# Homework Help: Dark matter density in fucntion of radius

1. Dec 23, 2012

### Fabio010

1. The problem statement, all variables and given/known data

Evidence for dark matter comes from “flat” rotation curves of galaxies. Assume
that the observed matter moves in circular orbits about the center of the galaxy
and that the velocity of the matter as a function of the radius v(r) is a constant.

Also assume (mass of luminous matter is negligible) and the dark matter is
distributed with spherical symmetry about the center of the galaxy.

What is the density ρ(r) of the dark matter as a function of radius?

2. Attempt

Critic density = ρ(r) = 3Ho^2 / 8piG Ho= Hubble constante

because Ho = v/d then

ρ(r) = 3v^2 / 8piGr^2 r= distance from the center of galaxy

So we can conclude that the dark matter density in a galaxy is proportional to 1/r^2.

But i found this wrong. Can somebody the me what is wrong?

2. Dec 23, 2012

### Staff: Mentor

There is no expansion of the universe involved in the problem, why do you use the Hubble constant and what is d?

Just use v as a constant and Newtonian gravity.
You could calculate M(r) (the total mass up to radius r) as intermediate step.

3. Dec 23, 2012

### Fabio010

like this:

$\vec{}$

$F=\frac{GMm}{R^{2}}$

$a=\frac{v^{2}}{R}$

3. The attempt at a solution
$\frac{F}{m}=\frac{GM}{R^{2}}$

$4\pi \frac{F}{m}=\frac{4\pi GM}{R^{2}}$

$\frac{v^{2}}{R}=\frac{GM}{R^{2}}$

$dM=4\pi\rho (R)R^{2}dR$

$dM=\frac{v^{2}}{G}dR$

$4\pi\rho (R)R^{2}dR=\frac{v^{2}}{G}dR$

$\rho (R)=\frac{v^{2}}{4\pi GR^{2}}$

so $\rho (R)$ is propitiational to $\frac{1}{r^{2}}$

4. Dec 26, 2012

### Fabio010

anyone?

5. Dec 26, 2012

### Staff: Mentor

Sorry, missed that post somehow.
I don't see an error.

6. Dec 26, 2012

### Fabio010

So with my result, i can conclude that the dark matter density decreases with the distance from the center of the galaxy.

But is not the dark matter suppose to increase with the distance? To maintain the galaxy rotation velocity constant?

7. Dec 26, 2012

### Staff: Mentor

The total amount of dark matter increases (M~R), the density decreases. If density would be constant everywhere, our galaxy would not have any border in terms of its gravitational attraction.

8. Dec 26, 2012

### Fabio010

Thanks for the help mfb!