In the standard picture of disc galaxy formation , baryons and dark matter receive the same tidal torques , and therefore approximately the same initial specific angular momentum .
However , observations indicate that disc galaxies typically have only about half as much specific angular momentum as their dark matter haloes .
We argue this does not necessarily imply that baryons lose this much specific angular momentum as they form galaxies .
It may instead indicate that galaxies are most directly related to the inner regions of their host haloes , as may be expected in a scenario where baryons in the inner parts of haloes collapse first .
A limiting case is examined under the idealised assumption of perfect angular momentum conservation .
Namely , we determine the density contrast \Delta , with respect to the critical density of the Universe , by which dark matter haloes need to be defined in order to have the same average specific angular momentum as the galaxies they host .
Under the assumption that galaxies are related to haloes via their characteristic rotation velocities , the necessary \Delta is \sim 600 .
This \Delta corresponds to an average halo radius and mass which are \sim 60 % and \sim 75 % , respectively , of the virial values ( i.e. , for \Delta = 200 ) .
We refer to this radius as the radius of baryonic collapse R _ { BC } , since if specific angular momentum is conserved perfectly , baryons would come from within it .
It is not likely a simple step function due to the complex gastrophysics involved , therefore we regard it as an effective radius .
In summary , the difference between the predicted initial and the observed final specific angular momentum of galaxies , which is conventionally attributed solely to angular momentum loss , can more naturally be explained by a preference for collapse of baryons within R _ { BC } , with possibly some later angular momentum transfer .