Title: Chemistry and Radiative Feedback of Early Galaxies
Description: The abundance of molecular hydrogen (H2), the primary coolant in primordial gas, is critical for the thermodynamic evolution and star–formation histories in early protogalaxies. Suppression of H2–cooling in early protogalaxies can occur via photodissociation of H2 (by ultraviolet Lyman–Werner [LW] photons) or by photodetachment of H−, a precursor in H2 formation (by infrared [IR] photons). It is widely believed that the formation of the first massive black hole “seeds,” with masses 104−6 M⊙, in primordial halos may be enabled if H2–cooling is suppressed. We study the radiative feedback processes that suppress H2–cooling in primordial proto- galaxies. Previous studies have typically adopted idealized spectra, with a blackbody or a power–law shape, in modeling the chemistry of metal–free protogalaxies, and utilized a single parameter, the critical UV flux, or Jcrit, to determine whether H2–cooling is prevented. This can be misleading, as independent of the spectral shape, there is a a critical curve in the (kLW,kH−) plane, where kLW and kH− are the H2–dissociation rates by LW and IR photons, which determines whether a protogalaxy can cool below ∼ 1000 Kelvin.

In Chapter 1, we use a one–zone model to follow the chemical and thermal evolution of gravitationally collapsing protogalactic gas, to compute this critical curve, and provide an accurate analytical fit for it. We improve on previous works by considering a variety of more realistic Pop III or Pop II-type spectra from population synthesis models and perform fully frequency–dependent calculations of the H2–photodissociation rates for each spectrum. We compute the ratio kLW/kH− for each spectrum, as well as the minimum stellar mass M∗, for various IMFs and metallicities, required to prevent cooling in a neighboring halo a distance d away. We provide critical M∗/d2 values for suppression of H2–cooling, with analytic fits, which can be used in future studies. Determining the photodissociation rate of H2 by an incident LW flux is crucial, but prohibitively expensive to calculate on the fly in simulations. The rate is sensitive to the H2 rovibrational distribution, which in turn depends on the gas density, temperature, and incident LW radiation field.

In Chapter 2, we use the publicly available cloudy package to model primordial gas clouds and compare exact photodissociation rate calculations to commonly–used fitting formulae. We find the fit from Wolcott-Green et al. (2011) is most accurate for moderate densities n ∼ 103cm−3 and temperatures, T ∼ 103K, and we provide a new fit, which captures the increase in the rate at higher densities and temperatures, owing to the increased excited rovibrational populations in this regime. Our new fit has typical errors of a few percent percent up to n ≤ 107 cm−3, T ≤ 8000K, and H2 column density NH2 ≤ 1017 cm−2, and can be easily utilized in simulations. We also show that pumping of the excited rovibrational states of H2 by a strong LW flux further modifies the level populations when the gas density is low, and noticeably decreases self-shielding for J21 > 103 and n < 102cm−3. This may lower the “critical flux” at which primordial gas remains H2–poor in some protogalaxies, enabling massive black hole seed formation.

In Chapter 3, we study the thermal evolution of UV–irradiated atomic cooling halos using high–resolution three–dimensional hydrodynamic simulations. We consider the effect of H− photodetachment by Lyα cooling radiation in the optically–thick cores of three such halos, a process which has not been included in previous simulations. H− is a precursor of molecular hydrogen, and therefore, its destruction can diminish the H2 abundance and cooling. We find that the critical UV flux for suppressing H2–cooling is decreased by up to a factor of a few when H− photodetachment by Lyα is included. In a more conservative estimate of the trapped Lyα energy density, we find the critical flux is decreased by ∼ 15 − 50 per cent. Our results suggest that Lyα radiation may have an important effect on the thermal evolution of UV–irradiated halos, and therefore on the potential for massive black hole formation.