1,898 research outputs found
Cross-correlations of the Lyman-alpha forest with weak lensing convergence I: Analytical Estimates of S/N and Implications for Neutrino Mass and Dark Energy
We expect a detectable correlation between two seemingly unrelated
quantities: the four point function of the cosmic microwave background (CMB)
and the amplitude of flux decrements in quasar (QSO) spectra. The amplitude of
CMB convergence in a given direction measures the projected surface density of
matter. Measurements of QSO flux decrements trace the small-scale distribution
of gas along a given line-of-sight. While the cross-correlation between these
two measurements is small for a single line-of-sight, upcoming large surveys
should enable its detection. This paper presents analytical estimates for the
signal to noise (S/N) for measurements of the cross-correlation between the
flux decrement and the convergence and for measurements of the
cross-correlation between the variance in flux decrement and the convergence.
For the ongoing BOSS (SDSS III) and Planck surveys, we estimate an S/N of 30
and 9.6 for these two correlations. For the proposed BigBOSS and ACTPOL
surveys, we estimate an S/N of 130 and 50 respectively. Since the
cross-correlation between the variance in flux decrement and the convergence is
proportional to the fourth power of , the amplitude of these
cross-correlations can potentially be used to measure the amplitude of
at z~2 to 2.5% with BOSS and Planck and even better with future data
sets. These measurements have the potential to test alternative theories for
dark energy and to constrain the mass of the neutrino. The large potential
signal estimated in our analytical calculations motivate tests with non-linear
hydrodynamical simulations and analyses of upcoming data sets.Comment: 24 pages, 9 figure
A Dual-Band Microwave Filter Design for Modern Wireless Communication Systems
Nowadays, modern communication system relies on the designs of high-performance devices to enhance communication effect for a high quality of life and smart city system. As a crucial signal processing step, microwave filter removes unwanted frequency components away from the received signal and enhances the useful ones. However, large loss, bulky size, and single-band greatly limit the practical applications in urban computing. Therefore, the filters with dual-band characteristic are highly desirable for modern wireless communication, such as device-to-device communication, environment monitoring, and automatic driving. In this paper, a dual-band microwave filter is designed and fabricated based on the theory of Mie-resonance extraordinary transmission. An electromagnetic wave cannot propagate through a subwavelength aperture drilled in a metallic film. By adding two dielectric cuboids of different sizes into the two apertures, two passbands appear in the frequency range of 10.0-12.0 GHz. In this range, the insertion loss is less than 0.4 dB, and 3-dB bandwidth is more than 48 MHz. Particularly, the two passband frequencies can be tuned by adjusting the size of the dielectric cuboids. This approach opens a way for designing tunable dual-band microwave bandpass filter, which is benefit for enhancing spectrum resource utilization
Fast, large volume, GPU enabled simulations for the Ly-alpha forest: power spectrum forecasts for baryon acoustic oscillation experiments
High redshift measurements of the baryonic acoustic oscillation scale (BAO)
from large Ly-alpha forest surveys represent the next frontier of dark energy
studies. As part of this effort, efficient simulations of the BAO signature
from the Ly-alpha forest will be required. We construct a model for producing
fast, large volume simulations of the Ly-alpha forest for this purpose.
Utilising a calibrated semi-analytic approach, we are able to run very large
simulations in 1 Gpc^3 volumes which fully resolve the Jeans scale in less than
a day on a desktop PC using a GPU enabled version of our code. The Ly-alpha
forest spectra extracted from our semi-analytical simulations are in excellent
agreement with those obtained from a fully hydrodynamical reference simulation.
Furthermore, we find our simulated data are in broad agreement with
observational measurements of the flux probability distribution and 1D flux
power spectrum. We are able to correctly recover the input BAO scale from the
3D Ly-alpha flux power spectrum measured from our simulated data, and estimate
that a BOSS-like 10^4 deg^2 survey with ~15 background sources per square
degree and a signal-to-noise of ~5 per pixel should achieve a measurement of
the BAO scale to within ~1.4 per cent. We also use our simulations to provide
simple power-law expressions for estimating the fractional error on the BAO
scale on varying the signal-to-noise and the number density of background
sources. The speed and flexibility of our approach is well suited for exploring
parameter space and the impact of observational and astrophysical systematics
on the recovery of the BAO signature from forthcoming large scale spectroscopic
surveys.Comment: 16 pages, 11 figures, accepted to MNRA
Line versus Flux Statistics -- Considerations for the Low Redshift Lyman-alpha Forest
The flux/transmission power spectrum has become a popular statistical tool in
studies of the high redshift () Lyman-alpha forest. At low redshifts,
where the forest has thinned out into a series of well-isolated absorption
lines, the motivation for flux statistics is less obvious. Here, we study the
relative merits of flux versus line correlations, and derive a simple condition
under which one is favored over the other on purely statistical grounds.
