magnitude smaller than the gravitational ‘pressure’ and therefore does not play a significant role in resisting the ram pressure.

Redistribution of angular momentum {#sec:am_redistribution_eda1}
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The redistribution of angular momentum in the disc is governed by two processes:(1) the accretion of material with no angular momentum material with respect to the disc and (2) the viscous evolution of the disc and consequent redistribution of its mass and angular momentum.

### Accretion of ISM

The accretion of material with no azimuthal angular momentum lowers the specific angular momentum of the disc. Since the total angular momentum of the disc has to be conserved, mass and angular momentum will be redistributed within the disc. We can estimate this redistribution to first order, if we consider the disc to consist of concentric rings with thickness $dr$ and mass $m_{\rm ring}(r) = 2 \pi r \Sigma(r) dr$. In a time interval $dt$, the ring will accrete an amount of mass from the ISM equal to $m_{\rm accr}(r) = 2 \pi r \rho_{\rm ISM} v_{\rm ISM} dt dr$. The specific angular momentum, $h$, in the ring will decrease by a factor $\Sigma(r)/(\Sigma(r) + \rho_{\rm ISM} v_{\rm ISM} dt)$ and the ring will migrate to a 