astrophysics - Mass-to-light ratio and rotation curve from brightness profile

This should probably be basic but I've been looking for days and I can't find how to (I'm probably over complicating, but still).

I want to calculate a rotation curve for some spiral galaxies. From what I understand I need to use the brightness profile (which I have) to determine the mass distribution and then use that with Poisson to calculated the $g$. Then I can get the rotation curve.

My problem (for now) is how do I convert the brightness profile to mass-distribution?

the units are $mag/pc^2$ and the $y$-axis scale is 24-->18. it's not a direct relation because the distribution $y$-axis scale goes from small to large (10-->10^2 for example) so I can't just use $100 L/pc^2 = 22.05$ been finding all kind of different equations for different things, just not this.

would appreciate any help.