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Open Access Publications from the University of California

Scholarly Works (36 results)

  • Thesis
  • Peer Reviewed
UC Santa Barbara Electronic Theses and Dissertations (2024)

One of the great successes of modern cosmology is the percent-level concordance between theory and observations of the intergalactic medium (IGM) at z ≳ 1.7. Yet, the Lyα forest at z < 1.7, which can only be studied via HST UV spectra, has pointed out a puzzling discrepancy, i.e., the Doppler b-parameters of these absorption lines are, on average, ∼ 10 km/s wider than those in any existing hydrodynamic simulation. This discrepancy implies that the low-z IGM might be substantially hotter than expected, contradicting one of the fundamental predictions in current cosmology that the IGMshould cool down owing to the Hubble expansion after He ii reionization ( z < 2.5). Moreover, the IGM thermal state degenerates with its ionization state characterized by the UV background (UVB) photoionization rate, ΓHI , which dictates the abundance of the Lyα absorbers, dN /dz. Such a degeneracy requires any reliable measurement to adopt a careful statistical inference procedure. To overcome these difficulties, in this thesis, a novel machine-learning-based inference framework is employed to jointly measure the thermal and ionization state of the IGM, using the 2D distribution of b-parameter and H i column density and dN /dz. This method effectively resolves the degeneracies between the thermal and ionization state of the IGM and achieves high precision, even with limited-sized data. I apply this method to 94 archival HST COS and STIS quasar spectra distributed across the seven redshift bins, yielding a comprehensive evolutionary history of the IGM thermal and ionization state at z < 1.5. The results suggest that the IGM may be significantly hotter than previously expected at low-z and is potentially isothermal, with IGM temperature at mean density, T0 ∼ 30, 000K and power-law index of the temperature-density, γ ∼ 1.0 at z = 0.1. The inferred thermal history suggests that this unexpected IGM temperature possibly emerges around z ∼ 1. Additionally, while the ΓHI measurements align with the theoretical model at z ∼ 1, the values measured at z < 0.5 are substantially lower than predicted, posing challenges to low-z UV background synthesis models.

    Cover page: Deciphering the Thermal and Ionization State of the Intergalactic Medium over the Past 10 Billion Years
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2013)
    Although the baryons in the intergalactic medium (IGM) trace dark matter fluctuations on megaparsec scales, on smaller scales ∼100 kpc, fluctuations are suppressed because the finite temperature gas is pressure supported against gravity, analogous to the classical Jeans argument. This Jeans filtering scale, which quantifies the small-scale structure of the IGM, has fundamental cosmological implications. First, it provides a thermal record of heat injected by ultraviolet photons during cosmic reionization events, and thus constrains the thermal and reionization history of the universe. Second, the Jeans scale determines the clumpiness of the IGM, a critical ingredient in models of cosmic reionization. Third, it sets the minimum mass scale for gravitational collapse from the IGM, and hence plays a pivotal role in galaxy formation. Unfortunately, it is extremely challenging to measure the Jeans scale via the standard technique of analyzing purely longitudinal Lyα forest spectra, because the thermal Doppler broadening of absorption lines along the line-of-sight, is highly degenerate with Jeans smoothing. In this work, we show that the Jeans filtering scale can be directly measured by characterizing the coherence of correlated Lyα forest absorption in close quasar pairs, with separations small enough ∼100 kpc to resolve it. We present a novel technique for this purpose, based on the probability density function (PDF) of phase angle differences of homologous longitudinal Fourier modes in close quasar pair spectra. A Bayesian formalism is introduced based on the phase angle PDF, and Markov Chain Monte Carlo techniques are used to characterize the precision of a hypothetical Jeans scale measurement, and explore degeneracies with other thermal parameters governing the IGM. A semi-analytical model of the Lyα forest is used to generate a large grid (500) of thermal models from a dark matter only simulation. Our full parameter study indicates that a realistic sample of only 20 close quasar pair spectra can pinpoint the Jeans scale to ≃ 5% precision, independent of the amplitude T 0 and slope γ of the temperature-density relation of the IGM . This exquisite sensitivity arises because even long-wavelength one-dimensional Fourier modes ∼10 Mpc, i.e., two orders of magnitude larger than the Jeans scale, are nevertheless dominated by projected small-scale three-dimensional (3D) power. Hence phase angle differences between all modes of quasar pair spectra actually probe the shape of the 3D power spectrum on scales comparable to the pair separation. We show that this new method for measuring the Jeans scale is unbiased and is insensitive to a battery of systematics that typically plague Lyα forest measurements, such as continuum fitting errors, imprecise knowledge of the noise level and/or spectral resolution, and metal-line absorption. © 2013. The American Astronomical Society. All rights reserved..
      Cover page: A NEW METHOD TO DIRECTLY MEASURE THE JEANS SCALE OF THE INTERGALACTIC MEDIUM USING CLOSE QUASAR PAIRS
      • Article
      • Peer Reviewed
      LBL Publications (2017)
      The ultraviolet background (UVB) emitted by quasars and galaxies governs the ionization and thermal state of the intergalactic medium (IGM), regulates the formation of high-redshift galaxies, and is thus a key quantity for modeling cosmic reionization. The vast majority of cosmological hydrodynamical simulations implement the UVB via a set of spatially uniform photoionization and photoheating rates derived from UVB synthesis models. We show that simulations using canonical UVB rates reionize and, perhaps more importantly, spuriously heat the IGM, much earlier (z ∼15) than they should. This problem arises because at z > 6, where observational constraints are nonexistent, the UVB amplitude is far too high. We introduce a new methodology to remedy this issue, and we generate self-consistent photoionization and photoheating rates to model any chosen reionization history. Following this approach, we run a suite of hydrodynamical simulations of different reionization scenarios and explore the impact of the timing of reionization and its concomitant heat injection on the thermal state of the IGM. We present a comprehensive study of the pressure smoothing scale of IGM gas, illustrating its dependence on the details of both hydrogen and helium reionization, and argue that it plays a fundamental role in interpreting Ly α forest statistics and the thermal evolution of the IGM. The premature IGM heating we have uncovered implies that previous work has likely dramatically overestimated the impact of photoionization feedback on galaxy formation, which sets the minimum halo mass able to form stars at high redshifts. We make our new UVB photoionization and photoheating rates publicly available for use in future simulations.
        Cover page: Self-consistent Modeling of Reionization in Cosmological Hydrodynamical Simulations
        • Thesis
        • Peer Reviewed
        UC Santa Barbara Electronic Theses and Dissertations (2024)

