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[1005.2416] Relative velocity of dark matter and baryonic fluids and the formation of the first structures

Authors:  Dmitriy Tseliakhovich, Christopher Hirata 
Abstract:  At the time of recombination, baryons and photons decoupled and the sound
speed in the baryonic fluid dropped from relativistic to the thermal velocities
of the hydrogen atoms. This is less than the relative velocities of baryons and
dark matter computed via linear perturbation theory, so we infer that there are
supersonic coherent flows of the baryons relative to the underlying potential
wells created by the dark matter. As a result, the advection of smallscale
perturbations (near the baryonic Jeans scale) by largescale velocity flows is
important for the formation of the first baryonic structures. This effect
involves a quadratic term in the cosmological perturbation theory equations and
hence has not been included in studies based on linear perturbation theory. We
show that the relative motion suppresses the abundance of the first bound
objects, even if one only investigates dark matter haloes, and leads to
qualitative changes in their spatial distribution, such as introducing
scaledependent bias and stochasticity. We discuss the possible observable
implications for highredshift galaxy clustering and reionization. 

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Antony Lewis
Joined: 23 Sep 2004 Posts: 1327 Affiliation: University of Sussex

Posted: May 17 2010 


This is a very interesting paper, which looks at the secondorder effect of baryon velcoties on the smallscale matter power spectrum and halo formation. The basic idea is that in regions of space where the baryon velocity was large at the end of recombination the fast bulk flow of the baryons relative to the smallscale dark matter perturbations washes away any baryon overdensity and hence suppresses growth of structure. The effect is therefore a bit like local nonGaussianity, where here the smallscale matter perturbations are modulated by the largescale baryon velocities. The paper finds a large effect on the bias, which become scaledependent.
I was confused by Fig 2, but I think this is rather than , so there is a falloff in the linear result about the Jeans scale though I can't quite reproduce the same shape for the linear result. 

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