5,467 research outputs found
The contribution of the IGM and minihalos to the 21 cm signal of reionization
We study the statistical properties of the cosmological 21 cm signal from
both the intergalactic medium (IGM) and minihalos, using a reionization
simulation that includes a self--consistent treatment of minihalo
photoevaporation. We consider two models for minihalo formation and three
typical thermal states of the IGM -- heating purely by ionization, heating from
both ionizing and photons, and a maximal "strong heating" model.
We find that the signal from the IGM is almost always dominant over that from
minihalos. In our calculation, the differential brightness temperature,
of minihalos is never larger than 2 mK. Although there are
indeed some differences in the signals from the minihalos and from the IGM,
even with the planned generation of radio telescopes it will be unfeasible to
detect them. However, minihalos significantly affect the ionization state of
the IGM and the corresponding 21 cm flux.Comment: 13 pages, 12 figures, accepted by MNRA
Enhanced Detectability of Pre-reionization 21-cm Structure
Before the universe was reionized, it was likely that the spin temperature of
intergalactic hydrogen was decoupled from the CMB by UV radiation from the
first stars through the Wouthuysen-Field effect. If the IGM had not yet been
heated above the CMB temperature by that time, then the gas would appear in
absorption relative to the CMB. Large, rare sources of X-rays could inject
sufficient heat into the neutral IGM, so that the differential brightness
temperature was greater than zero at comoving distances of tens to hundreds of
Mpc, resulting in large 21-cm fluctuations with amplitudes of about 250 mK on
arcminute to degree angular scales, an order of magnitude larger in amplitude
than that caused by ionized bubbles during reionization, about 25 mK. This
signal could therefore be easier to detect and probe higher redshifts than that
due to patchy reionization. For the case in which the first objects to heat the
IGM are QSOs hosting 10^7-solar mass black holes with an abundance exceeding
about 1 per Gpc^3 at z~15, observations with either the Arecibo Observatory or
the Five Hundred Meter Aperture Spherical Telescope (FAST) could detect and
image their fluctuations at greater than 5-sigma significance in about a month
of dedicated survey time. Additionally, existing facilities such as MWA and
LOFAR could detect the statistical fluctuations arising from a population of
10^5-solar mass black holes with an abundance of about 10^4 per Gpc^3 at
z~10-12.Comment: 5 pages, 4 figures, submitted to ApJ Letters; comments welcom
Fossil HII Regions: Self-Limiting Star Formation at High Redshift
Recent results by the WMAP satellite suggest that the intergalactic medium
was significantly reionized at redshifts as high as z~17. At this early epoch,
the first ionizing sources likely appeared in the shallow potential wells of
mini-halos with virial temperatures T < 10^4 K. Once such an ionizing source
turns off, its surrounding HII region Compton cools and recombines.
Nonetheless, we show that the ``fossil'' HII regions left behind remain at high
adiabats, prohibiting gas accretion and cooling in subsequent generations of
mini-halos. Thus, early star formation is self-limiting. We quantify this
effect to show that star formation in mini-halos cannot account for the bulk of
the electron scattering opacity measured by WMAP, which must be due to more
massive objects. We argue that gas entropy, rather than IGM metallicity,
regulates the evolution of the global ionizing emissivity, and impedes full
reionization until lower redshifts. We discuss several important consequences
of this early entropy floor for reionization. It reduces gas clumping,
curtailing the required photon budget for reionization. An entropy floor also
prevents H2 formation and cooling, due to reduced gas densities: it greatly
enhances feedback from UV photodissociation of H2. An early X-ray background
would also furnish an entropy floor to the entire IGM; thus, X-rays impede
rather than enhance H2 formation. Future 21cm observations may probe the
topology of fossil HII regions.Comment: Submitted to MNRA
The First Miniquasar
We investigate the environmental impact of the first active galactic nuclei
that may have formed ~150 Myr after the big bang in low-mass ~10^6 Msun
minihaloes. Using Enzo, an adaptive-mesh refinement cosmological hydrodynamics
code, we carry out three-dimensional simulations of the radiative feedback from
`miniquasars' powered by intermediate-mass black holes. We follow the
non-equilibrium multispecies chemistry of primordial gas in the presence of a
point source of X-ray radiation, which starts shining in a rare high-sigma peak
at z=21 and emits a power-law spectrum in the 0.2-10 keV range. We find that,
after one Salpeter time-scale, the miniquasar has heated up the simulation box
to a volume-averaged temperature of 2800 K. The mean electron and H2 fractions
are now 0.03 and 4e-5: the latter is 20 times larger than the primordial value,
and will delay the buildup of a uniform UV photodissociating background. The
net effect of the X-rays is to reduce gas clumping in the IGM by as much as a
factor of 3. While the suppression of baryonic infall lowers the gas mass
fraction at overdensities delta in the range 20-2000, enhanced molecular
cooling increases the amount of dense material at delta>2000. In many haloes
within the proximity of our miniquasar the H2-boosting effect of X-rays is too
weak to overcome heating, and the cold and dense gas mass actually decreases.
