Star velocity

Question by MeadowyRavine Reward: 500 Pond Points + 10 Knowledge Tokens Status: Pending a resolution
What is the average velocity and range of velocities of stars in elliptical galaxies such as NGC 4621?

Clarification by MeadowyRavine
I am wondering about the velocity distribution of the individual stars with respect to the center of the galaxy, not the rotational velocity of the galaxy itself.
The last link posted was helpful, it referenced this paper: http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1983ApJ...266..516D&data_type=PDF_HIGH&whole_paper=YES&type=PRINTER&filetype=.pdf
which does graph velocity distributions, under the variable sigma, but I cannot tell if these are star velocity distributions or the distribution of the rotational velocity of the galaxy as a function of its radius. Do you have any thoughts?



Reply from razer21 User Rating:  370 Knowledge Tokens
Velocity is a relative measurement. Velocity varies based on your point of observation. For example, the velocity of a galaxy from the standpoint of an observer which the galaxy is moving towards will be a negative value, whereas an observer located on the opposite side of the galaxy will observe a positive value.

Normally we use Earth as a reference point for "the observer", but since the Earth (and our galaxy) are constantly moving, it only makes sense to use them as reference points for nearby objects.

Because of this, I assume you are asking about the rotational velocity of elliptical galaxies. The following link mentions a rotational velocity of 120 km/s for NGC 4621.

http://adsabs.harvard.edu/abs/1996PASJ...48..591M

A couple of other links that may assist you:

http://www.stsci.edu/stsci/meetings/shst2/jaffew.html
http://www.astr.ua.edu/keel/galaxies/ellipdyn.html
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Reply from AeroSpear User Rating:  80 Knowledge Tokens
Things to take into consideration:
When talking about orbital mechanics, the closer an orbit is to the center of the gravitational field (center of the elliptical galaxy) the faster it has to go to stay in orbit. Likewise, the farther out it is the slower is needs to go to stay in orbit. A basic orbital equation can tell you the velocity of an object in a two body system (example: the Earth and Moon or a galaxy and a star). You will have to make mass assumptions when dealing with a galaxy because the masses are distributed rather than centralized, but it's generally the same.

V(orbital) = sqrt((G*M)/r); G - gravitational constant, M - mass of the greater body (the smaller mass is usually negligible), r - the radial distance between the two
Key point I sometimes overlook, make sure your units match.

Some sites for reference on this:
http://en.wikipedia.org/wiki/Gravitational_constant
http://www.daviddarling.info/encyclopedia/O/orbital_velocity.html
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