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28 October 2010

SMBH formation by direct collapse: keeping protogalactic gas H2 free in dark matter haloes with Tvir>10^4 K

Authors: Shang et. al.

The paper presents the estimates of J(crit) values needed for a halo to undergo direct collapse. They perform 3-D hydrodynamical adaptive mesh refinement (AMR) simulations of gas collapse in three different protogalactic halos with Tvir >10e4 K, irradiated by a UV flux with various intensities and spectra.

They then determine the J(crit) required to suppress molecular (hydrogen) cooling in each of the three halos simulated above and find that;[i] for a hard spectrum (metal free stars): J(crit) is between 10e4 to 10e5,[ii] for a softer spectrum (normal stellar population) J(crit) lies between 30 to 300.

The values are ~ 3 to 10 percent lower than previous estimates. They argue that this improved estimate resulted from a better hydrogen molecule-dissociational rate that they adopted. As seen in the Dijkstra paper (see previous post), the reduction in J(crit) exponentially increases the number of rare halos exposed critical radiation; there by preventing fragmentation and ensuing direct collapse. This might give rise to 10e5 solar mass objects at the centre of these haloes- progenitors for SMBH.

1 comment:

  1. The highlight of this paper would be the implementation of a better H2 dissociation rate (as the authors argue).

    Although the change in the corresponding value of Jcrit is only a few percent, from the Dijkstra et. al. 2008 paper (see previous post), this implies an exponential change in the fraction of the haloes that are exposed to this critical value of J.

    The paper can be viewed as a motivation for implementing precise physics (eg. dissociation of H2 molecules, escape fractions etc.) in the simulations, to obtain better Jcrit estimates and ultimately saying something conclusive on the SMBH formation channels.