# Galaxy lensing and dark matter

When we observe galaxy lensing of background objects taking place, is it possible to estimate the total mass of the galaxy carrying out the lensing and from that confirm that dark matter is needed to be present to provide sufficient mass to bend the light by the amount observed? Is there agreement?

http://www.space.com/28489-dark-matter-light-halos.html

This is just another article which I thought interesting:
http://phys.org/news/2015-02-big-quantum-equation-universe.html

Chalnoth
The evidence comes from galaxy clusters rather than individual galaxies. This is because most of the normal matter mass in galaxy clusters is visible in the x-ray band: it exists as a diffuse, high-temperature gas throughout the cluster. The reason why the temperature is in the X-ray range is because of the depth of the gravitational potential well. You can figure out the mass of the galaxy cluster from either the temperature of the gas, or from gravitational lensing. And the luminosity of the gas gives you how much gas there is, so it's easy to see that there isn't near enough mass in the cluster gas to produce the deep gravitational potential well of the galaxy cluster.

To get a really stark picture, it's sufficient to look at collisions between galaxy clusters:
http://www.preposterousuniverse.com/blog/2006/08/21/dark-matter-exists/

In general, yes, there is quite good agreement that the cosmological data as a whole strongly supports the existence of dark matter. There are still a few holdouts that are trying to produce alternative gravity models that do not require dark matter, but their attempts have become ever more feeble, and seem highly unlikely to ever succeed.

Tanelorn
phinds
Gold Member
2021 Award
I'm not aware that there has ever been any theory that says that dark matter exists only at the galactic rim. What HAS been said that because of the yo-yo motion of dark matter through the middle of galaxies and back out the other side, and the way this motion works, most dark matter spends most of its time in the halo.

Chalnoth
I'm not aware that there has ever been any theory that says that dark matter exists only at the galactic rim. What HAS been said that because of the yo-yo motion of dark matter through the middle of galaxies and back out the other side, and the way this motion works, most dark matter spends most of its time in the halo.
Indeed. The largest density is near the center of the galaxy.

I think the problem is that the term "halo" misleadingly implies that dark matter surrounds the galaxy. A better visualization is that a galaxy is made up of a large cloud of dark matter, and in the center of that cloud is a small but relatively dense bunch of normal matter (the visible part of the galaxy).

phinds
Gold Member
2021 Award
Indeed. The largest density is near the center of the galaxy.

I think the problem is that the term "halo" misleadingly implies that dark matter surrounds the galaxy. A better visualization is that a galaxy is made up of a large cloud of dark matter, and in the center of that cloud is a small but relatively dense bunch of normal matter (the visible part of the galaxy).
Wait, do you mean the largest density of DARK matter is in the center? I though the halo was slightly more dense than the center in terms of dark matter only. I've got that wrong?

Chalnoth
Yup. One of the more commonly-used density profiles is the NFW profile:
https://en.wikipedia.org/wiki/Navarro–Frenk–White_profile

$$\rho(r) = {\rho_0 \over {r \over R_s}\left(1 + {r \over R_s}\right)^2}$$

It's not quite correct, but neither is it horribly wrong. If I recall correctly, one of the unsolved problems with dark matter simulations is that they predict a dark matter density profile that is much denser at the galactic center than observations suggest. But either way, yes, the density continues to increase. The normal matter has such a large impact because it is more dense.

phinds
Gold Member
2021 Award
OK, thanks for that correction.

Chronos
Gold Member
The density profile of DM, from halo to core, is a valuable clue about its collisional cross section.

phinds