2,574 research outputs found
Precision Cosmology from the Lyman-alpha Forest: Power Spectrum and Bispectrum
We investigate the promise of the Ly-alpha forest for high precision
cosmology in the era of the Sloan Digital Sky Survey using low order N-point
statistics. We show that with the existing data one can determine the
amplitude, slope and curvature of the slope of the matter power spectrum with a
few percent precision. Higher order statistics such as the bispectrum provide
independent information that can confirm and improve upon the statistical
precision from the power spectrum alone. The achievable precision is comparable
to that from the cosmic microwave background with upcoming satellites, and
complements it by measuring the power spectrum amplitude and shape at smaller
scales. Since the data cover the redshift range 2<z<4, one can also extract the
evolution of the growth factor and Hubble parameter over this range, and
provide useful constraints on the presence of dark energy at z>2.Comment: 14 pages, 17 figures, accepted to MNRAS; minor changes made (section
2) and references adde
Supernova Feedback and the Hot Gas Filling Fraction of the Interstellar Medium
Supernovae (SN), the most energetic stellar feedback mechanism, are crucial
for regulating the interstellar medium (ISM) and launching galactic winds. We
explore how supernova remnants (SNRs) create a multiphase medium by performing
3D hydrodynamical simulations at various SN rates, , and ISM average
densities, . The evolution of a SNR in a self-consistently generated
three-phase ISM is qualitatively different from that in a uniform or a
two-phase warm/cold medium. By travelling faster and further in the low-density
hot phase, the domain of a SNR increases by . Varying and
, we find that a steady state can only be achieved when the hot gas volume
fraction . Above that level, overlapping
SNRs render connecting topology of the hot gas, and the ISM is subjected to
thermal runaway. Photoelectric heating (PEH) has a surprisingly strong impact
on . For \bar{n}\gtrsim 3 \cm-3 , a reasonable PEH rate is
able to suppress the thermal runaway. Overall, we determine the critical SN
rate for the onset of thermal runaway to be S_{\rm{crit}} = 200
(\bar{n}/1\cm-3)^k (E_{\rm{SN}}/10^{51}\erg)^{-1} \kpc^{-3} \myr-1, where for and > 1\cm-3 , respectively. We present a
fitting formula of the ISM pressure , ), which can be used as an
effective equation of state in cosmological simulations. Despite the 5 orders
of magnitude span of , the average Mach number varies little:
for the hot, warm
and cold phases, respectively.Comment: 57 pages, 16 figures, 3 tables. ApJ accepte
A Fast, Accurate and Robust Algorithm For Transferring Radiation in Three-Dimensional Space
We have developed an algorithm for transferring radiation in
three-dimensional space. The algorithm computes radiation source and sink terms
using the Fast Fourier Transform (FFT) method, based on a formulation in which
the integral of any quantity (such as emissivity or opacity) over any volume
may be written in the classic convolution form. The algorithm is fast with the
computational time scaling as N (log N)^2, where N is the number of grid points
of a simulation box, independent of the number of radiation sources.
Furthermore, in this formulation one can naturally account for both local
radiation sources and diffuse background as well as any extra external sources,
all in a self-consistent fashion. Finally, the algorithm is completely stable
and robust.
While the algorithm is generally applicable, we test it on a set of problems
that encompass a wide range of situations in cosmological applications,
demonstrating that the algorithm is accurate. These tests show that the
algorithm produces results that are in excellent agreement with analytic
expectations in all cases. In particular, radiation flux is guaranteed to
propagate in the right direction, with the ionization fronts traveling at the
correct speed with an error no larger than one cell for all the cases tested.
The total number of photons is conserved in the worst case at 10% level and
typically at 1-5% level over hundreds of time steps. As an added advantage, the
accuracy of the results depends weakly on the size of the time step, with a
typical cosmological hydrodynamic time step being sufficient.Comment: submitted to ApJ, 50 pages, added comparisons to ray tracing metho
An X-ray WHIM metal absorber from a Mpc-scale empty region of space
We report a detection of an absorption line at ~44.8 {\AA} in a > 500 ks
Chandra HRC-S/LETG X-ray grating spectrum of the blazar H 2356-309. This line
can be identified as intervening CV-K{\alpha} absorption, at z\approx0.112,
produced by a warm (log T = 5.1 K) intergalactic absorber. The feature is
significant at a 2.9{\sigma} level (accounting for the number of independent
redshift trials). We estimate an equivalent hydrogen column density of log
N_H=19.05 (Z/Zsun)^-1 cm^-2. Unlike other previously reported FUV/X-ray metal
detections of warm-hot intergalactic medium (WHIM), this CV absorber lies in a
region with locally low galaxy density, at ~2.2 Mpc from the closest galaxy at
that redshift, and therefore is unlikely to be associated with an extended
galactic halo. We instead tentatively identify this absorber with an
intervening Warm-Hot Intergalactic Medium filament possibly permeating a
large-scale, 30 Mpc extended, structure of galaxies whose redshift centroid,
within a cylinder of 7.5 Mpc radius centered on the line of sight to H
2356-309, is marginally consistent (at a 1.8{\sigma} level) with the redshift
of the absorber.Comment: ApJ accepted, 6 pages, 3 figure
Studying the WHIM Content of the Galaxy Large-Scale Structures along the Line of Sight to H 2356-309
We make use of a 500ks Chandra HRC-S/LETG spectrum of the blazar H2356-309,
combined with a lower S/N spectrum of the same target, to search for the
presence of warm-hot absorbing gas associated with two Large-Scale Structures
(LSSs) crossed by this sightline at z=0.062 (the Pisces-Cetus Supercluster,
PCS) and at z=0.128 ("Farther Sculptor Wall", FSW). No statistically
significant (>=3sigma) individual absorption is detected from any of the strong
He- or H-like transitions of C, O and Ne at the redshifts of the structures.
