532 research outputs found
The impact of helium reionization on the structure of the intergalactic medium
We examine the impact of helium reionization on the structure of the
intergalactic medium (IGM). We model the reionization using a radiative
transfer (RT) code coupled to the combined gravity hydrodynamics code Enzo.
Neutral hydrogen and helium are initially ionized by a starburst spectrum,
which is allowed to gradually evolve into a power law spectrum over the
redshift interval 3 < z < 4. The temperature-density relation of the gas is
found to fan out and flatten following HeII reionization, with an inversion for
overdensities above 5. Peculiar velocities of up to 10 km/s are induced by the
increased pressure, with the gas density field distorted over large coherent
regions by 10-20%, and the dark matter by levels of 1%. The
photoionization-induced flows may thus distort the matter power spectrum at
comoving wavenumbers k > 0.5 h/Mpc by a few percent by z = 2. Absorption
spectra for HI and HeII are drawn from the simulations, and absorption lines
are fit to the spectra. A median Doppler parameter of 35 km/s is obtained for
the HI absorption systems at z = 3. Dividing into subsamples optically thick
and optically thin at line centre reveals that the optically thick systems
undergo only mild evolution while the optically thin systems evolve rapidly
following HeII reionization. A comparison between HeII and HI absorption
features shows a broad distribution in the HeII and HI column density ratio,
peaking near the measured value and only slightly narrower than measured. A
comparison with approximate simulation methods shows moderately good agreement
in the absorption line properties, but not to the precision to which they may
be measured.Comment: 19 pages, 25 figures. Submitted to MNRA
The Growth of Correlations in the Matter Power Spectrum
We find statistically significant correlations in the cosmological matter
power spectrum over the full range of observable scales. While the correlations
between individual modes are weak, the band-averaged power spectrum shows
strong non-trivial correlations. The correlations are significant when the
modes in either one or both bands are in the non-linear regime, and approach
100% for pairs of bands in which all the modes are non-linear. The correlations
are weaker, but not absent, when computed in redshift space. Since estimates of
the power spectrum from galaxy surveys require band-averaging, the correlations
must be taken into account when comparing a measured power spectrum with
theoretical models.Comment: 7 pages, 6 Figures, accepted for publication to MNRAS. Revise
Reionisation scenarios and the temperature of the IGM
We examine the temperature structure of the IGM due to the passage of
individual ionisation fronts using a radiative transfer (RT) code coupled to a
particle-mesh (PM) N-body code. Multiple simulations were performed with
different spectra of ionising radiation: a power law (goes as nu^{-0.5}),
miniquasar, starburst, and a time-varying spectrum that evolves from a
starburst spectrum to a power law. The RT is sufficiently resolved in time and
space to correctly model both the ionisation state and the temperature across
the ionisation front. We find the post-ionisation temperature of the reionised
intergalactic medium (IGM) is sensitive to the spectrum of the source of
ionising radiation, which may be used to place strong constraints on the nature
of the sources of reionisation. Radiative transfer effects also produce large
fluctuations in the HeII to HI number density ratio eta. The spread in values
is smaller than measured, except for the time-varying spectrum. For this case,
the spread evolves as the spectral nature of the ionising background changes.
Large values for eta are found in partially ionised HeII as the power-law
spectrum begins to dominate the starburst, suggesting that the large eta values
measured may be indicating the onset of the HeII reionisation epoch.Comment: Accepted to MNRAS. Version with high resolution colour figures
available at http://www.roe.ac.uk/~ert/Publications/Tittley_Meiksin_07.pd
Helium reionization and the thermal proximity effect
We examine the temperature structure of the intergalactic medium IGM)
surounding a hard radiation source, such as a Quasi-Stellar Object (QSO), as it
responds to the onset of helium reionization by the source. We model the
reionization using a radiative transfer (RT) code coupled to a particle-mesh
(PM) N-body code. Neutral hydrogen and helium are initially ionized by a
starburst spectrum, which is allowed to gradually evolve into a power law
spectrum (fnu ~ nu^(-0.5)). Multiple simulations were performed with different
times for the onset and dominance of the hard spectrum, with onset redshifts
ranging from z = 3.5 to 5.5. The source is placed in a high-density region to
mimic the expected local environment of a QSO. Simulations with the source
placed in a low-density environment were also performed as control cases to
explore the role of the environment on the properties of the surrounding IGM.