        Understanding the reionization of the intergalactic medium (IGM) by the first luminous sources remains an important open problem in cosmology. During Reionization ionization fronts propagate through the IGM, heating the reionized gas. This heat injection can be observed over a redshift interval of ∆z ∼ 1 due to the long cooling times in the low-density IGM. Simultaneously, the mean free path of ionizing photons (λmfp) describing the ultraviolet background (UVB) rapidly evolves as bubbles of reionized gas, where the UVB is stronger, merge. Thus, constraining the thermal state of the IGM and the evolution of λmfp can, in turn, be used to constrain reionization. Transmission in the Lyman-α (Lyα) forest, the ubiquitous Lyα absorption lines produced by residual neutral hydrogen in the IGM along quasar sightlines, offers a powerful tool to investigate these phenomena.

        Here I present studies that look into utilizing the z > 5 Lyα forest to constrain Reionization via the thermal state of the IGM and λmfp. First, I use wavelet analysis as an initial clustering statistic and shows improvement on simulated temperature constraints when compared to measurements from the 1D Lyα forest power spectrum. This work also notes the importance of careful accounting of the correlations between datasets to ensure observational constraints are not over-confident. The next two studies use the Lyα forest auto-correlation function at z ≥ 5.4 as the clustering statistic. The second study demonstrates the potential in constraining λmfp from simulated data while the third does the same for the temperature. Both of these do careful statistical inference tests to ensure the results are not over-confident and describe a method to guarantee statistical robustness of measurements. The final study makes measurements of the Lyα forest auto-correlation functions at z > 5 from the XQR-30 data set. Preliminary comparisons of these measurements to the models from the second study are done, showing agreement with previous measurements of λmfp. Each of these contribute to our understanding of the high-z Lyα forest and how it can be used to constrain Reionization.

