Browsing by Subject "cosmic reionization"

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The First Galaxies: Chemical Enrichment, Mixing, And Star Formation
(2010-06) Greif, Thomas H.; Glover, Simon C. O.; Bromm, Volker; Klessen, Ralf S.; Bromm, Volker
Using three-dimensional cosmological simulations, we study the assembly process of one of the first galaxies, with a total mass of similar to 10(8) M(circle dot), collapsing at z similar or equal to 10. Our main goal is to trace the transport of the heavy chemical elements produced and dispersed by a pair-instability supernova exploding in one of the minihalo progenitors. To this extent, we incorporate an efficient algorithm into our smoothed particle hydrodynamics code that approximately models turbulent mixing as a diffusion process. We study this mixing with and without the radiative feedback from Population III (Pop III) stars that subsequently form in neighboring minihalos. Our simulations allow us to constrain the initial conditions for second-generation star formation, within the first galaxy itself, and inside of minihalos that virialize after the supernova explosion. We find that most minihalos remain unscathed by ionizing radiation or the supernova remnant, while some are substantially photoheated and enriched to supercritical levels, likely resulting in the formation of low-mass Pop III or even Population II (Pop II) stars. At the center of the newly formed galaxy, similar to 10(5) M(circle dot) of cold, dense gas uniformly enriched to similar to 10(-3) Z(circle dot) is in a Stateof collapse, suggesting that a cluster of Pop II stars will form. The first galaxies, as may be detected by the James Webb Space Telescope, would therefore already contain stellar populations familiar from lower redshifts.
Rapid Decline Of Ly Alpha Emission Toward The Reionization Era
(2014-10) Tilvi, Vithal; Papovich, Casey; Finkelstein, Steven L.; Long, J.; Song, Mimi; Dickinson, Mark; Ferguson, Henry C.; Koekemoer, A. M.; Giavalisco, Mauro; Mobasher, Bahram; Finkelstein, Steven L.; Song, Mimi
The observed deficit of strongly Ly alpha emitting galaxies at z > 6.5 is attributed to increasing neutral hydrogen in the intergalactic medium (IGM) and/or to the evolving galaxy properties. To investigate this we have performed very deep near-IR spectroscopy of z greater than or similar to 7 galaxies using MOSFIRE on the Keck-I Telescope. We measure the Lya fraction at z similar to 8 using two methods. First, we derived N-Ly alpha/N-tot directly, using extensive simulations to correct for incompleteness. Second, we used a Bayesian formalism (introduced by Treu et al.) that compares the z > 7 galaxy spectra to models of the Ly alpha equivalent width (WLy alpha) distribution at z similar to 6. We explored two simple evolutionary scenarios: pure number evolution where Lya is blocked in some fraction of galaxies (perhaps due to the IGM being opaque along only some fraction of sightlines) and uniform dimming evolution where Ly alpha is attenuated in all galaxies by a constant factor (perhaps owing to processes from galaxy evolution or a slowly increasing IGM opacity). The Bayesian formalism places stronger constraints compared with the direct method. Combining our data with that in the literature, we find that at z similar to 8 the Lya fraction has dropped by a factor of > 3 (84% confidence interval) using both the dimming and number evolution scenarios, compared to the z similar to 6 values. Furthermore, we find a tentative positive Bayesian evidence favoring the number evolution scenario over dimming evolution, extending trends observed at z less than or similar to 7 to higher redshift. A comparison of our results with theoretical models implies the IGM volume averaged neutral hydrogen fraction greater than or similar to 0.3, suggesting that we are likely witnessing reionization in progress at z similar to 8.
The Theory And Simulation Of The 21-Cm Background From The Epoch Of Reionization
(2008-08) Shapiro, P. R.; Iliev, I. T.; Mellerna, G.; Pen, U. L.; Merz, H.; Shapiro, Paul R.
The redshifted 21-cm line of distant neutral H atoms provides a probe of the cosmic >dark ages> and the epoch of reionization (>EOR>) which ended them, within die first billion years of cosmic time. The radio continuum produced by this redshifted line can be seen in absorption or emission against die cosmic microwave background (>CMB>) at meterwaves, yielding information about tire thermal and ionization history of the universe and the primordial density perturbation spectrum that led to galaxy and large-scale structure formation. Observing this 21-cm background is a great challenge,as it is necessary to detect a diffuse signal at a brightness temperature that differs from that of die CM B at millikelvin levels and distinguish this from foreground continuum sources. A new generation of low-frequency radio arrays is currently under development to search for this background. Accurate theoretical predictions of tire spectrum and anisotropy of this back-ground, necessary to guide and interpret future observations, are also quite challenging. Toward this end, it is necessary to model the inhomogeneous reionization of the intergalactic medium and determine the spin temperature of the 21-cm transition and its variations in time and space as it decouples from the temperature of the CMB, In my talk, I summarized some of the theoretical progress in this area. Here, I will focus on just a few of the predictions for the 21-cm background from the EOR, based on our newest, large-scale simulations of patchy reionization. These simulations are the first with enough N-body particles (from 5 to 29 billion) and radiative transfer rays to resolve the formation of and trace the ionizing radiation front each of the millions of dwarf galaxies believed responsible for reionization, down to 10(8) M-circle dot, in a cubic volume large enough (90 and 163 comoving Mpc on a side) to make meaningful statistical predictions of the fluctuating 21-cm background.