885 research outputs found
Quasar H II Regions During Cosmic Reionization
Cosmic reionization progresses as HII regions form around sources of ionizing
radiation. Their average size grows continuously until they percolate and
complete reionization. We demonstrate how this typical growth can be calculated
around the largest, biased sources of UV emission, such as quasars, by further
developing an analytical model based on the excursion set formalism. This
approach allows us to calculate the sizes and growth of the HII regions created
by the progenitors of any dark matter halo of given mass and redshift with a
minimum of free parameters. Statistical variations in the size of these
pre-existing HII regions are an additional source of uncertainty in the
determination of very high redshift quasar properties from their observed HII
region sizes. We use this model to demonstrate that the transmission gaps seen
in very high redshift quasars can be understood from the radiation of only
their progenitors and associated clustered small galaxies. The fit sets a lower
limit on the redshift of overlap at z = 5.8 +/- 0.1. This interpretation makes
the transmission gaps independent of the age of the quasars observed. If this
interpretation were correct it would raise the prospects of using radio
interferometers currently under construction to detect the epoch of
reionization.Comment: 6 pages, 3 figures, accepted by MNRAS, revised to match published
versio
UV driven evaporation of close-in planets: energy-limited; recombination-limited and photon-limited flows
We have investigated the evaporation of close-in exoplanets irradiated by
ionizing photons. We find that the properties of the flow are controlled by the
ratio of the recombination time to the flow time-scale. When the recombination
time-scale is short compared to the flow time-scale the the flow is in
approximate local ionization equilibrium with a thin ionization front, where
the photon mean free path is short compared to flow scale. In this
"recombination limited" flow the mass-loss scales roughly with the square root
of the incident flux. When the recombination time is long compared to the flow
time-scale the ionization front becomes thick and encompasses the entire flow,
with the mass-loss rate scaling linearly with flux. If the planet's potential
is deep the flow is approximately "energy-limited"; however, if the planet's
potential is shallow we identify a new limiting mass-loss regime, which we term
"photon-limited". In this scenario the mass-loss rate is purely limited by the
incoming flux of ionizing photons. We have developed a new numerical approach
that takes into account the frequency dependence of the incoming ionizing
spectrum and performed a large suite of 1D simulations to characterise UV
driven mass-loss around low mass planets. We find the flow is
"recombination-limited" at high fluxes but becomes "energy-limited" at low
fluxes; however, the transition is broad occurring over several order of
magnitude in flux. Finally, we point out the transitions between the different
flow types does not occur at a single flux value, but depends on the planet's
properties, with higher mass planets becoming "energy-limited" at lower fluxes.Comment: Published in Ap
The Impact of baryonic physics on the kinetic Sunyaev-Zel'dovich Effect
Poorly understood "baryonic physics" impacts our ability to predict the power
spectrum of the kinetic Sunyaev-Zel'dovich (kSZ) effect. We study this in one
sample high resolution simulation of galaxy formation and feedback, Illustris.
The high resolution of Illustris allows us to probe the kSZ power spectrum on
multipoles . Strong AGN feedback in Illustris nearly
wipes out gas fluctuations at and at late times,
likely somewhat under predicting the kSZ power generated at . The
post-reionization kSZ power spectrum for Illustris is well-fit by
over
, somewhat lower than most other reported values
but consistent with the analysis of Shaw et al. Our analysis of the bias of
free electrons reveals subtle effects associated with the multi-phase gas
physics and stellar fractions that affect even linear scales. In particular
there are fewer electrons in biased galaxies, due to gas cooling and star
formation, and this leads to an electron bias less than one even at low
wavenumbers. The combination of bias and electron fraction that determines the
overall suppression is relatively constant, , but more
simulations are needed to see if this is Illustris-specific. By separating the
kSZ power into different terms, we find at least of the signal at
comes from non-Gaussian connected four-point density and
velocity correlations, \left_{c}, even without
correcting for the Illustris simulation box size. A challenge going forward
will be to accurately model long-wave velocity modes simultaneously with
Illustris-like high resolution to capture the complexities of galaxy formation
and its correlations with large scale flows.Comment: 12 pages, 9 figure, submitted to Ap
Recognizing the First Radiation Sources Through Their 21-cm Signature
At the beginning of the reionization epoch, radiation sources produce
fluctuations in the redshifted 21-cm background. We show that different types
of sources (such as miniquasars, Pop II and III stars, supernovae, etc.)
produce distinct signatures in the 21-cm signal radial profiles and statistical
fluctuations, through which they can be identified. Further, we show that the
21-cm signal from X-ray emitting sources is much easier to observe than was
expected, due to a previously neglected pumping mechanism.Comment: 4 pages, 4 figures, accepted by ApJ
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