Browsing by Subject "First stars"
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Item The kinetic Sunyaev-Zel’dovich effect as a probe of the physics of cosmic reionization : the effect of self-regulated reionization(2014-12) Park, Hyunbae; Shapiro, Paul R.We calculate the angular power spectrum of the cosmic microwave background temperature fluctuations induced by the kinetic Sunyaev-Zel'dovich (kSZ) effect from the epoch of reionization (EOR). We use detailed N-body+radiative-transfer simulations to follow inhomogeneous reionization of the intergalactic medium. For the first time, we take into account the "self-regulation" of reionization: star formation in low-mass dwarf galaxies or minihalos is suppressed if these halos form in the regions that were already ionized or Lyman-Werner dissociated. Some previous work suggested that the amplitude of the kSZ power spectrum from the EOR can be described by a two-parameter family: the epoch of half-ionization and the duration of reionization. However, we argue that this picture applies only to simple forms of the reionization history which are roughly symmetric about the half-ionization epoch. In self-regulated reionization, the universe begins to be ionized early, maintains a low level of ionization for an extended period, and then finishes reionization as soon as high-mass atomically cooling halos dominate. While inclusion of self-regulation affects the amplitude of the kSZ power spectrum only modestly (~10%), it can change the duration of reionization by a factor of more than two. We conclude that the simple two-parameter family does not capture the effect of a physical, yet complex, reionization history caused by self-regulation. When added to the post-reionization kSZ contribution, our prediction for the total kSZ power spectrum is below the current upper bound from the South Pole Telescope. Therefore, the current upper bound on the kSZ effect from the EOR is consistent with our understanding of the physics of reionization.Item New insights into primordial star formation(2011-08) Stacy, Athena Ranice; Bromm, Volker; Milosavljevic, Milos; Wheeler, John C.; Evans, Neal J.; Dinerstein, Harriet; Loeb, AbrahamThe formation of the first stars, also known as Population III (Pop III), marked a pivotal point in the universe's evolution from relative smoothness and homogeneity to its current highly structured state. In this dissertation we study key aspects of Pop III star formation. We utilize three-dimensional cosmological simulations to follow the evolution of gas and DM from z ~100 until the first minihalo forms. Once the gas infalls toward the center of the minihalo and condenses, we implement the 'sink particle' method to represent regions that will form a star, and we follow the evolution of the metal-free, star-forming gas for many free-fall times. A disk forms around the initial Pop III star and fragments to form secondary stars with a range of masses (1 - 50 [solar mass]). This is markedly different from the previous paradigm of one single, massive star forming per minihalo. Using a ray-tracing technique, we also examine the effect of radiative feedback on protostellar growth and disk fragmentation. This feedback will not prevent the formation of secondary stars within the disk, but will reduce the final mass reached by the largest Pop III star. Measuring the angular momentum of the gas that falls onto the sink regions, we also find that the more massive Pop III stars accrete sufficient angular momentum to rotate at nearly break-up speeds, and can potentially end their lives as collapsar gamma-ray bursts or hypernovae. We furthermore numerically examine the recently discovered relative streaming motions between dark matter and baryons, originating from the era of recombination. Relative streaming will slightly delay the redshift at which Pop III stars first form, but will otherwise have little impact on Pop III star formation and the history of reionization. We finally evaluate the possible effect of a cosmic ray (CR) background generated by the supernova deaths of massive Pop III stars. A sufficiently large CR background could indirectly enhance the H₂ cooling within the affected minihalos. The resulting lower temperatures would lead to a reduced characteristic stellar mass (~ 10 [solar mass]), providing another possible pathway to form low-mass Pop III stars.Item Prospects for directly detecting the first supernovae, and their impact on early star formation(2016-05) Hummel, Jacob Alexander; Bromm, Volker; Milosavljevic, Milos; Wheeler, J. Craig; Finkelstein, Steven; Yoshida, NaokiThe formation of the first stars in the Universe marked a pivotal moment in cosmic history, initiating the transition from the simple initial conditions of the big bang to the complex structures we see today. Ionizing radiation produced by these so-called Population III stars began the process of reionization, and the supernovae marking their deaths initiated the process of chemical enrichment. We assess the prospects for direct detection of the first supernovae should they happen to end their lives as extremely energetic pair-instability supernovae, which should be within the detection limits of the upcoming James Webb Space Telescope. Using a combination of semi-analytic models and cosmological simulations to estimate their source density, we find that the primary obstacle to observing such events is their scarcity, not their faintness. The first supernovae and the compact remnants they leave behind also produce significant amounts of high-energy X-rays and cosmic rays able to travel through the predominantly neutral intergalactic medium and build up a cosmic background. To better understand how these violent explosions impact subsequent episodes of metal-free star formation, we employ ab-initio, cosmological hydrodynamics simulations to model the formation of stars in a minihalo at z = 20-30 under the influence of both an X-ray and cosmic ray background. The presence of an ionizing background---whether X-rays or cosmic rays---serves to expedite the collapse of gas to high densities by enhancing molecular hydrogen cooling, thus allowing stars to form at substantially earlier epochs in strongly irradiated minihalos. The mass of the stars thus formed however appears to be quite robust, maintaining a characteristic mass of order a few tens of solar masses even as the strength of the ionizing background varies by several orders of magnitude. Finally, we describe the novel software developed to enable this research. These tools for manipulating and analyzing simulation data have been released as the open-source GAdget DataFrame Library: gadfly.