          Cover page: Constraining Reionization with the High-z Lyman-α Forest
          • Article
          • Peer Reviewed
          LBL Publications (2016)
          Cosmological hydrodynamic simulations can accurately predict the properties of the intergalactic medium (IGM), but only under the condition of retaining the high spatial resolution necessary to resolve density fluctuations in the IGM. This resolution constraint prohibits simulating large volumes, such as those probed by BOSS and future surveys, like DESI and 4MOST. To overcome this limitation, we present "Iteratively Matched Statistics" (IMS), a novel method to accurately model the Lyα forest with collisionless N-body simulations, where the relevant density fluctuations are unresolved. We use a small-box, high-resolution hydrodynamic simulation to obtain the probability distribution function (PDF) and the power spectrum of the real-space Lyα forest flux. These two statistics are iteratively mapped onto a pseudo-flux field of an N-body simulation, which we construct from the matter density. We demonstrate that our method can reproduce the PDF, line of sight and 3D power spectra of the Lyα forest with good accuracy (7%, 4%, and 7% respectively). We quantify the performance of the commonly used Gaussian smoothing technique and show that it has significantly lower accuracy (20%-80%), especially for N-body simulations with achievable mean inter-particle separations in large-volume simulations. In addition, we show that IMS produces reasonable and smooth spectra, making it a powerful tool for modeling the IGM in large cosmological volumes and for producing realistic "mock" skies for Lyα forest surveys.
            Cover page: MODELING THE Lyα FOREST IN COLLISIONLESS SIMULATIONS
            • Article
            • Peer Reviewed
            LBL Publications (2018)
            Galaxy formation depends critically on the physical state of gas in the circumgalactic medium (CGM) and its interface with the intergalactic medium (IGM), determined by the complex interplay between inflow from the IGM and outflows from supernovae and/or AGN feedback. The average Lyα absorption profile around galactic halos represents a powerful tool to probe their gaseous environments. We compare predictions from Illustris and Nyx hydrodynamical simulations with the observed absorption around foreground quasars, damped Lyα systems, and Lyman-break galaxies. We show how large-scale BOSS and small-scale quasar pair measurements can be combined to precisely constrain the absorption profile over three decades in transverse distance . Far from galaxies, , the simulations converge to the same profile and provide a reasonable match to the observations. This asymptotic agreement arises because the ΛCDM model successfully describes the ambient IGM and represents a critical advantage of studying the mean absorption profile. However, significant differences between the simulations, and between simulations and observations, are present on scales , illustrating the challenges of accurately modeling and resolving galaxy formation physics. It is noteworthy that these differences are observed as far out as , indicating that the "sphere of influence" of galaxies could extend to approximately ∼7 times the halo virial radius. Current observations are very precise on these scales and can thus strongly discriminate between different galaxy formation models. We demonstrate that the Lyα absorption profile is primarily sensitive to the underlying temperature-density relationship of diffuse gas around galaxies, and argue that it thus provides a fundamental test of galaxy formation models.
              Cover page: A Fundamental Test for Galaxy Formation Models: Matching the Lyman-α Absorption Profiles of Galactic Halos Over Three Decades in Distance
              • Article
              • Peer Reviewed
              LBL Publications (2017)
              We present a new Bayesian algorithm making use of Markov Chain Monte Carlo sampling that allows us to simultaneously estimate the unknown continuum level of each quasar in an ensemble of high-resolution spectra, as well as their common probability distribution function (PDF) for the transmitted Lyα forest flux. This fully automated PDF regulated continuum fitting method models the unknown quasar continuum with a linear principal component analysis (PCA) basis, with the PCA coefficients treated as nuisance parameters. The method allows one to estimate parameters governing the thermal state of the intergalactic medium (IGM), such as the slope of the temperature-density relation γ -1, while marginalizing out continuum uncertainties in a fully Bayesian way. Using realistic mock quasar spectra created from a simplified semi-numerical model of the IGM, we show that this method recovers the underlying quasar continua to a precision of ≃7% and ≃10% at z = 3 and z = 5, respectively. Given the number of principal component spectra, this is comparable to the underlying accuracy of the PCA model itself. Most importantly, we show that we can achieve a nearly unbiased estimate of the slope γ - 1 of the IGM temperature-density relation with a precision of ± 8.6% at z = 3 and ± 6.1% at z = 5, for an ensemble of ten mock high-resolution quasar spectra. Applying this method to real quasar spectra and comparing to a more realistic IGM model from hydrodynamical simulations would enable precise measurements of the thermal and cosmological parameters governing the IGM, albeit with somewhat larger uncertainties, given the increased flexibility of the model.
                Cover page: Joint Bayesian Estimation of Quasar Continua and the Lyα Forest Flux Probability Distribution Function
                • Article
                • Peer Reviewed
                LBL Publications (2019)
