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Thread: Galaxies: Questions about radial velocity and age

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    Galaxies: Questions about radial velocity and age

    The tangential component of velocity, of a star around a galaxy, can be measured by looking for a Doppler redshift or blueshift for two sides, (after cosmological redshift has been subtracted), best when the galaxy is edge on.

    But how is the radial component (towards the centre) of a star in a galaxy measured?

    If it's edge on, won't the Doppler effect of the radial component be masked by the cosmological redshift, and in any case just the radial component of one side (nearest) would be visible. Or has a combination of the above been used? If face on, there is no radial component towards or away from the observer. Does a 45 degree situation help?

    If the radial component has been measured (not deduced), then how and what are typical values?

    How is the age of a galaxy measured, and does it involve assumptions about the age of stars?

    Thankyou.
    Last edited by john hunter; 2018-Jun-25 at 09:47 PM.
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    Will this help you with an astronomy class?
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    Quote Originally Posted by George View Post
    Will this help you with an astronomy class?
    It will help decide whether an ATM proposal about galaxies is worthwhile.

    As the standard of some recent ATM threads has been a bit low, thought it would be best to do some research first.
    Last edited by john hunter; 2018-Jun-25 at 09:46 PM.
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    Quote Originally Posted by john hunter View Post
    But how is the radial component (towards the centre) of a star in a galaxy measured?

    If it's edge on, won't the Doppler effect of the radial component be masked by the cosmological redshift, and in any case just the radial component of one side (nearest) would be visible. Or has a combination of the above been used? If face on, there is no radial component towards or away from the observer. Does a 45 degree situation help?

    If the radial component has been measured (not deduced), then how and what are typical values?
    The answers to your questions are beyond me, off hand, but I think you want to look up velocity dispersion.
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    I'd like to point out that except for a few very close neighbors, we do not observe the velocities of individual stars going around a galaxy, we observe light from many stars, and we see a spectrum representing many similar but not equal velocities. This means we wouldn't be looking at any one star's velocity toward or away from the center of that galaxy.

    As to the age of the galaxy, pretty much all galaxies are 13.6 billion years old, and have all gone through many periods of star formation as they merge with new gas clouds and other galaxies. When you read that some object is an old galaxy, they usually mean that it has been a long time since it had significant star formation (perhaps its gas was blown out by the monster galaxy in the center of its cluster), and doesn't have any massive (bright) stars.
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    In determining the rotation curves of galaxies usually several measurements are made at different wavelengths - typically things like Ha, HII, NII. The velocity dispersion is measured for each and the low dispersion ones are the ones used to get a rotation curve (this method effectively filters out the contribution from the least circular orbits). The 21cm HI line is also used to look at the extended rotation curve.

    This paper: https://arxiv.org/pdf/1608.08350.pdf has a fairly good review of methods and some references on them.

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    Quote Originally Posted by john hunter View Post
    Does a 45 degree situation help?

    If the radial component has been measured (not deduced), then how and what are typical values?
    I think I understand what you're asking--you're considering the possibility that stars are either being pulled into or thrown out of the center of galaxies. It's interesting and I think the way to look at it would, as you suggested, be to look at the near side and far side of the stars along the center line of a galaxy with some kind of angle. I'm not sure if 45 degrees would be ideal, maybe 30 or maybe 60. I think that I tried to look it up once (because I was interested in a similar ATM idea, I suppose), and couldn't find any positive evidence for a radial velocity. But I could be wrong.
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    As Antoniseb stated, we can only measure the radial velocities of individual stars for the very nearest external galaxies -- within our Local Group. For other galaxies, we measure the combined contribution of millions of stars together: some moving toward us, some away from us, some across our line of slight. The "velocity dispersion" of a galaxy is the result of that addition of millions of individual radial velocities. We can learn quite a bit from a careful study of the velocity dispersion, but it doesn't tell us anything about any particular star of interest in that galaxy. Alas.