Systematic errors probably play an important role in this discussion, and they
are outlined as well.Comment: 6 pages, to appear in "The IGM/Galaxy Connection: The Distribution of
Baryons at z=0", eds. J. L. Rosenberg and M. E. Putma
Pressure Support vs. Thermal Broadening in the Lyman-alpha Forest I: Effects of the Equation of State on Longitudinal Structure
In the low density intergalactic medium (IGM) that gives rise to the
Lyman-alpha forest, gas temperature and density are tightly correlated. The
velocity scale of thermal broadening and the Hubble flow across the gas Jeans
scale are of similar magnitude (Hlambda_J ~ sigma_th). To separate the effects
of gas pressure support and thermal broadening on the Lya forest, we compare
spectra extracted from two smoothed particle hydrodynamics (SPH) simulations
evolved with different photoionization heating rates (and thus different Jeans
scales), imposing different temperature-density relations on the evolved
particle distributions. The turnover scales in the flux power spectrum and flux
autocorrelation function are determined mainly by thermal broadening rather
than pressure. However, the insensitivity to pressure arises partly from a
cancellation effect with a sloped temperature-density relation (T ~ rho^{0.6}
in our simulations): the high density peaks in the colder, lower pressure
simulation are less smoothed by pressure support than in the hotter simulation,
and it is this higher density gas that experiences the strongest thermal
broadening. Changes in thermal broadening and pressure support have comparably
important effects on the flux probability distribution (PDF), which responds
directly to the gas overdensity distribution rather than the scale on which it
is smooth. Tests on a lower resolution simulation show that our statistical
results are converged even at this lower resolution. While thermal broadening
generally dominates the longitudinal structure in the Lya forest, we show in
Paper II that pressure support determines the transverse coherence of the
forest observed towards close quasar pairs. [ABRIDGED]Comment: 12 figures, MNRAS in pres
Cross-correlation of the HI 21-cm Signal and Lyman-alpha Forest: A Probe Of Cosmology
Separating the cosmological redshifted 21-cm signal from foregrounds is a
major challenge. We present the cross-correlation of the redshifted 21-cm
emission from neutral hydrogen (HI) in the post-reionization era with the
Ly-alpha forest as a new probe of the large scale matter distribution in the
redshift range z=2 to 3 without the problem of foreground contamination. Though
the 21-cm and the Ly-alpha forest signals originate from different
astrophysical systems, they are both expected to trace the underlying dark
matter distribution on large scales. The multi-frequency angular
cross-correlation power spectrum estimator is found to be unaffected by the
discrete quasar sampling, which only affects the noise in the estimate. We
consider a hypothetical redshifted 21-cm observation in a single field of view
1.3 degrees (FWHM) centered at z=2.2 where the binned 21-cm angular power
spectrum can be measured at an SNR of 3 sigma or better across the range 500 <
l < 4000 . Keeping the parameters of the 21-cm observation fixed, we have
estimated the SNR for the cross-correlation signal varying the quasar angular
number density n of the Ly-alpha forest survey. Assuming that the spectra have
SNR ~5 in pixels of length 44 km/s, we find that a 5 sigma detection of the
cross-correlation signal is possible at 600 < l < 2000 with n=4 deg^{-2}. This
value of n is well within the reach of upcoming Ly-alpha forest surveys. The
cross-correlation signal will be a new, independent probe of the astrophysics
of the diffuse IGM, the growth of structure and the expansion history of the
Universe.Comment: Revised paper, accepted to MNRA
Pressure Support vs. Thermal Broadening in the Lyman-alpha Forest II: Effects of the Equation of State on Transverse Structure
We examine the impact of gas pressure on the transverse coherence of
high-redshift (2 <= z <= 4) Lyman-alpha forest absorption along neighboring
lines of sight that probe the gas Jeans scale (projected separation Delta r <=
500 kpc/h comoving; angular separation Delta theta <= 30"). We compare
predictions from two smoothed particle hydrodynamics (SPH) simulations that
have different photoionization heating rates and thus different
temperature-density relations in the intergalactic medium (IGM). We also
compare spectra computed from the gas distributions to those computed from the
pressureless dark matter. The coherence along neighboring sightlines is
markedly higher for the hotter, higher pressure simulation, and lower for the
dark matter spectra. We quantify this coherence using the flux
cross-correlation function and the conditional distribution of flux decrements
as a function of transverse and line-of-sight (velocity) separation. Sightlines
separated by Delta theta <= 15" are ideal for probing this transverse
coherence. Higher pressure decreases the redshift-space anisotropy of the flux
correlation function, while higher thermal broadening increases the anisotropy.
