how long does it take to note a shift in the distances--or relative positions--between some of these stars as observed from earth?

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neufer
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Re: Can we observe a galaxy rotating?

Post by neufer » Sat Jun 11, 2011 6:57 pm

ddale51 wrote:I was looking at the recent, side-by-side APOD pictures of the M51 Spiral Galaxy that were taken six years apart, revealing two supernova explosions. Looking at the two, they of course looked identical except for the two supernovas. I began wondering: How long would it take to actually observe the rotation of a galaxy? Our Milky Way galaxy rotates once every million years, I've read;
Our Milky Way galaxy rotates once every ~ 240 million years so you are off by about two orders of magnitude.
ddale51 wrote:
let's say M51 takes a similar length of time. So if we place a circle around M51 and divide it into 360 degrees, each degree would represent the distance traveled by the galaxy's outer stars every 2,778 years. But if we look at a hundredth of a degree, the time involved is only about 28 years. Since M51 has been photographed for at least 80 years (I'm guessing) mightn't it be possible to observe a tiny bit of rotation in the galaxy? 80 years would equal a little under 3/100th of a degree. This is a tiny arc, but could it be seen in superimposed photos today? Of course I realize photography 80-some years ago did not have near the resolution power of today's astro-photos. So maybe 100 years from now, when the galaxy has rotated, let's say, 5/100ths or 1/20th of a degree since the earliest clear photo, astronomers could note a definite rotational shift between that earlier photo and one taken in their time? Or is this still too tiny of an arc to be observed?
Hipparcos ("High precision parallax collecting satellite") operated between 1989 and 1993 with a measuring accuracy of ~0.1 milliarc-sec or ~0.015 light years at the distance of M51.

But a star orbiting M51 would have moved only about 0.003 light years in four years time so it would be about 5 times too small to see.
Art Neuendorffer

jarle
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Re: Can we observe a galaxy rotating?

Post by jarle » Mon Jun 27, 2011 1:42 pm

It is an interesting question. While it is still too challenging to do this in the optical using stars, note that this has been done already using radio observations using very long baseline interferometry (VLBI). See for instance Figure 1 in Reid et al's White Paper: http://fr.arxiv.org/abs/0902.3932

However you are limited to maser sources for now. Using stars is a long way away, even with something like GAIA you would simply not be able to resolve stars efficiently enough to do the rotation measurement very accurately.

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