Velocity curve of galaxy rotation

[Copernicus]

I noticed that the velocity of rotation is much slower nearer the center of the galaxy. Does anyone know the reason for this?

[korjik]

Orbital velocity is dependent on the amount of mass contained inside the orbit. When you get to the very center of the galaxy, the amount of mass inside the orbit is very small.

That and the graph you were looking at dosent include the SMBH in the center of the galaxy

[tusenfem]

Why not ask Mr. Kepler about his third law?

[forrest noble]

Copernicus,

Velocity curve of galaxy rotation

I noticed that the velocity of rotation is much slower nearer the center of the galaxy. Does anyone know the reason for this?

The center of spiral galaxies like our own are called galactic bulges. The bulge of a spiral galaxy is generally spheroidal in shape and some of these bulges have most orbiting stars on the galactic plane moving in the same direction as the outer galaxy. These bulges have characteristics similar to the rest of the galaxy excepting that stars are closer together with interacting gravitational influences which overall slows down orbital velocities.

Other spiral galaxy bulges have characteristics more like elliptical galaxies with stars moving in many different directions and planes including orbits contrary to the direction of the rest of the galaxy. This is thought to be due to the merger of clusters or dwarf galaxies with many different inertial motions.

With all these counter acting gravitational influences speeds slow down and orbits do not necessarily orbit the galactic center for either type of bulge.

Inner disc stars can also have slower orbital speeds than the outer-most disk stars, or their rates could be similar (a flat disc rotation curve). These rotation rates are presently attributed to hypothetical dark matter.

http://en.wikipedia.org/wiki/Bulge_(astronomy)

[grav]

As Korjik also mentioned, it is because those stars are interior to the body of mass. For instance, if we were to take a uniformly dense spherical mass, then the acceleration of gravity lessens with greater distance from the surface of the body and so does the orbital speed. One might tend to think this implies that the orbital speed increases with lesser distance from the center of mass, which is true outside of the surface of the body of mass itself, but if a particle were to orbit within the body of mass, say at 1/3 of the distance to the surface of the body r, so that r_x = 1/3 r, then the acceleration there is only a_x / a = [G D ((4 pi / 3) r_x^3) / r_x^2] / [G D ((4 pi / 3) r^3) / r^2] = r_x / r = 1/3 as great as the acceleration at the surface a, where D is the density, and would become zero at the center, because all of the gravity from the mass contained outside of that radius of orbit cancels itself out in the case of a uniformly dense sphere, so since a_x = v_x^2 / r_x for the acceleration of (circular) orbit, then (1/3 a) = v_x^2 / (1/3 r), 1/9 a r = v_x^2, whereas a = v^2 / r, a r = v^2 at the surface, so v_x = 1/3 v, giving one third of the orbital speed at one third of the radius of the body of mass than at the surface. The mass distribution of a disk galaxy at various orbital distances and therefore the gravity involved would be much different than that of a uniformly dense sphere, though, and all of the gravity of the mass contained outside of the orbital radius wouldn't completely cancel out for a disk as it does with a sphere, but the general principle involved otherwise remains the same.

[Copernicus]

Why not ask Mr. Kepler about his third law?

Yeah

You were right. I should have done the calculation first. I just assumed it was for some odd reason.