Motivated by recent observations of galaxies dominated by emission lines, which show evidence of being metal poor with young stellar populations, we present calculations of multiple model grids with a range of abundances, ionization parameters, and s
tellar ages, finding that the predicted spectral line diagnostics are heavily dependent on all three parameters. These new model grids extend the ionization parameter to larger values than typically explored. We compare these model predictions with previous observations of such objects, including two new Lyman-$alpha$ emitting galaxies (LAE) that we have observed. Our models give improved constraints on the metallicity and ionization parameter of these previously studied objects, as we are now able to consider high ionization parameter models. However, similar to previous work, these models have difficulty predicting large line diagnostics for high ionization potential species, requiring future work refining the modelling of FUV photons. Our model grids are also able to constrain the metallicity and ionization parameter of our LAEs, and give constraints on their Ly$alpha$ escape fractions, all of which are consistent with recent lower redshift studies of LAEs.
The early Universe hosted a large population of small dark matter `minihalos that were too small to cool and form stars on their own. These existed as static objects around larger galaxies until acted upon by some outside influence. Outflows, which h
ave been observed around a variety of galaxies, can provide this influence in such a way as to collapse, rather than disperse the minihalo gas. Gray & Scannapieco performed an investigation in which idealized spherically-symmetric minihalos were struck by enriched outflows. Here we perform high-resolution cosmological simulations that form realistic minihalos, which we then extract to perform a large suite of simulations of outflow-minihalo interactions including non-equilibrium chemical reactions. In all models, the shocked minihalo forms molecules through non-equilibrium reactions, and then cools to form dense chemically homogenous clumps of star-forming gas. The formation of these high-redshift clusters will be observable with the next generation of telescopes, and the largest of them should survive to the present day, having properties similar to halo globular clusters.
In the early universe, substantial relative stream velocities between the gas and dark matter arise due to radiation pressure and persist after recombination. To asses the impact of these velocities on high-redshift structure formation, we carry out
a suite of high-resolution Adaptive Mesh Refinement (AMR) cosmological simulations, which use Smoothed Particle Hydrodynamic datasets as initial conditions, converted using a new tool developed for this work. These simulations resolve structures with masses as small as a few 100 M$_odot$, and we focus on the $10^6$ M$_odot$ mini-halos in which the first stars formed. At $z approx 17,$ the presence of stream velocities has only a minor effect on the number density of halos below $10^6$ M$_odot$, but it greatly suppresses gas accretion onto all halos and the dark matter structures around them. Stream velocities lead to significantly lower halo gas fractions, especially for $approx 10^5$ M$_odot$ objects, an effect that is likely to depend on the orientation of a halos accretion lanes. This reduction in gas density leads to colder, more compact radial profiles, and it substantially delays the redshift of collapse of the largest halos, leading to delayed star formation and possibly delayed reionization. These many differences suggest that future simulations of early cosmological structure formation should include stream velocities to properly predict gas evolution, star-formation, and the epoch of reionization.
