Resolving the Formation of Protogalaxies
John Wise (Stanford Univ.) - Sep 25
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Numerous cosmological hydrodynamic studies have addressed the
formation of galaxies. Here we expand the standard model of galaxy
formation to include molecular hydrogen cooling and primordial stellar
feedback with a suite of cosmological Eulerian adaptive mesh
refinement simulations that resolve the Jeans length by at least 16
cells. We gradually introduce molecular hydrogen cooling, radiative
transfer, metal enrichment, and radiative backgrounds to determine the
importance of each process. In simulations that consider the standard
galaxy formation model with only hydrogen and helium cooling,
gravitationally unstable central objects with masses more than 105
solar masses within a radius of 1 pc form within dark matter halos
~108 solar masses. These cores do not fragment down in sub-solar
scales and could form a massive black hole. We also observe that no
rotationally supported disk forms before this central collapse. Then
we introduce molecular hydrogen cooling while suppressing the residual
electron fraction in order to restrict our focus to protogalaxies.
Here protogalactic halos with masses greater than ~8 x 106 solar
masses at z ~ 20 can cool and collapse. Due to the exponential nature
of Press-Schechter formalism, this corresponds to an order of
magnitude increase in protogalaxy number density at z = 20, and galaxy
formation may start earlier than previously thought. Next we consider
radiative feedback from primordial stars using time dependent adaptive
ray tracing that is solved self-consistently with the hydrodynamics,
chemistry, and radiative cooling. This technique retains the time
derivative and is photon conserving. We follow more than 20
primordial stars as they photo-ionize their host halos and the
surrounding few kpc and expel all baryons from their dark matter
halos. The increased electron fraction in the relic HII regions of
primordial stars causes molecular hydrogen to become more abundant and
increases the number of primordial stars by a factor of a few. The
dynamical and thermal feedback from primordial stars affect the
protogalaxy in several ways, e.g. decreasing the baryon content,
skewing the angular momentum distribution to higher values, increasing
the temperature of accreting gas, and enriching the IGM with the first
metals. Our results highlight the importance of the inclusion of
primordial stars and molecular hydrogen cooling in high redshift
galaxy formation models.
The seminar will be held in 544 Campbell Hall.
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