Suppression and Spatial Variation of Early Galaxies and Minihalos

TL;DR

This paper investigates how the relative velocity between dark matter and baryonic fluids after recombination influences early galaxy and minihalo formation, significantly affecting their abundance, gas content, and spatial distribution.

Contribution

It provides a detailed analytical and simulation-validated study of the impact of relative velocities on early halo properties and their gas fractions across different masses and redshifts.

Findings

Characteristic mass for gas-rich halos increases by an order of magnitude.

Baryon fraction in star-less minihalos is reduced by a factor of 4 at z=20.

Spatial variations in baryon fractions are significantly enhanced.

Abstract

We study the effect of the relative velocity of dark matter and baryonic fluids after the epoch of recombination on the evolution of the first bound objects in the early universe. Recent work has shown that, although relative motion of the two fluids is formally a second order effect in density, it has a dramatic impact on the formation and distribution of the first cosmic structures. Focusing on the gas content, we analyze the effect of relative velocity on the properties of halos over a wide range of halo masses and redshifts. We calculate accurately the linear evolution of the baryon and dark matter fluctuations, and quantify the resulting effect on halos based on an analytical formalism that has been carefully checked with simulations in the case with no relative velocity. We estimate the effect on the abundance of and gas fraction in early halos. We find that the relative velocity effect causes several changes: (i) the characteristic mass that divides gas-rich and gas-poor halos is increased by roughly an order of magnitude, from 2 10^4 Msun to about 2 10^5 Msun; (ii) this characteristic mass has a large scatter (full width at half maximum is ~ 1.5 10^5 Msun at z=20); (iii) the fraction of baryons in star-less gas minihalos is suppressed by a factor of 4 at z=20; (iv) the fraction of baryons in halos that can cool and form stars is suppressed by a factor of 1.5 at z=20; and (v) there are enhanced spatial variations of these various fractions.