The formation and gas content of high-redshift galaxies and minihaloes

Abstract

We investigate the suppression of the baryon density fluctuations compared to the dark matter in the linear regime. Previous calculations predict that the suppression occurs up to a characteristic mass scale of ∼106 M⊙, which suggests that pressure has a central role in determining the properties of the first luminous objects at early times. We show that the expected characteristic mass scale is in fact substantially lower (by a factor of ∼3–10, depending on redshift), and thus the effect of baryonic pressure on the formation of galaxies up to reionization is only moderate. This reduction in the characteristic mass scale is a result of including the fact that the baryons have smoother initial conditions than the dark matter. This smoothing is due to the aftereffects of the coupling with the radiation before recombination. This yields lower pressure gradients than previous estimations in the period from cosmic recombination to z∼ 200, after which the gas cooled down adiabatically and the pressure dropped. At z∼ 10 the suppression of the baryon fluctuations is still sensitive to the history of pressure in this high‐redshift era. We calculate the fraction of the cosmic gas that is in minihaloes and find that it is substantially higher than would be expected with the previously estimated characteristic mass. Expanding our investigation to the non‐linear regime, we calculate in detail the spherical collapse of high‐redshift objects in a Λ cold dark matter universe. We include the gravitational contributions of the baryons and radiation and the memory of their kinematic coupling before recombination. We use our results to predict a more accurate halo mass function as a function of redshift.

Small-scale structure and the Lyman-α forest baryon acoustic oscillation feature

“…Naoz & Barkana 2007), which is ∼ 2 × 10 5 M⊙ depending on the baryon streaming velocity (Tseliakhovich et al 2011). On the other hand, haloes at M > 3 × 10 7 M⊙ have virial temperatures exceeding 10 4 K, and can undergo Lyman-α cooling (and hence may be able to form stars, even in the presence of an H2-dissociating UV background).…”

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

“…Several previous works have extended this theory down to the post-recombination baryon Jeans scale [8,9]. Interest in direct observations of the high-redshift Universe via absorption in the redshifted 21 cm line [10] has motivated more detailed investigation of the clustering of baryons during the epoch between recombination and reionization [11,12], including the entropy and ionization fluctuations in the baryons [13][14][15]. A deficiency of linear perturbation theory is that it does not describe the collapse of perturbations to form bound haloes, although analytical models such as the Press-Schechter formalism [16,17] are often used to estimate the halo mass function and clustering.…”

The rise of the first stars: Supersonic streaming, radiative feedback, and 21-cm cosmology

“…This approach can be described through their relative difference. If we approximately include only the dominant modes, this key quantity decays as [62] …”

“…To define it, we start from the regime of large-scale structure (i.e., scales too large to be affected by pressure, but much smaller than the horizon and the scale of baryon acoustic oscillations), where, as noted above, r LSS does not depend on k, and is simply a function of redshift. On smaller scales, the next-order term describing the difference between the baryons and dark matter is the k 2 term [71], and the filtering wavenumber k F and corresponding mass scale M F are defined through [62] …”