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Open Access Publications from the University of California

Scholarly Works (5 results)

  • Thesis
  • Peer Reviewed
UCLA Electronic Theses and Dissertations (2012)

Recent observations suggest that quasars fully ionize helium around redshift z ~ 3. Most directly, the optical depth derived from the He II Lyman-α (Lyα) forest fluctuates significantly above and near z ~ 2.8 and then evolves smoothly at lower redshifts. These fluctuations may be indicative of patchy He II reionization, which is expected (primarily) due to the rarity and brightness of quasars and the inhomogeneous intergalactic medium. This dissertation presents approximate methods for interpreting observations of this epoch and quantifying the magnitude of fluctuations.

From these opacity measurements, we compute, using a semi-analytic model, the evolution of the mean photoionization rate and attenuation length for helium over the redshift range 2.0 ≤ z ≤ 3.2. The average effective optical depth indicates a sharp decrease in the photoionization rate and, provided the quasar emissivity evolves smoothly, in the effective attenuation length at z ~ 2.8. A simple model with reionization ending at z ~ 2.7 is most consistent with the data, although the constraints are weak thanks to sparse, uncertain data.

We further present a fast, semi-numeric procedure to approximate detailed cosmological simulations. We produce the distribution of dark matter halos, ionization state of helium, and density field at z = 3 in broad agreement with recent simulations. Given our speed and flexibility, we investigate a range of source and active quasar prescriptions. We find order-of-magnitude fluctuations in the He II ionization rate in the post-reionization Universe. During reionization, the fluctuations are even stronger and develop a bimodal distribution, in contrast with semi-analytic models and the hydrogen equivalent. We also examine large-scale fluctuations in the He II Lyα forest transmission during and after He II reionization. Like previous studies, we show that the rarity of quasars and the inhomogeneous intergalactic medium induce order unity fluctuations in the mean optical depth after reionization, even when averaged over large segments (~10-100 Mpc in length). During reionization, we find a broader distribution with an extended high-opacity tail. These conclusions are fairly insensitive underlying model assumptions, including quasar properties and the mean free path of photons. We compare to existing data along ~10 He II Lyα forest lines of sight spanning z ~ 2 - 3.2, demonstrating that the observed fluctuations are likely due to ongoing reionization.

