Rotation curves of galaxies as a function of isolation.

[WCOLtd]

Which galaxy has the largest angular momentum of all galaxies observed? What does its rotation curve look like?
Which galaxy has the smallest angular momentum of all galaxies observed? What does its rotation curve look like?
Which two galaxies are closest to one another? What do the rotation curves of these galaxies look like?
What is the most isolated galaxy ever observed? What does its rotation curve look like?

Where do I get data on this?

 

[DrSteve]

Where are these questions coming from? A homework set? They're not terribly scientifically meaningful they way they are posed, in that their answers would do little to shed light on the nature of dark matter halos, etc.

 

[WCOLtd]

I want to graph rotation curves as a function of non-dark-matter angular momentum and isolation of galaxies. You might be right that it won't shed any light I just have a suspicion and want to investigate it.

 

[DrSteve]

There is no such thing as the non dark matter angular momentum - there is just one total angular momentum. One way this is gotten is by constructing rotation curves. Read up on the Tully-Fisher relationship and perhaps some related papers to see how they derived the rotation rates of the galaxies they used in their studies.

 

[WCOLtd]

I need help understanding this. Is the reasoning behind the Tully-Fischer relation anything like the following; the luminosity of the galaxy indicates some estimate of mass that is less than what the angular velocity of the galaxy would allow for, so therefore there must be mass which does not emit light? Is this an accurate summation of the line of reasoning?

 

[WCOLtd]

How I understood it, the luminosity or the light that is given off by the galaxy can be used to find some estimate for the mass of the galaxy, and then there is the emission line width which allows the observer to determine the angular velocity of the galaxy. Since these two values do not match therefore we suppose in non glowing matter, or matter that does not interact with light. I wasn't sure by the link if they could determine the mass by the luminosity or apparent brightness alone or whether they derive the mass of galaxies by the angular velocities. I looked at the graph on the link, and I think the Y axis is the angular velocity and the X axis is mass. My interest I suppose is whether galaxies which are close to one another, or which have larger or smaller luminosities, how they would vary on the graph of the Tully Fisher relation. Thank you for you help, keep in mind I am not the most knowledgeable in the area of comology or astrophysics so it may require some patience on your part to deal with my stupidity in the area. I value your feedback Dr.Steve if you could correct any misconceptions I have or have any other links you think might help me understand I would appreciate it.

 

[DrSteve]

The Tully Fisher relationship relates the line width of the 21 cm line (or some surrogate) to its intrinsic luminosity. The width is proportional to the angular velocity, which is presumably dictated by the massive dark matter halo. The luminosity is, of course, driven by the luminous matter. Notice that there is no rotation curve here, as these galaxies are too far away to conduct a radial scan of the 21 cm line. In effect, the radius dependent rotation velocity is replaced with a single value.

The mass, hence the angular momentum of a galaxy, is not a well defined quantity, due to the difficulty of determining the extent of the dark matter halo.

 

[WCOLtd]

Other than angular velocity, is there a reason to suppose dark matter halos around galaxies? Like for instance gravitational lens?

 

[DrSteve]

Yes, weak gravitational lensing is an excellent example. Alos, the velocity of polar stars, disk stability and, most importantly, their sheer existence, which would not have been possible had dark matter not started clumping before baryonic matter prior to decoupling.

 

[WCOLtd]

The Tully Fisher relationship relates the line width of the 21 cm line (or some surrogate) to its intrinsic luminosity. The width is proportional to the angular velocity, which is presumably dictated by the massive dark matter halo. The luminosity is, of course, driven by the luminous matter. Notice that there is no rotation curve here, as these galaxies are too far away to conduct a radial scan of the 21 cm line. In effect, the radius dependent rotation velocity is replaced with a single value.

The mass, hence the angular momentum of a galaxy, is not a well defined quantity, due to the difficulty of determining the extent of the dark matter halo.

Lets ignore dark matter and try to get estimates of a galaxies baryonic matter, would that be in proportion to its luminosity? In other words, could you figure out the baryonic mass of a galaxy by knowing the luminosity?

What is there to suppose the dark matter, what about the angular velocity makes scientists suspect a dark matter halo? I understand the reasoning behind rotation curves and the discrepancy between observed and theoretical values, but I do not understand the reasoning behind Tully Fisher.

 

[WCOLtd]

I have a hypothesis that the greater the luminosity of a galaxy the greater the angular velocity, also the more isolated a galaxy is (how far away it is from other massive celestial bodies) the greater the angular velocity.

 

[DrSteve]

I have a hypothesis that the greater the luminosity of a galaxy the greater the angular velocity, also the more isolated a galaxy is (how far away it is from other massive celestial bodies) the greater the angular velocity.

So, your first hypothesis is the Tully Fisher relationship. I see no physics at all to support the latter proposition.

 

[WCOLtd]

So, your first hypothesis is the Tully Fisher relationship. I see no physics at all to support the latter proposition.

The reason you don't see the physics is because I am operating on a different set of assumptions than most physicists do. I have an idea but its very tenuous it doesn't add up. The idea is wrong but I want to figure out how wrong. I want to know how observation deviates from this idea. About the only prediction I can make that isn't already predicted by current models is that isolated galaxies will have a greater rotational curve discrepancy between theoretical and observed values - since that is not possible to know for more distant galaxies (galaxies outside the cluster) the next safest prediction I can make is that angular velocity of isolated galaxies is greater than galaxies which are close to one another assuming the two galaxies have identical or similar values for luminosity.

Apart from the assumptions made in my personal theory I can't talk about. The assumption I am making is that baryonic mass of a galaxy is a function of luminosity, and that rotational curve discrepancies can be expressed as a function of angular velocity.