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    I don't know if anybody has done any studies on face-on galaxies alone.

    Any shift from these galaxies would have to be mostly cosmological shift, regardless of the radial velocity of their stars, as the plane of their rotation would be 90 degrees to the observer. As such all light emitted at the same time, from stars all around the galaxy disk, will largely arrive at the observer at the same time if they are not blocked or distorted in transit.

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    Thankyou all for the useful comments and links.

    and yes Jens, the ATM thread will be along those lines. From Cougars link it seems that there is a significant radial dispersion "Face-on spiral galaxies have a central σ ≲ 90 km/s; slightly more if viewed edge-on.", which was surprising as the constant (with radius) tangential velocity is of similar magnitude.

    It's probably not known, from what you've all said, whether this is due to stars travelling towards the centre on both sides, or some towards and some away on each side.

    Any more comments would be welcome before the ATM is started shortly. Hope it's not thrown out too quickly!
    "...when you have eliminated the impossible, whatever remains, however improbable, must be the truth." Sherlock Holmes

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    The velocity dispersion for a face-on spiral galaxy is a measure of the stars bobbling up and down away from and towards the disk plane. This is all randomised.

    When you look at a spiral edge-on, one side is moving towards you and the other away, due to the rotation of the galaxy. This is not the really the same as velocity dispersion, it is a systematic average orbital velocity.

    Built on top of that rotational velocity will be a smaller random component, due to orbits not being precisely circular, exactly in plane, and to interactions between stars and other objects. These random motions are generally much smaller than the rotational velocity, but you can find the odd exception like hypervelocity stars.

    Viewing at 45 degree angle is just going to complicate how to sort out the different motions.

    That's my understanding of it anyway.

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    It seems as though there are three types of velocity and probably three different 'best ways' to measure them.

    1) The bobbling just mentioned, where the motion is into and out of the plane of the galaxy. Best measured face on and a velocity dispersion is what's measured.

    2) Tangential motion in the plane. i.e the type which leads to the famous flat rotation curves and so to the dark matter theory. Best measured edge on.

    3) Radial motion in the plane, probably towards the centre. Presumably best measured at an angle, so both sides can be seen. This was really what the original question was about.

    So does the velocity dispersion measure the bobbling, or has the radial motion also been found separately as a velocity dispersion? If so are there any typical values? Is it towards the centre, or has the velocity dispersion (radial) not been isolated from the bobbling. Another possibility is that it has been measured, but edge on, where it might be difficult to decide which part of the dispersion is from the near side and which from the far side...?
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    Quote Originally Posted by john hunter View Post
    It seems as though there are three types of velocity and probably three different 'best ways' to measure them.

    1) The bobbling just mentioned, where the motion is into and out of the plane of the galaxy. Best measured face on and a velocity dispersion is what's measured.

    2) Tangential motion in the plane. i.e the type which leads to the famous flat rotation curves and so to the dark matter theory. Best measured edge on.

    3) Radial motion in the plane, probably towards the centre. Presumably best measured at an angle, so both sides can be seen. This was really what the original question was about.

    So does the velocity dispersion measure the bobbling, or has the radial motion also been found separately as a velocity dispersion? If so are there any typical values? Is it towards the centre, or has the velocity dispersion (radial) not been isolated from the bobbling. Another possibility is that it has been measured, but edge on, where it might be difficult to decide which part of the dispersion is from the near side and which from the far side...?
    I'm not really clear what you are trying to measure to be honest.

    If it is the component of stellar motions towards and away from the galaxy centre, it is the velocity dispersion along that vector you need to measure.

    As to the practicalities or doing that, I'm not so sure. I would think edge on is simplest conceptually but on the other hand the inner stars would be obscured.

    Anyway, I do not know why other spiral galaxies should be much different to our own galaxy in this respect. Look at the data for the Milky Way. In our locality the relative motions are a few km/s in all directions.

