October 12, 2001
The temporal evolution of the ionizing UV background radiation field at high redshift provides a probe of the evolution of the early star formation rate. By comparing the observed levels of absorption in the highest redshift quasar spectra to the predictions of a hydrodynamic simulation, we determine the evolution of the photoionization rate Gamma for neutral hydrogen in the intergalactic medium, over the redshift range 4.9 < z < 6.1. After accounting for sampling variance, we infer a sharp increase in Gamma from z~6.1 to z~5.8, probably implying reionization at this redshift. We find a decrease in Gamma from z~5.6 to 5.2, at 3sigma significance. This feature is a generic signature in the aftermath of reionization, entirely consistent with the cosmological reionization process being completed at z~6.1. This generic feature is a result of a significant change in the star formation rate subsequent to the cosmological reionization. There is an abrupt rise of the temperature of the intergalactic medium due to photo-heating, when it is reionized. This translates to a correspondingly sudden jump in the Jeans mass and a dramatic suppression of gas accretion onto the most abundant (sub-galactic) halos at the epochs of interest. The star formation rate suffers a temporary setback in the aftermath of reionization, resulting in a temporary decrease in the amplitude of the ionizing radiation field.
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November 9, 2001
We study the process of cosmic reionization and estimate the ionizing background in the IGM using the Lyman series absorption in the spectra of the four quasars at 5.7<z<6.3 discovered by the SDSS. We derive the evolution of the ionizing background at high redshifts, using both semi-analytic techniques and cosmological simulations to model the density fluctuations in the IGM. The existence of the complete Ly alpha Gunn-Peterson trough in the spectrum of the z=6.28 quasar SDSS...
January 28, 2009
The ionizing background determines the ionization balance and the thermodynamics of the cosmic gas. It is therefore a fundamental ingredient to theoretical and empirical studies of both the IGM and galaxy formation. We present here a new calculation of its spectrum that satisfies the empirical constraints we recently obtained by combining state-of-the-art luminosity functions and intergalactic opacity measurements. In our preferred model, star-forming galaxies and quasars eac...
October 8, 2014
The ionizing background of cosmic hydrogen is an important probe of the sources and absorbers of ionizing radiation in the post-reionization universe. Previous studies show that the ionization rate should be very sensitive to changes in the source population: as the emissivity rises, absorbers shrink in size, increasing the ionizing mean free path and, hence, the ionizing background. By contrast, observations of the ionizing background find a very flat evolution from z~2-5, b...
April 27, 1998
The history of the transition from a neutral intergalactic medium to one that is almost fully ionized can reveal the character of cosmological ionizing sources. In this talk I will discuss the implications for rival reionization scenarios of the rapid decline observed in the space density of quasars and star-forming galaxies at redshifts z>3. The hydrogen component in a highly inhomogeneous universe is completely reionized when the number of ionizing photons emitted in one re...
July 14, 2016
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 UV...
February 16, 1999
There has been remarkable progress recently in both observational and theoretical studies of galaxy formation and evolution. Largely due to a combination of deep Hubble Space Telescope imaging, Keck spectroscopy, and COBE far-IR background measurements, new constraints have emerged on the emission history of the galaxy population as a whole. While the bulk of the stars present today appears to have formed relatively recently, the existence of a decline in the star-formation d...
July 8, 2013
We present new measurements of the intensity of the ionizing ultraviolet background and the global emissivity of ionizing photons over 2 < z < 5. Our results are based on a suite of updated measurements of physical properties of the high-redshift intergalactic medium (IGM), including gas temperatures and the opacity of the IGM to Ly-alpha and ionizing photons. Consistent with previous works, we find a relatively flat hydrogen photoionization rate over 2 < z < 5, although our ...
November 13, 2008
The cosmological background of ionizing radiation has been dominated by quasars once the Universe aged by ~2 billion years. At earlier times (redshifts z>3), the observed abundance of bright quasars declined sharply, implying that cosmic hydrogen was reionized by stars instead. Here, we explain the physical origin of the transition between the dominance of stars and quasars as a generic feature of structure formation in the concordance LCDM cosmology. At early times, the frac...
July 26, 2008
We investigate the implications of the intergalactic opacity for the evolution of the cosmic UV luminosity density and its sources. Our main constraint is our measurement of the Lya forest opacity at redshifts 2<z<4.2 from 86 high-resolution quasar spectra. In addition, we impose the requirements that HI must be reionized by z=6 and HeII by z~3, and consider estimates of the hardness of the ionizing background from HI to HeII column density ratios. The derived hydrogen photoi...
July 15, 2019
The universe goes through several phase transitions during its formative stages. Cosmic reionization is the last of them, where ultraviolet and X-ray radiation escape from the first generations of galaxies heating and ionizing their surroundings and subsequently the entire intergalactic medium. There is strong observational evidence that cosmic reionization ended approximately one billion years after the Big Bang, but there are still uncertainties that will be clarified with ...