The Role of H₂ Molecules in Cosmological Structure Formation

Summary

We review the relevance of H₂ molecules for structure formation in cosmology. Molecules are important at high–redshifts, when the first collapsed structures appear with typical temperatures of a few hundred Kelvin. In these chemically pristine clouds, radiative cooling is dominated by H₂ molecules. As a result, H₂ “astro–Chemistry” is likely to determine the epoch when the first astrophysical objects appear. We summarize results of recent three–dimensional simulations. A discussion of the effects of feedback, and implications for the reionization of the universe is also given.

Introduction

In current “best–fit” cosmological models, cold dark matter (CDM) dominates the dynamics of structure formation, and processes the initial density fluctuation power spectrum P(K) ∝ kⁿ with n = 1 to predict n = 1 on large scales and n ≈ −3 on small scales (Peebles 1982). The r.m.s. density fluctuation σ_M then varies inversely with the mass-scale (σ_M ∝ M^−2/3 for M » 10¹²M_⊙, while the dependence is only logarithmic for M « 10¹²M_⊙). The more overdense a region, the earlier it collapses, implying that the present structure was built from the bottom up, with smaller objects appearing first, and subsequently merging and/or clustering together to assemble the larger objects (Peebles 1980). The predicted formation epochs of “objects” (i.e. collapsed dark matter halos) with various masses in the so-called standard CDM cosmology (Bardeen et al. 1986) are shown in Figure 1.