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    Let's look at some situations with hypothetical unperturbed disk galaxies.

    1. Face on. The tangential and radial components in the orbital plane will be crossways to our line of sight and thus undetectable. The "bobbing", which is a consequence of an inclined orbit, will be the only detectable component, and the dispersion will tell us something about the distribution of orbital inclinations to the galactic equator.

    2. Edge on. The bobbing will be crossways and thus undetectable. Samples in conjunction with the core will show radial components, if any, with the tangential component undetectable. At the maximum elongations for any given orbital radius, it will be the opposite. The radial components are consequences of eccentric orbits, as are variations in the tangential components. Samples at intermediate apparent positions would show a blend of these conditions. The whole thing would have to be analyzed statistically.

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    Quote Originally Posted by kzb View Post
    I'm not really clear what you are trying to measure to be honest.

    If it is the component of stellar motions towards and away from the galaxy centre, it is the velocity dispersion along that vector you need to measure.

    As to the practicalities or doing that, I'm not so sure. I would think edge on is simplest conceptually but on the other hand the inner stars would be obscured.

    Anyway, I do not know why other spiral galaxies should be much different to our own galaxy in this respect. Look at the data for the Milky Way. In our locality the relative motions are a few km/s in all directions.
    Yes, it is the component of stellar motions towards and away from the galaxy centre. As described in post 12 and Hornblower's summary. The obscuring of some stars is why an angle (tilt of axis towards or away from observer) of 45 degrees (or 30 degrees etc.) was suggested.

    Does anyone know if it's been done and any typical values for radial velocity?

    The Milky Way point is a good one, has the average radial component for the Milky Way been measured, by looking towards or away from the Milky Way's centre? Has the suns radial component been determined? The relative velocity of stars would need to be combined with the sun's radial velocity to try and get an average radial component for the Milky way.
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    I would expect there to be tons of papers on such motions in the astronomy libraries of the world. Whether or not they can easily be found online is anybody's guess. I would expect the mean value for an unperturbed disk galaxy to be near zero, because the orientations of the individual eccentric stellar orbits would largely cancel out. If we find something otherwise, my prime suspect would be a relatively recent merger or close encounter with another galaxy.

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    Quote Originally Posted by john hunter View Post
    It's probably not known, from what you've all said, whether this is due to stars travelling towards the centre on both sides, or some towards and some away on each side.
    The
    problem with interpreting velocity dispersion as all the stars on one side going one way, and all the stars on the other side going the other way, is that it requires the object change significantly in the time it would take the stars to cross the object. But we don't find object in various stages of change, the whole population seems to be in equilibrium. Dynamical equilibrium means the age of the object is much longer than the time it takes stars to move across it, and interpreting galaxies in that way makes a lot more sense than imagining they have ordered velocities that we don't see in our own galaxy. What's more, we expect stars to interact gravitationally with each other, which should randomize order like you are describing given the huge age of these objects. What prevents that from happening is spiral galaxies is the angular momentum that is present and needs to be conserved, but that's not present in a structure where all the stars are going inward. Instead, when there is not angular momentum, you get an elliptical galaxy, where the individual stars are at random points in their orbits, and are going in all different directions everywhere you look.
    Last edited by Ken G; 2018-Jun-27 at 03:20 AM.

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    Quote Originally Posted by kzb View Post
    I'm not really clear what you are trying to measure to be honest.
    I think a simple way is to put it like this: is there any evidence that galaxies are like vacuum cleaners, that pull stars into the centers, or like sprinklers that eject the stars outward?

    I think that I asked a question like that a long time ago, probably in Q&A, and if my memory is correct I think that there wasn't any evidence for this. I think the evidence would be a galaxy tilted to us at say 45 degrees, with the stars nearest to us being red-shifted and the stars on the far side of the galaxy being blue-shifted (in the case where the galaxy is acting as a vacuum cleaner). But I don't think we see anything like that...
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    Quote Originally Posted by kzb View Post
    I'm not really clear what you are trying to measure to be honest.
    I think a simple way is to put it like this: is there any evidence that galaxies are like vacuum cleaners, that pull stars into the centers, or like sprinklers that eject the stars outward?

