# Could dark matter assist with star formation?

This is related to the question: "Can dark matter decrease the Jeans length?" If dark matter assists in any way with star formation, shouldn't it be detectable by precisely measuring velocities in young open clusters? In particular, could the motions of stars in the Pleiades all be explained without the presence of dark matter?

• Duplicate: I found a very similar question after I posted. I would, however be curious to know if the motions of stars in newly formed clusters (like the Pliedes) can be explained without dark matter. – Jack R. Woods Mar 9 '16 at 16:28
• If there is a duplicate you can link to it, or edit the question to the one about the motion of the pleiades, which has an excellent answer below. – James K Mar 9 '16 at 19:15

Strange you should ask - I am currently working on a paper on NGC 2516 - "the southern Pleiades" and looking at the dynamical status of the stars compared with the distribution of visible mass.

Our conclusion is that the radial velocities of the stars are in virial equilibrium, with velocity dispersions that are entirely consistent with the mass that is present in the stars that are visible. So, no need for dark matter there - you could probably rule out any contribution of more than about 50% of the visible stars. (NB the point of the work is not to look for dark matter, is is not expected - see below).

This is one of the first very detailed measurement for an open cluster (like the Pleiades), because the velocity precisions required have to be considerably better than the velocity dispersion. And for a sparse cluster like the Pleiades, this is $<1$ km/s, which is challenging.

There is one old paper by Jones (1970) that uses proper motions in the Pleiades to estimate a dynamical mass based on virial equilibrium of $690\ M_{\odot}$, which compares with $470\ M_{\odot}$ from directly counting stars. But Jones points out that the latter number is a lower limit (they certainly couldn't see very low mass stars and brown dwarfs in their study) and the former number has large uncertainties. So there is no real evidence for any "dark matter".

The dynamics of globular clusters are easier to measure - they are more massive and have much higher velocity dispersions. The general picture is reasonable agreement between velocity dispersions and inferred mass distributions from counting the visible stars. There are of course uncertainties, but there can be no large dark matter component. Limits as small as $<6$% dark matter have been set in some clusters (e.g. Ibata et al. 2012).

The first Gaia satellite results, due in about 18 months time, will blow this field apart. We will have exquisitely precise tangential velocities for $\sim 1000$ stars in the Pleiades.

Dark matter is not expected to play a role in small scale (star cluster) formation. When clusters form, the dissipation of energy in gas interactions is what allows them to end up as bound systems. Dark matter is dissipationless. As the escape velocity of these clusters is 1-10 km/s, compared to velocities of hundreds of km/s for datk matter, there is not even expected to be any gravitational concentration of dark matter. This is quite different from galaxy formation, which is associated with pre-existing dark matter structures.

• So warm or hot dark matter, if it's there. Nothing that hangs around long enough to exert significant gravitational acceleration? – Wayfaring Stranger Mar 9 '16 at 18:59
• @ Rob Jeffries I'm not completely surprised about a null finding in NGC 2516 as dark matter seems to be quite disperse, but I'm kind of shocked that it isn't found in globular clusters containing hundreds of thousands of stars. Any ideas as to why??? – Jack R. Woods Mar 11 '16 at 5:44
• @JackR.Woods see edit. – Rob Jeffries Mar 11 '16 at 7:40