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Title: Measurement of the small-scale structure of the intergalactic medium using close quasar pairs

Abstract

The distribution of diffuse gas in the intergalactic medium (IGM) imprints a series of hydrogen absorption lines on the spectra of distant background quasars known as the Lyman-α forest. Cosmological hydrodynamical simulations predict that IGM density fluctuations are suppressed below a characteristic scale where thermal pressure balances gravity. We measured this pressure-smoothing scale by quantifying absorption correlations in a sample of close quasar pairs. We compared our measurements to hydrodynamical simulations, where pressure smoothing is determined by the integrated thermal history of the IGM. Lastly, our findings are consistent with standard models for photoionization heating by the ultraviolet radiation backgrounds that reionized the universe.

Authors:
ORCiD logo [1];  [2]; ORCiD logo [3]; ORCiD logo [4]; ORCiD logo [5]; ORCiD logo [1]; ORCiD logo [3];  [6]; ORCiD logo [7]
  1. Inst. of Astronomy, Cambridge (United Kingdom); Max-Planck-Inst. fur Astronomie, Konigstuhl (Germany)
  2. Max-Planck-Inst. fur Astronomie, Konigstuhl (Germany); Univ. of California, Santa Barbara, CA (United States). Dept. of Physics
  3. Max-Planck-Inst. fur Astronomie, Konigstuhl (Germany)
  4. Univ. of California, Berkeley, CA (United States). Dept. of Astronomy; Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
  5. Univ. of California, Santa Cruz, CA (United States). Lick Observatory
  6. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
  7. Univ. of California, Berkeley, CA (United States). Dept. of Astronomy
Publication Date:
Research Org.:
Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
Sponsoring Org.:
USDOE Office of Science (SC); National Science Foundation (NSF)
OSTI Identifier:
1393226
Grant/Contract Number:
AC02-05CH11231; AST-1010004
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Science
Additional Journal Information:
Journal Volume: 356; Journal Issue: 6336; Journal ID: ISSN 0036-8075
Publisher:
AAAS
Country of Publication:
United States
Language:
English
Subject:
79 ASTRONOMY AND ASTROPHYSICS

Citation Formats

Rorai, Alberto, Hennawi, Joseph F., Oñorbe, Jose, White, Martin, Prochaska, J. Xavier, Kulkarni, Girish, Walther, Michael, Lukić, Zarija, and Lee, Khee-Gan. Measurement of the small-scale structure of the intergalactic medium using close quasar pairs. United States: N. p., 2017. Web. doi:10.1126/science.aaf9346.
Rorai, Alberto, Hennawi, Joseph F., Oñorbe, Jose, White, Martin, Prochaska, J. Xavier, Kulkarni, Girish, Walther, Michael, Lukić, Zarija, & Lee, Khee-Gan. Measurement of the small-scale structure of the intergalactic medium using close quasar pairs. United States. doi:10.1126/science.aaf9346.
Rorai, Alberto, Hennawi, Joseph F., Oñorbe, Jose, White, Martin, Prochaska, J. Xavier, Kulkarni, Girish, Walther, Michael, Lukić, Zarija, and Lee, Khee-Gan. 2017. "Measurement of the small-scale structure of the intergalactic medium using close quasar pairs". United States. doi:10.1126/science.aaf9346.
@article{osti_1393226,
title = {Measurement of the small-scale structure of the intergalactic medium using close quasar pairs},
author = {Rorai, Alberto and Hennawi, Joseph F. and Oñorbe, Jose and White, Martin and Prochaska, J. Xavier and Kulkarni, Girish and Walther, Michael and Lukić, Zarija and Lee, Khee-Gan},
abstractNote = {The distribution of diffuse gas in the intergalactic medium (IGM) imprints a series of hydrogen absorption lines on the spectra of distant background quasars known as the Lyman-α forest. Cosmological hydrodynamical simulations predict that IGM density fluctuations are suppressed below a characteristic scale where thermal pressure balances gravity. We measured this pressure-smoothing scale by quantifying absorption correlations in a sample of close quasar pairs. We compared our measurements to hydrodynamical simulations, where pressure smoothing is determined by the integrated thermal history of the IGM. Lastly, our findings are consistent with standard models for photoionization heating by the ultraviolet radiation backgrounds that reionized the universe.},
doi = {10.1126/science.aaf9346},
journal = {Science},
number = 6336,
volume = 356,
place = {United States},
year = 2017,
month = 4
}

Journal Article:
Free Publicly Available Full Text
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  • 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.more » 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{sub 0} and slope γ of the temperature-density relation of the IGM T=T{sub 0}(ρ/ ρ-bar ){sup γ-1}. 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.« less
  • Early in the reionization process, the intergalactic medium (IGM) would have been quite inhomogeneous on small scales, due to the low Jeans mass in the neutral IGM and the hierarchical growth of structure in a cold dark matter universe. This small-scale structure acted as an important sink during the epoch of reionization, impeding the progress of the ionization fronts that swept out from the first sources of ionizing radiation. Here we present results of high-resolution cosmological hydrodynamics simulations that resolve the cosmological Jeans mass of the neutral IGM in representative volumes several Mpc across. The adiabatic hydrodynamics we follow aremore » appropriate in an unheated IGM, before the gas has had a chance to respond to the photoionization heating. Our focus is determination of the resolution required in cosmological simulations in order to sufficiently sample and resolve small-scale structure regulating the opacity of an unheated IGM. We find that a dark matter particle mass of m {sub dm} {approx}< 50 M {sub Sun} and box size of L {approx}> 1 Mpc are required. With our converged results we show how the mean free path of ionizing radiation and clumping factor of ionized hydrogen depend on the ultraviolet background flux and redshift. We find, for example at z = 10, clumping factors typically of 10-20 for an ionization rate of {Gamma} {approx} (0.3-3) Multiplication-Sign 10{sup -12} s{sup -1}, with corresponding mean free paths of {approx}3-15 Mpc, extending previous work on the evolving mean free path to considerably smaller scales and earlier times.« less
  • We present an updated determination of the z {approx} 4 QSO luminosity function (QLF), improving the quality of the determination of the faint end of the QLF presented by Glikman et al. (2010). We have observed an additional 43 candidates from our survey sample, yielding one additional QSO at z = 4.23 and increasing the completeness of our spectroscopic follow-up to 48% for candidates brighter than R = 24 over our survey area of 3.76 deg{sup 2}. We study the effect of using K-corrections to compute the rest-frame absolute magnitude at 1450 A compared with measuring M{sub 1450} directly frommore » the object spectra. We find a luminosity-dependent bias: template-based K-corrections overestimate the luminosity of low-luminosity QSOs, likely due to their reliance on templates derived from higher luminosity QSOs. Combining our sample with bright quasars from the Sloan Digital Sky Survey and using spectrum-based M{sub 1450} for all the quasars, we fit a double power law to the binned QLF. Our best fit has a bright-end slope, {alpha} = 3.3 {+-} 0.2, and faint-end slope, {beta} = 1.6{sup +0.8}{sub -0.6}. Our new data revise the faint-end slope of the QLF down to flatter values similar to those measured at z {approx} 3. The break luminosity, though poorly constrained, is at M* = -24.1{sup +0.7}{sub -1.9}, approximately 1-1.5 mag fainter than at z {approx} 3. This QLF implies that QSOs account for about half the radiation needed to ionize the intergalactic medium at these redshifts.« less