    I think that I asked a question like that a long time ago, probably in Q&A, and if my memory is correct I think that there wasn't any evidence for this. I think the evidence would be a galaxy tilted to us at say 45 degrees, with the stars nearest to us being red-shifted and the stars on the far side of the galaxy being blue-shifted (in the case where the galaxy is acting as a vacuum cleaner). But I don't think we see anything like that...

    Here's the thread where it sort of came up:

    https://forum.cosmoquest.org/showthr...-spiral-galaxy
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    Ken, Yes, angular momentum is something to consider...

    Quote Originally Posted by Jens View Post
    I think a simple way is to put it like this: is there any evidence that galaxies are like vacuum cleaners, that pull stars into the centers, or like sprinklers that eject the stars outward?

    I think the evidence would be a galaxy tilted to us at say 45 degrees, with the stars nearest to us being red-shifted and the stars on the far side of the galaxy being blue-shifted (in the case where the galaxy is acting as a vacuum cleaner). But I don't think we see anything like that...
    Yes, that just about sums it up. But the question of whether there is evidence for or against it, is what the OP was about.

    The ATM thread will have some new ideas and propose ejection perpendicular to the disc.

    Does anyone know, or have any links to papers which have measured radial velocity or found that it doesn't exist? Or is it something that may exist - but hasn't or can't be isolated from the bobbling or obscuring problems other effects?

    There isn't much point starting the proposed ATM thread if someone says straight away. This...paper shows there is zero (within certain limits) radial velocity.
    "...when you have eliminated the impossible, whatever remains, however improbable, must be the truth." Sherlock Holmes

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    Actually, this image came up in the other thread. It was explained at that time that it shows a normal rotation curve. But looking at it, since the galaxy is tilted 54 degrees to us, it seems to me like it is showing that the stars far away (at the top of the image) are coming toward us, since they are blueshifted, while those close to us are redshifted, meaning that they are being pulled in to the galaxy. Is my reasoning wrong? I'm pretty sure it is because otherwise it would have been concluded that stars are being pulled inward! If so, what is the blue and red showing?

    ETA: Actually, I can understand it shows a rotation curve if we are looking at the disk from the rim rather than from the top. Because then the part at the top is the part rotating toward us and the red the part rotating away from us. I was assuming that we were looking at it from a relatively flat perspective looking from the top. Sort of like this image.
    Last edited by Jens; 2018-Jun-27 at 07:09 AM.
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    Quote Originally Posted by john hunter View Post

    Does anyone know, or have any links to papers which have measured radial velocity or found that it doesn't exist? Or is it something that may exist - but hasn't or can't be isolated from the bobbling or obscuring problems other effects?
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    Quote Originally Posted by Hornblower View Post
    I would expect the mean value for an unperturbed disk galaxy to be near zero, because the orientations of the individual eccentric stellar orbits would largely cancel out. If we find something otherwise, my prime suspect would be a relatively recent merger or close encounter with another galaxy.
    ...that would apply to lenticular galaxy discs?
    Do density waves of spiral arms involve collective average radial motions of stars?

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    Quote Originally Posted by john hunter View Post
    The tangential component of velocity, of a star around a galaxy, can be measured by looking for a Doppler redshift or blueshift for two sides, (after cosmological redshift has been subtracted), best when the galaxy is edge on.

    But how is the radial component (towards the centre) of a star in a galaxy measured?

    If it's edge on, won't the Doppler effect of the radial component be masked by the cosmological redshift, and in any case just the radial component of one side (nearest) would be visible. Or has a combination of the above been used? If face on, there is no radial component towards or away from the observer. Does a 45 degree situation help?