We find in both cases that the IGM temperature within the HeIII region produced
exceeds the IGM temperature before full helium reionization, resulting in a
"thermal proximity effect", but that the temperature in the HeIII region
increases systematically with distance from the source. With time the
temperature relaxes with a reduced spread as a function of impact parameter
along neighbouring lines of sight, although the trend continues to persist
until z = 2. Such a trend could be detected using the widths of intervening
metal absorption systems using high resolution, high signal-to-noise ratio
spectra.Comment: 17 pages, 12 figures, for publication in MNRA
Two-Phase Cooling Flows with Magnetic Reconnection
Motivated by the observations of high Faraday rotation measures measured in
cooling flow clusters we propose a model relevant to plasmas with comparable
thermal and magnetic pressures. Magnetic field reconnection may play a major
role in changing the topology of the magnetic field in the central cooling flow
regions. The effect of the topology change is that cool flux loops can
reconnect to hot flux loops that are connected to the overall thermal reservoir
of the cluster. There can be a rapid recycling of mass between hot and cold
phases on a time scale of 3 x 10^8-10^9 yr which may reduce the inferred inflow
and mass condensation rates by at least an order of magnitude. A central
multiphase medium is a direct consequence of such a model. Throughout the
cooling flow the filling factor of the hot loops (T > 2 x 10^7 K) is of order
unity. The filling factor of the cool loops (T < 2 x 10^7 K) is 0.1-1% with a
corresponding mass fraction of cold phase of 1-10%. A crucial parameter is the
coherence length of the field relative to the cooling radius and the
distribution of field energy with scale. When the cooling radius is greater
than the field coherence length then cooling flows proceed as usual. When the
coherence length is greater than the central cooling radius, however, the
thermal energy of the reservoir can be tapped and the mass condensation rates
may be very significantly reduced. Three additional conditions must be
satisfied: I. Cold loops must be able to fall at least as far as the mean
distance between hot loops in a cooling time; II. Loops must enter an
evaporative phase on reconnecting; and III. A sufficient number of hot loops
penetrate the cold phase region to power the radiative losses.Comment: 16 pages, uses AAS macro aasm
Can quasars photoionize the intergalactic medium at high redshift?
The reionization of the intergalactic medium (IGM) by quasar sources at high redshift are discussed. The integrated UV background from observed QSO's, taking into account the hydrogen opacity associated with intervening Ly-alpha clouds and Lyman limit systems are computed. It is noted that the published data appear to indicate a significant underdensity of absorption systems in the Ly-alpha forest with column densities N(sub HI) greater than 10(exp 15) cm(sup -2). This deficit results in a reduction of the opacity of the universe by a factor of 1.5-3 at z = 3-5 relative to previous estimates. The QSO contribution to the metagalactic flux at the Lyman edge may be as large as J(sub 912)(z) is approximately 6((1 + z)/4.5)(sup 0.5) x 10(exp -22) erg cm(sup -2) s(sup -1) Hz(sup -1) sr(sup -1) for q(sup o) = O, and slightly lower for q(sub o) = 1/2. For a density of the diffuse component of the IGM of omega(sub D)(h(sub 50)(sup 2)) less than 0.025, QSO's could photoionize a smooth IGM sufficiently to satisfy the constraints imposed by the Gunn-Peterson effect. The epoch of reionization could be as recent as z is approximately greater than 5. As a result, neutral patches of IGM would be detectable in the spectra of high redshift quasars. The patches would appear as absorption line systems with typical column densities of 10(exp 19) - 10(exp 20) cm(sup -2), and velocity widths of 100 - 1000 km s(sup -1)
Radiative transfer through the Intergalactic Medium
We use a probabilistic method to compute the propagation of an ionization
front corresponding to the re-ionization of the intergalactic medium in a LCDM
cosmology, including both hydrogen and helium. The effects of radiative
transfer substantially boost the temperature of the ionized gas over the case
of uniform re-ionization. The resulting temperature-density relation of the
ionized gas is both non-monotonic and multiple-valued, reflecting the non-local
character of radiative transfer and suggesting that a single polytropic
relation between local gas density and temperatue is a poor description of the
thermodynamic state of baryons in the post-reionization universe.Comment: 5 pages, 7 figures, letter accepted for publication in MNRA
Intergalactic heating by Lyα photons including hyperfine structure corrections
Lyman-alpha photons from the first radiating sources in the Universe play a
pivotal role in 21-cm radio detections of Cosmic Dawn and the Epoch of
Reionization. Comments are provided on the effect of the hyperfine structure of
hydrogen on the rate of heating or cooling of the Intergalactic Medium. It is
shown that heating of the still neutral hydrogen by the Cosmic Microwave
Background is negligible, with a characteristic heating time of 1e27 s/ (1+z)
at redshift z.Comment: Accepted for publication in Research Notes of the AAS. (Missing
reference added.
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