We find little evidence for an entropy floor in gas at intermediate densities
preventing gas contraction and H2 formation. Overall, the radiative feedback
from X-rays enhances gas cooling in lower-sigma peaks that are far away from
the initial site of star formation, thus decreasing the clustering bias of the
early pregalactic population, but does not appear to dramatically reverse or
promote the collapse of pregalactic clouds as a whole. (abridged)Comment: 15 pages, 13 figures, accepted for publication in MNRAS. Minor
modifications in response to the referee's comments. A high resolution
version of this paper, as well as movies, can be found at
http://www.ucolick.org/~mqk/miniqs
Plasma actuator: influence of dielectric surface temperature
Plasma actuators have become the topic of interest of many researchers for the purpose of flow control. They have the advantage of manipulating the flow without the need for any moving parts, a small surface profile which does not disturb the free stream flow, and the ability to switch them on or off depending on the particular situation (active flow control). Due to these characteristics they are becoming very popular for flow control over aircraft wings. The objective of the current study is to examine the effect of the actuator surface temperature on its performance. This is an important topic to understand when dealing with real life aircraft equipped with plasma actuators. The temperature variations encountered during a flight envelope may have adverse effects in actuator performance. A peltier heater along with dry ice are used to alter the actuator temperature, while particle image velocimetry (PIV) is utilised to analyse the flow field. The results show a significant change in the induced flow field by the actuator as the surface temperature is varied. It is found that for a constant peak-to-peak voltage the maximum velocity produced by the actuator depends directly on the dielectric surface temperature. The findings suggest that by changing the actuator temperature the performance can be maintained or even altered at different environmental conditions
Lyalpha heating and its impact on early structure formation
In this paper we have calculated the effect of Lyalpha photons emitted by the
first stars on the evolution of the IGM temperature. We have considered both a
standard Salpeter IMF and a delta-function IMF for very massive stars with mass
300 M_sun. We find that the Lyalpha photons produced by the stellar populations
considered here are able to heat the IGM at z<25, although never above ~100 K.
Stars with a Salpeter IMF are more effective as, due to the contribution from
small-mass long-living stars, they produce a higher Lyalpha background. Lyalpha
heating can affect the subsequent formation of small mass objects by producing
an entropy floor that may limit the amount of gas able to collapse and reduce
the gas clumping.We find that the gas fraction in halos of mass below ~ 5 x
10^6 M_sun is less than 50% (for the smallest masses this fraction drops to 1%
or less) compared to a case without Lyalpha heating. Finally, Lyalpha photons
heat the IGM temperature above the CMB temperature and render the 21cm line
from neutral hydrogen visible in emission at z<15.Comment: 7 pages, 5 figures, to be printed in MNRA
The 21 Centimeter Forest
We examine the prospects for studying the pre-reionization intergalactic
medium (IGM) through the so-called 21 cm forest in spectra of bright
high-redshift radio sources. We first compute the evolution of the mean optical
depth for models that include X-ray heating of the IGM gas, Wouthuysen-Field
coupling, and reionization. Under most circumstances, the spin temperature T_S
grows large well before reionization begins in earnest. As a result, the
optical depth is less than 0.001 throughout most of reionization, and
background sources must sit well beyond the reionization surface in order to
experience measurable absorption. HII regions produce relatively large
"transmission gaps" and may therefore still be observable during the early
stages of reionization. Absorption from sheets and filaments in the cosmic web
fades once T_S becomes large and should be rare during reionization. Minihalos
can produce strong (albeit narrow) absorption features. Measuring their
abundance would yield useful limits on the strength of feedback processes in
the IGM as well as their effect on reionization.Comment: 9 pages, 5 figures, submitted to MNRA
X-ray emission from high-redshift miniquasars: self-regulating the population of massive black holes through global warming
Observations of high-redshift quasars at z>6 imply that supermassive black
holes (SMBHs) with masses over a billion solar masses were in place less than 1
Gyr after the Big Bang. If these SMBHs assembled from "seed" BHs left behind by
the first stars, then they must have accreted gas at close to the Eddington
limit during a large fraction (>50%) of the time. A generic problem with this
scenario, however, is that the mass density in million-solar-mass SMBHs at z=6
already exceeds the locally observed SMBH mass density by several orders of
magnitude; in order to avoid this overproduction, BH seed formation and growth
must become significantly less efficient in less massive protogalaxies, while
proceeding uninterrupted in the most massive galaxies that formed first. Using
Monte-Carlo realizations of the merger and growth history of BHs, we show that
X-rays from the earliest accreting BHs can provide such a feedback mechanism.