    If the radial component has been measured (not deduced), then how and what are typical values?

    How is the age of a galaxy measured, and does it involve assumptions about the age of stars?

    Thankyou.
    Doppler shift gives the radial component of velocity relative to the observer; the radial and tangential components of an object’s may be determined if the angle the plane of the orbit relative to the line of sight can be found, so maybe for nearby galaxies or for eclipsing binaries.

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    Quote Originally Posted by Hornblower View Post
    I would expect there to be tons of papers on such motions in the astronomy libraries of the world. Whether or not they can easily be found online is anybody's guess. I would expect the mean value for an unperturbed disk galaxy to be near zero, because the orientations of the individual eccentric stellar orbits would largely cancel out. If we find something otherwise, my prime suspect would be a relatively recent merger or close encounter with another galaxy.
    Agreed.

    One area this has been examined in detail is in the galaxy rotation curve. Coming out with figures for that requires you to evaluate motions in all three dimensions.

    The "peculiar motion" of the sun has to be established as well, in the course of such studies.

    The relative motions of stars in our galactic locality have been studied extensively. There is also a "local standard of rest".

    I don't think there is any evidence in any of these studies that there is a systematic expansion or contraction of the galaxy disk over time.

    Here's one recent article picked at random:

    The local rotation curve of the Milky Way based on SEGUE and RAVE data

    https://arxiv.org/abs/1802.07658

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    Quote Originally Posted by kzb View Post
    One area this has been examined in detail is in the galaxy rotation curve. Coming out with figures for that requires you to evaluate motions in all three dimensions.

    The "peculiar motion" of the sun has to be established as well, in the course of such studies.

    The relative motions of stars in our galactic locality have been studied extensively. There is also a "local standard of rest".

    I don't think there is any evidence in any of these studies that there is a systematic expansion or contraction of the galaxy disk over time.
    Not over times large compared to galactic rotation period, sure.
    Do spiral arm density waves show up in rotation curve?

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    Thankyou everyone,

    As the evidence for or against radial motion doesn't seem conclusive, the ATM proposal will be started now. It's going to be a difficult one to defend, but there are a couple of points to raise in it's favour, which might encourage others to develop the proposal.

    Maybe see you there.
    "...when you have eliminated the impossible, whatever remains, however improbable, must be the truth." Sherlock Holmes

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    Quote Originally Posted by john hunter View Post
    Thankyou everyone,

    As the evidence for or against radial motion doesn't seem conclusive, the ATM proposal will be started now. It's going to be a difficult one to defend, but there are a couple of points to raise in it's favour, which might encourage others to develop the proposal.

    Maybe see you there.
    Usually, people come up with alternative theories to explain actual observations or experimental results.

    This could be one step beyond; we going to have an alternative theory to explain something that is not even happening !

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    Quote Originally Posted by chornedsnorkack View Post
    Not over times large compared to galactic rotation period, sure.
    Do spiral arm density waves show up in rotation curve?
    Probably the spiral arms themselves do, there is fine structure in the curve. The up and down wiggles in the curve are because the distribution of mass with radius is not a completely smooth decrease. The spiral arms are probably major contributors to this lack of smoothness.

    But if you mean stars going at different speeds at the same radius, i.e because they are or are not in a spiral arm, I think they will have been averaged out in the generation of the curve.

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    Quote Originally Posted by kzb View Post
    Usually, people come up with alternative theories to explain actual observations or experimental results.

    This could be one step beyond; we going to have an alternative theory to explain something that is not even happening !
    Its an alternative theory to explain things which are definitely happening such as:

    The mass distribution of the dark matter, which leads to the very flat rotation curves.

    The spiral shape of the spiral galaxies.

    It has as a kind of starting point that there is radial motion towards the nucleus, and as that hasn't been ruled out so far in this thread the ATM thread has started.
    "...when you have eliminated the impossible, whatever remains, however improbable, must be the truth." Sherlock Holmes

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