Abstract
(abridged) We study high redshift structure formation and reionization in a LCDM universe under the assumption that the spectral power index of primordial density fluctuations is a function of length scale. We adopt a particular formulation of the running spectral index (RSI) model as suggested by the recent WMAP data. While early structure forms hierarchically in the RSI model, the reduced power on small scales causes a considerable delay in the formation epoch of low mass (~ 10^6 Msun) ``mini-halos'' compared to the LCDM model. The extremely small number of gas clouds in the RSI model indicates that reionization is initiated later than z<15, generally resulting in a smaller total Thomson optical depth than in the LCDM model. By carrying out radiative transfer calculations, we also study reionization by stellar populations formed in galaxies. Even with a top-heavy intial mass function representing an early population of massive stars and/or an extraordinarily high photon emission rate from galaxies, the total optical depth can only be as large as tau ~ 0.1 for reasonable models of early star-formation. The RSI model is thus in conflict with the large Thomson optical depth inferred by the WMAP satellite.
Highlights
The origin of matter density fluctuations in the Universe is one of the fundamental problems in cosmology
We have shown that an early generation of massive, metal-free stars in mini-halos supplemented by “ordinary” star formation at lower redshifts to account for the spectra of high redshift quasars can yield a Thomson optical depth approaching that of the WMAP measurement in a ΛCDM cosmology
Our numerical simulations of early structure formation show that primordial gas cloud formation in low-mass “mini-halos” is very inefficient at z > 15 in the running” spectral index (RSI) model, making it unlikely that the “first stars” contribute significantly to reionization in this scenario
Summary
Structure formation (Zentner & Bullock 2002). The origin of matter density fluctuations in the Universe is one of the fundamental problems in cosmology. We study early structure formation in RSI and ΛCDM universes using cosmological N -body/hydrodynamic simulations By combining these dynamical models with radiative transfer calculations, we compute the epoch of reionization by various stellar sources in the two cosmologies and compare the Thomson optical depth to that measured by the WMAP satellite. We show that the reduced small scale power in the RSI model causes a considerable delay in the formation epoch of low mass halos and the primordial gas clouds within them compared to a ΛCDM counterpart In view of this result, it appears unlikely that massive stars in mini-halos can contribute significantly to reionization in cosmologies with a running spectral index, at least for those with parameters similar to what is inferred from the joint WMAPext + 2dFGRS + Lyman-α forest analysis (Spergel et al 2003). The solid line is the Press-Schechter mass function for the ΛCDM model, and the dashed line is that for the RSI model
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