Browsing by Subject "Intergalactic medium"
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Item Constraining the end of reionization with ly [alpha] spectroscopy(2019-07-29) Jung, Intae; Finkelstein, Steven L.; Dickinson, Mark; Bromm, Volker; Gebhardt, Karl; Casey, Caitlin MThe reionization of the intergalactic medium (IGM) marks the time in the early universe when the first stars and galaxies began to affect the universe around them, as during this last major phase transition high-energy ultraviolet photons from these objects ionized the gas in the IGM, and it remains ionized to the present day. Studying reionization is a key frontier in observational cosmology, as it can therefore provide key insights into the formation and evolution of galaxies in the early universe. As Lyα photons are resonantly scattered by neutral hydrogen in the IGM, an analysis of this line can be used to trace the existence of neutral hydrogen in the IGM at different points in the history of the universe (i.e., when the IGM becomes neutral, we should stop seeing these photons, as they are likely scattered out of our line-of-sight). The work in this dissertation focuses on completing a spectroscopic survey of galaxies in the early universe, to measure the Lyα equivalent width (EW) distribution into the epoch of reionization and investigate the evolution of the IGM during reionization, pinning down the late time-evolution of reionization. To measure the Lyα EW distribution in the reionization era, we utilize deep spectroscopic observations of candidate galaxies from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) using both the DEIMOS (optical) and MOSFIRE (near-infrared; NIR) spectrographs on the Keck telescopes. Our large spectroscopic dataset compiled with both Keck telescope observations represent the deepest and most complete spectroscopic survey for galaxies in the epoch of reionization. We study the Lyα emission strength through constraining the Lyα equivalent width (EW) distribution with our spectroscopic dataset by constructing detailed simulations of mock emission lines, accounting for the observational conditions (e.g., exposure time, wavelength coverage, and sky emission) and galaxy photometric redshift probability distribution functions. The measurements of the EW distribution with the detected Lyα emission lines from our DEIMOS and MOSFIRE observations provide additional evidence that the Lyα EW distribution declines at z > 6, suggesting an increasing fraction of neutral hydrogen in the IGM. Thanks to plenty of high-quality observational data from space telescopes such as the Hubble Space Telescope and the Spitzer Space Telescope, observational studies of the evolution of galaxies in the early universe have been performed over the past decade. Those studies have revealed statistical trends of the star formation history and the evolution of those galaxies over the cosmic time. However, since the star formation happens in a complex way, an analysis of the integrated properties of galaxies is not enough to grasp the physical processes governing the star formation and the growth of galaxies. The advent of a spatially resolved study of each individual galaxy has provided us an excellent approach to explore star formation processes inside a galaxy and to examine the inside-out / outside-in growth scenarios of galaxies. We perform a spatially resolved study of galaxies in the early universe at z ≳ 4 using the CANDELS Survey and HAWK-I UDS GOODS (HUGS) survey data. We estimate stellar mass, star formation rate, and dust extinction for galaxy inner and outer regions via spatially resolved spectral energy distribution fitting based on a Markov Chain Monte Carlo algorithm. By comparing specific star formation rates (sSFRs) between inner and outer parts of the galaxies we find that the majority of galaxies with high central mass densities show evidence for a preferentially lower sSFR in their centers than in their outer regions, indicative of reduced sSFRs in their central regionsItem Development of a new low resolution spectrograph for probing Lyman-alpha emitters in the HETDEX survey(2011-08) Chonis, Taylor Steven; Gebhardt, Karl; Hill, Gary J.The Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) will map the power spectrum of 0.8 million blindly discovered Lyman-alpha Emitting Galaxies (LAE) using a revolutionary new array of massively replicated, fiber-fed spectrographs dubbed the Visible Integral-Field Replicable Unit Spectrograph (VIRUS). In the era of the Hobby-Eberly Telescope wide-field upgrade, the current Low Resolution Spectrograph (LRS) must be replaced with a fiber instrument. In this thesis, I discuss the development of the second generation LRS (LRS2), which is an R>1200 multi-channel instrument based on the VIRUS design and fed by a 287 fiber, 7” x 12” microlens coupled integral field unit. I focus on the blue optimized version of the instrument (3720<[lamda] (Angstroms)<7000), specifically on the opto-mechanical design of the VPH grisms. With the purpose of making the instrument ideal for the follow-up of LAE in the HETDEX survey, I discuss the science drivers for selecting the spectral resolution of the instrument. To test the utility of such an instrument, I present R~2400 spectra of two LAE that were originally discovered in the HETDEX Pilot Survey (Adams et al. 2011). These data were taken with the VIRUS prototype spectrograph in a high-resolution mode at the McDonald Observatory Harlan J. Smith 2.7 m telescope. The Lyman-alpha line profiles are constrained by near-infrared observations of rest-frame optical emission lines from Finkelstein et al. (2011), which set the systemic redshift of the galaxies. I discuss the velocity offsets of the Lyman-alpha line from the systemic line center and the implications for the HETDEX survey. I compare the line profiles to theory, specifically to those describing dust attenuation, outflows or inflows of neutral gas on the galactic scale, and attenuation in the intergalactic medium. This study provides an example of how LRS2 can be used to probe Lyman-alpha emission in 2Item 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 Maximal X-ray feedback in the pre-reionization universe(2023-08-04) Jeon, Junehyoung; Bromm, Volker; Finkelstein, Steven LX-ray feedback in the pre-reionization Universe provided one of the major energy sources for reionization and the thermal evolution of the early intergalactic medium. However, X-ray sources at high redshift have remained largely inaccessible to observations. One alternative approach to study the overall effect of X-ray feedback in the early Universe is a full cosmological simulation. Toward this goal, in this paper we create an analytic model of X-ray feedback from accretion onto supermassive black holes (SMBHs), to be used as a sub-grid model in future cosmological simulations. Our analytic model provides a relation between the mass of a dark matter halo and the SMBH it hosts, where the efficiency is governed by an energy balance argument between thermal feedback and the confining gravitational potential of the halo. To calibrate the model, we couple the halo-level recipe with the Press-Schechter halo mass function and derive global mass and energy densities. We then compare our model to various observational constraints, such as the resulting soft X-ray and IR cosmic radiation backgrounds, to test our choice of model parameters. We in particular derive model parameters that do not violate any constraints, while providing maximal X-ray feedback prior to reionization. In addition, we consider the contribution of SMBH X-ray sources to reionization and the global 21 cm absorption signal.