I am having a problem wrapping my mind around orbital velocities.

Keplers 3rd law indicates the orbital velocity of planets in the solar system, which decreases from 48 km/sec for Mercury to 5.4 km/sec for Neptune.

http://www.windows.ucar.edu/tour/lin...ets_table.html

Galaxies appear to be different.

Orbital speeds actually increase from the galaxy core out to a point where they are constant due to dark matter.
http://en.wikipedia.org/wiki/Image:G...cRotation2.svg

My question is: what causes the increasing orbital speeds on the above curve (prior to it flattening out) in galaxies?

in the solar system orbital speeds decrease with distance.

My only thoughts are that the further from the central bulge the more matter there is towards the galaxy core which increases orbital speed, but the central bulge would have to contain most of the mass of the inner disk(?).

The orbital speeds in the outer disk are, of course, influenced by dark matter.

ta

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You answered the question yourself: in the inner portions of some galaxies, the distribution of mass continues to grow with radius in some way which causes the rotation curve to increase. In simplest terms, it means that all the mass can't be concentrated in a point at the galaxy's very center (as the mass is concentrated in the Solar System).

To explain the wide variety of observed rotation curves, astronomers find that each galaxy needs a particular blend of nuclear mass distribution plus disk mass distribution plus dark matter distribution. In some galaxies, the dark matter doesn't dominate until one reaches very large radii; in others, the dark matter dominates even close to the center.

You can find a couple of recent papers showing plots of these components and the overall shape of rotation curves on astro-ph: I searched and found

http://arxiv.org/pdf/astro-ph/0603622

http://arxiv.org/pdf/astro-ph/0602027

Read them (or just look at the figures) and see for yourself.

yep, matter concentration (whether dark or visible) makes the curve the way it is.

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