In contrast to the longitudinal (line-of-sight) structure of the Lya forest,
the transverse structure on these scales is dominated by pressure effects
rather than thermal broadening. With the rapid recent growth in the number of
known close quasar pairs, paired line-of-sight observations offer a promising
new route to probe the IGM temperature-density relation and test the
unexpectedly high temperatures that have been inferred from single sightline
analyses.Comment: 11 figures, submitted to MNRA
A Search for Oxygen in the Low-Density Lyman-alpha Forest Using the Sloan Digital Sky Survey
We use 2167 Sloan Digital Sky Survey (SDSS) quasar spectra to search for
low-density oxygen in the Intergalactic Medium (IGM). Oxygen absorption is
detected on a pixel-by-pixel basis by its correlation with Lyman-alpha forest
absorption. We have developed a novel Locally Calibrated Pixel (LCP) search
method that uses adjacent regions of the spectrum to calibrate interlopers and
spectral artifacts, which would otherwise limit the measurement of OVI
absorption. Despite the challenges presented by searching for weak OVI within
the Lyman-alpha forest in spectra of moderate resolution and signal-to-noise,
we find a highly significant detection of absorption by oxygen at 2.7 < z < 3.2
(the null hypothesis has a chi^2=80 for 9 data points).
We interpret our results using synthetic spectra generated from a lognormal
density field assuming a mixed quasar-galaxy photoionizing background (Haardt &
Madau 2001) and that it dominates the ionization fraction of detected OVI. The
LCP search data can be fit by a constant metallicity model with [O/H] =
-2.15_(-0.09)^(+0.07), but also by models in which low-density regions are
unenriched and higher density regions have a higher metallicity. The
density-dependent enrichment model by Aguirre et al. (2008) is also an
acceptable fit. All our successful models have similar mass-weighted oxygen
abundance, corresponding to [_MW] = -2.45+-0.06. This result can be used
to find the cosmic oxygen density in the Lyman-alpha forest, Omega_(Oxy, IGM) =
1.4(+-0.2)x10^(-6) = 3x10^(-4) Omega_b. This is the tightest constraint on the
mass-weighted mean oxygen abundance and the cosmic oxygen density in the
Lyman-alpha forest to date and indicates that it contains approximately 16% of
metals produced by star formation (Bouch\'e et al. 2008) up to z = 3.Comment: 12 pages, 9 figures. Accepted by ApJ (minor changes
Glimpsing through the high redshift neutral hydrogen fog
We analyze the transmitted flux in a sample of 17 QSOs spectra at
5.74<zem<6.42 to obtain tighter constraints on the volume-averaged neutral
hydrogen fraction, xHI, at z~6. We study separately the narrow transmission
windows (peaks) and the wide dark portions (gaps) in the observed absorption
spectra. By comparing the statistics of these spectral features with Lyalpha
forest simulations, we conclude that xHI evolves smoothly from 10^{-4.4} at
z=5.3 to 10^{-4.2} at z=5.6, with a robust upper limit xHI<0.36 at z=6.3. The
frequency and physical sizes of the peaks imply an origin in cosmic underdense
regions and/or in HII regions around faint quasars or galaxies. In one case
(the intervening HII region of the faint quasar RD J1148+5253 at z=5.70 along
the LOS of SDSS J1148+5251 at z=6.42) the increase of the peak spectral density
is explained by the first-ever detected transverse proximity effect in the HI
Lyalpha forest; this indicates that at least some peaks result from a locally
enhanced radiation field. We then obtain a strong lower limit on the foreground
QSO lifetime of tQ>11 Myr. The observed widths of the peaks are found to be
systematically larger than the simulated ones. Reasons for such discrepancy
might reside either in the photoionization equilibrium assumption or in
radiative transfer effects.Comment: 12 pages, 9 figures, revised to match the accepted version including
a detailed analysis of the foreground QSO redshift and of the relativistic
effects on the HII region shape; MNRAS in pres
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