                We determine the thermal evolution of the intergalactic medium (IGM) over 3 Gyr of cosmic time 1.8 < z < 5.4 by comparing measurements of the Ly forest power spectrum to a suite of 70 hydrodynamical simulations. We conduct Bayesian inference of IGM thermal parameters using an end-to-end forward modeling framework whereby mock spectra generated from our simulation grid are used to build a custom emulator that interpolates the power spectrum between thermal grid points. The temperature at mean density T0 rises steadily from T0 ∼ 6000 K at z=5.4, peaks at 14,000 K for z3.4, and decreases at lower redshift, reaching T07000 K by z1.8. This evolution provides conclusive evidence for photoionization heating resulting from the reionization of He II, as well as the subsequent cooling of the IGM due to the expansion of the universe after all reionization events are complete. Our results are broadly consistent with previous measurements of thermal evolution based on a variety of approaches, but the sensitivity of the power spectrum, the combination of high-precision measurements of largescale modes (k 0.02 s km-1) from the Baryon Oscillation Spectroscopic Survey with our recent determination of the small-scale power, our large grid of models, and our careful statistical analysis allow us to break the wellknown degeneracy between the temperature at mean density T0 and the slope of the temperature-density relation that has plagued previous analyses. At the highest redshifts, z5, we infer lower temperatures than expected from the standard picture of IGM thermal evolution leaving little room for additional smoothing of the Ly forest by free streaming of warm dark matter.
                  Cover page: New Constraints on IGM Thermal Evolution from the Lyα Forest Power Spectrum
                  • Article
                  • Peer Reviewed
                  UC Berkeley Previously Published Works (2015)
                  We present a new method of finding protoclusters using tomographic maps of Ly α forest flux. We review our method of creating tomographic flux maps and discuss our new highperformance implementation, which makes large reconstructions computationally feasible. Using a large N-body simulation, we illustrate how protoclusters create large-scale flux decrements, roughly 10 h-1 Mpc across, and how we can use this signal to find them in flux maps. We test the performance of our protocluster finding method by running it on the ideal, noiseless map and tomographic reconstructions from mock surveys, and comparing to the halo catalogue. Using the noiseless map, we find protocluster candidates with about 90 per cent purity, and recover about 75 per cent of the protoclusters that form massive clusters (>3 × 1014 h-1M⊙). We construct mock surveys similar to the ongoing COSMOS Lyman-Alpha Mapping And Tomography Observations survey. While the existing data have an average sightline separation of 2.3 h-1 Mpc, we test separations of 2-6 h-1 Mpc to see what can be tolerated for our application. Using reconstructed maps from small separation mock surveys, the protocluster candidate purity and completeness are very close to what was found in the noiseless case. As the sightline separation increases, the purity and completeness decrease, although they remain much higher than we initially expected. We extended our test cases to mock surveys with an average separation of 15 h-1 Mpc, meant to reproduce high source density areas of the Baryon Oscillation Spectroscopic Survey. We find that even with such a large sightline separation, the method can still be used to find some of the largest protoclusters.
                    Cover page: Protocluster discovery in tomographic Ly α forest flux maps
                    • Article
                    • Peer Reviewed
                    UC Berkeley Previously Published Works (2019)
                    The intense radiation emitted by luminous quasars dramatically alters the ionization state of their surrounding IGM. This so-called proximity effect extends out to tens of Mpc, and manifests as large coherent regions of enhanced Ly (Ly) forest transmission in absorption spectra of background sightlines. Here, we present a novel method based on Ly forest tomography, which is capable of mapping these quasar "light echoes" in three dimensions. Using a dense grid (10-100) of faint () background galaxies as absorption probes, one can measure the ionization state of the IGM in the vicinity of a foreground quasar, yielding detailed information about the quasar's radiative history and emission geometry. An end-to-end analysis-combining cosmological hydrodynamical simulations post-processed with a quasar emission model, realistic estimates of galaxy number densities, and instrument + telescope throughput-is conducted to explore the feasibility of detecting quasar light echoes. We present a new, fully Bayesian statistical method that allows one to reconstruct quasar light echoes from thousands of individual low-S/N transmission measurements. Armed with this tool, we undertake an exhaustive parameter study and show that light echoes can be convincingly detected for luminous (M 1450 <-27.5 mag, corresponding to m 1450 < 18.4 mag at ) quasars at redshifts 3 < z QSO < 5, and that a relative precision better than 20% on the quasar age can be achieved for individual objects in the expected range of ages between 1 and 100 Myr. The observational requirements are relatively modest: Moderate-resolution (R 750), multiobject spectroscopy at a low signal-to-noise ratio (S/N > 5) is sufficient, requiring three-hour integrations using existing instruments on 8 m class telescopes.
                      Cover page: Mapping Quasar Light Echoes in 3D with Lyα Forest Tomography
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