Our calculations paint a self-consistent picture of black-hole-made climate
change, in which the first miniquasars---among them the ancestors of the z>6
quasar SMBHs---globally warm the IGM and suppress the formation and growth of
subsequent generations of BHs. We present two specific models with global
miniquasar feedback that provide excellent agreement with recent estimates of
the z=6 SMBH mass function. For each of these models, we estimate the rate of
BH mergers at z>6 that could be detected by the proposed gravitational-wave
observatory eLISA/NGO.Comment: 15 pages, 6 figures, accepted to MNRAS; v2 includes minor changes,
mostly to references, to match version to be publishe
The Impact of HI in Galaxies on 21-cm Intensity Fluctuations During the Reionisation Epoch
We investigate the impact of neutral hydrogen (HI) in galaxies on the
statistics of 21-cm fluctuations using analytic and semi-numerical modelling.
Following the reionisation of hydrogen the HI content of the Universe is
dominated by damped absorption systems (DLAs), with a cosmic density in HI that
is observed to be constant at a level equal to ~2% of the cosmic baryon density
from z~1 to z~5. We show that extrapolation of this constant fraction into the
reionisation epoch results in a reduction of 10-20% in the amplitude of 21-cm
fluctuations over a range of spatial scales. The assumption of a different
percentage during the reionisation era results in a proportional change in the
21-cm fluctuation amplitude. We find that consideration of HI in galaxies/DLAs
reduces the prominence of the HII region induced shoulder in the 21-cm power
spectrum (PS), and hence modifies the scale dependence of 21-cm fluctuations.
We also estimate the 21cm-galaxy cross PS, and show that the cross PS changes
sign on scales corresponding to the HII regions. From consideration of the
sensitivity for forthcoming low-frequency arrays we find that the effects of HI
in galaxies/DLAs on the statistics of 21-cm fluctuations will be significant
with respect to the precision of a PS or cross PS measurement. In addition,
since overdense regions are reionised first we demonstrate that the
cross-correlation between galaxies and 21-cm emission changes sign at the end
of the reionisation era, providing an alternative avenue to pinpoint the end of
reionisation. The sum of our analysis indicates that the HI content of the
galaxies that reionise the universe will need to be considered in detailed
modelling of the 21-cm intensity PS in order to correctly interpret
measurements from forthcoming low-frequency arrays.Comment: 11 pages, 6 figures. Submitted to MNRA
Suppression of HD-cooling in protogalactic gas clouds by Lyman-Werner radiation
It has been shown that HD molecules can form efficiently in metal-free gas
collapsing into massive protogalactic halos at high redshift. The resulting
radiative cooling by HD can lower the gas temperature to that of the cosmic
microwave background, T_CMB=2.7(1+z)K, significantly below the temperature of a
few 100 K achievable via H_2-cooling alone, and thus reduce the masses of the
first generation of stars. Here we consider the suppression of HD-cooling by UV
irradiation in the Lyman-Werner (LW) bands. We include photo-dissociation of
both H_2 and HD, and explicitly compute the self-shielding and shielding of
both molecules by neutral hydrogen as well as the shielding of HD by H_2. We
use a simplified dynamical collapse model, and follow the chemical and thermal
evolution of the gas, in the presence of a UV background. We find that a LW
flux of J_crit = 1e-22 erg/cm^2/sr/s/Hz is able to suppress HD cooling and thus
prevent collapsing primordial gas from reaching temperatures below 100 K. The
main reason for the lack of HD cooling for J>J_crit is the partial
photo-dissociation of H_2, which prevents the gas from reaching sufficiently
low temperatures (T<150K) for HD to become the dominant coolant; direct HD
photo-dissociation is unimportant except for a narrow range of fluxes and
column densities. Since the prevention of HD-cooling requires only partial H_2
photo-dissociation, the critical flux J_crit is modest, and is below the UV
background required to reionize the universe at redshift z=10-20. We conclude
that HD-cooling can reduce the masses of typical stars only in rare halos
forming well before the epoch of reionization.Comment: 14 pages with 9 figures, submitted to MNRA
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