    Cover page: Inhomogeneous He II Reionization and its Implications for the Intergalactic Medium
    • Article
    • Peer Reviewed
    UCLA Previously Published Works (2014)
    Recent He II Lyman a forest observations within the range 2.0 ≤ z ≤ 3.2 show large fluctuations in the optical depth at z ≥ 2.7. These results point to a fluctuating He-ionizing background, which may be due to the end of helium reionization of this era. We present a fast, semi-numeric procedure to approximate detailed cosmological simulations. We compute the distribution of dark matter haloes, ionization state of helium, and density field at z = 3, which are in broad agreement with recent simulations. Given our speed and flexibility, we investigate a range of ionizing source and active quasar prescriptions. Spanning a large area of parameter space, we find order-of-magnitude fluctuations in the He II ionization rate in the post-reionization regime. During reionization, the fluctuations are even stronger and develop a bimodal distribution, in contrast to semi-analytic models and the hydrogen equivalent. These distributions indicate a low-level ionizing background even at significant He II fractions. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.
      Cover page: Semi-numeric simulations of helium reionization and the fluctuating radiation background
      • Article
      • Peer Reviewed
      UCLA Previously Published Works (2017)
      Large variations in the effective optical depth of the He II Ly$\alpha$ forest have been observed at $z\gtrsim2.7$, but the physical nature of these variations is uncertain: either the Universe is still undergoing the process of He II reionization, or the Universe is highly ionized but the He II-ionizing background fluctuates significantly on large scales. In an effort to build upon our understanding of the latter scenario, we present a novel model for the evolution of ionizing background fluctuations. Previous models have assumed the mean free path of ionizing photons to be spatially uniform, ignoring the dependence of that scale on the local ionization state of the intergalactic medium (IGM). This assumption is reasonable when the mean free path is large compared to the average distance between the primary sources of He II-ionizing photons, $\gtrsim L_\star$ quasars. However, when this is no longer the case, the background fluctuations become more severe, and an accurate description of the average propagation of ionizing photons through the IGM requires additionally accounting for the fluctuations in opacity. We demonstrate the importance of this effect by constructing 3D semi-analytic models of the helium ionizing background from $z=2.5$-$3.5$ that explicitly include a spatially varying mean free path of ionizing photons. The resulting distribution of effective optical depths at large scales in the He II Ly$\alpha$ forest is very similar to the latest observations with HST/COS at $2.5 \lesssim z \lesssim 3.5$.
        Cover page: A self-consistent 3D model of fluctuations in the helium-ionizing background
        • Article
        • Peer Reviewed
        LBL Publications (2021)
        ABSTRACT: The 21-cm signal from the Cosmic Dawn (CD) is likely to contain large fluctuations, with the most extreme astrophysical models on the verge of being ruled out by observations from radio interferometers. It is therefore vital that we understand not only the astrophysical processes governing this signal, but also other inherent processes impacting the signal itself, and in particular line-of-sight effects. Using our suite of fully numerical radiative transfer simulations, we investigate the impact on the redshifted 21-cm from the CD from one of these processes, namely the redshift-space distortions (RSDs). When RSDs are added, the resulting boost to the power spectra makes the signal more or equally detectable for our models for all redshifts, further strengthening hopes that a power spectra measurement of the CD will be possible. RSDs lead to anisotropy in the signal at the beginning and end of the CD, but not while X-ray heating is underway. The inclusion of RSDs, however, decreases detectability of the non-Gaussianity of fluctuations from inhomogeneous X-ray heating as measured by the skewness and kurtosis. On the other hand, mock observations created from all our simulations that include telescope noise corresponding to 1000 h of observation with the Square Kilometre Array telescope show that we may be able to image the CD for all heating models considered and suggest RSDs dramatically boost fluctuations coming from the inhomogeneous Ly α background.
          Cover page: Redshift-space distortions in simulations of the 21-cm signal from the cosmic dawn
          • Article
          • Peer Reviewed
          LBL Publications (2019)
          The upcoming radio interferometer Square Kilometre Array (SKA) is expected to directly detect the redshifted 21-cm signal from the neutral hydrogen present during the Cosmic Dawn. Temperature fluctuations from X-ray heating of the neutral intergalactic medium can dominate the fluctuations in the 21-cm signal from this time. This heating depends on the abundance, clustering, and properties of the X-ray sources present, which remain highly uncertain. We present a suite of three new large-volume, 349\,Mpc a side, fully numerical radiative transfer simulations including QSO-like sources, extending the work previously presented in Ross et al. (2017). The results show that our QSOs have a modest contribution to the heating budget, yet significantly impact the 21-cm signal. Initially, the power spectrum is boosted on large scales by heating from the biased QSO-like sources, before decreasing on all scales. Fluctuations from images of the 21-cm signal with resolutions corresponding to SKA1-Low at the appropriate redshifts are well above the expected noise for deep integrations, indicating that imaging could be feasible for all the X-ray source models considered. The most notable contribution of the QSOs is a dramatic increase in non-Gaussianity of the signal, as measured by the skewness and kurtosis of the 21-cm probability distribution functions. However, in the case of late Lyman-$\alpha$ saturation, this non-Gaussianity could be dramatically decreased particularly when heating occurs earlier. We conclude that increased non-Gaussianity is a promising signature of rare X-ray sources at this time, provided that Lyman-$\alpha$ saturation occurs before heating dominates the 21-cm signal.
            Cover page: Evaluating the QSO contribution to the 21-cm signal from the Cosmic Dawn
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