However we are still able to constrain the physical and geometrical parameters
of the associated putative absorbing gas, by performing joint spectral fit of
marginal detections and upper limits of the strongest expected lines with our
self-consistent hybrid ionization WHIM spectral model. At the redshift of the
PCS we identify a warm phase with logT=5.35_-0.13^+0.07 K and log N_H
=19.1+/-0.2 cm^-2 possibly coexisting with a hotter and less significant phase
with logT=6.9^+0.1_-0.8 K and log N_H=20.1^+0.3_-1.7 cm^-2 (1sigma errors). For
the FSW we estimate logT=6.6_-0.2^+0.1 K and log N_H=19.8_-0.8^+0.4 cm^-2. Our
constraints allow us to estimate the cumulative number density per unit
redshifts of OVII WHIM absorbers. We also estimate the cosmological mass
density obtaining Omega_b(WHIM)=(0.021^+0.031_-0.018) (Z/Z_sun)^-1, consistent
with the mass density of the intergalactic 'missing baryons' for high
metallicities.Comment: 29 pages, 8 figures, 4 tables. Accepted for publication in Ap
A Comparison of Cosmological Hydrodynamic Codes
We present a detailed comparison of the simulation results of various
cosmological hydrodynamic codes. Starting with identical initial conditions
based on the Cold Dark Matter scenario for the growth of structure, we
integrate from redshift to to determine the physical state within
a representative volume of size where . Five
independent codes are compared: three of them Eulerian mesh based and two
variants of the Smooth Particle Hydrodynamics "SPH" Lagrangian approach. The
Eulerian codes were run at cells,
the SPH codes at and particles. Results were then rebinned
to a grid with the expectation that the rebinned data should converge,
by all techniques, to a common and correct result as . We
find that global averages of various physical quantities do, as expected, tend
to converge in the rebinned model, but that uncertainties in even primitive
quantities such as , persists
at the 3\%-17\% level after completion of very large simulations. The two SPH
codes and the two shock capturing Eulerian codes achieve comparable and
satisfactory accuracy for comparable computer time in their treatment of the
high density, high temperature regions as measured in the rebinned data; the
variance among the five codes (at highest resolution) for the mean temperature
(as weighted by ) is only 4.5\%. Overall the comparison allows us to
better estimate errors, it points to ways of improving this current generation
of hydrodynamic codes and of suiting their use to problems which exploit their
individually best features.Comment: 20p plaintex to appear in The Astrophysical Journal on July 20, 199
Controllable coherent population transfers in superconducting qubits for quantum computing
We propose an approach to coherently transfer populations between selected
quantum states in one- and two-qubit systems by using controllable
Stark-chirped rapid adiabatic passages (SCRAPs). These {\it evolution-time
insensitive} transfers, assisted by easily implementable single-qubit
phase-shift operations, could serve as elementary logic gates for quantum
computing. Specifically, this proposal could be conveniently demonstrated with
existing Josephson phase qubits. Our proposal can find an immediate application
in the readout of these qubits. Indeed, the broken parity symmetries of the
bound states in these artificial "atoms" provide an efficient approach to
design the required adiabatic pulses.Comment: 4 pages, 6 figures. to appear in Physical Review Letter
Galaxy Size Problem at z=3: Simulated Galaxies Are Too Small
Using state-of-the-art adaptive mesh refinement cosmological hydrodynamic
simulations with a spatial resolution of proper 0.21kpc/h in refined subregions
embedded within a comoving cosmological volume (27.4Mpc/h)^3, we investigate
the sizes of galaxies at z=3 in the standard cold dark matter model where
reionization is assumed to complete at zri~6. Our simulated galaxies are found
to be significantly smaller than the observed ones: while more than one half of
the galaxies observed by HST and VLT ranging from rest-frame UV to optical
bands with stellar masses larger than 2E10 Msun have half-light radii larger
than 2kpc/h, none of the simulated massive galaxies in the same mass range have
half-light radii larger than 2kpc/h, after taking into account dust extinction.
Corroborative evidence is provided by the rotation curves of the simulated
galaxies with total masses of 1E11-1E12Msun, which display values 300-1000km/s
at small radii (0.5kpc/h) due to high stellar concentration in the central
regions, larger than those of any well observed galaxies. Possible physical
mechanisms to resolve this serious problem include: (1) an early reionization
at zri>>6 to suppress gas condensation hence star formation, (2) a strong,
internal energetic feedback from stars or central black holes to reduce the
overall star formation efficiency, or (3) a substantial small-scale cutoff in
the matter power spectrum.Comment: high resolution pdf file is available at
http://www.astro.princeton.edu/~cen/galaxysize.pdf 15 pages, 3 figures, in
press of ApJ Letter
Comparisons of Cosmological MHD Galaxy Cluster Simulations to Radio Observations
Radio observations of galaxy clusters show that there are G magnetic
fields permeating the intra-cluster medium (ICM), but it is hard to accurately
constrain the strength and structure of the magnetic fields without the help of
advanced computer simulations. We present qualitative comparisons of synthetic
VLA observations of simulated galaxy clusters to radio observations of Faraday
Rotation Measure (RM) and radio halos. The cluster formation is modeled using
adaptive mesh refinement (AMR) magneto-hydrodynamic (MHD) simulations with the
assumption that the initial magnetic fields are injected into the ICM by active
galactic nuclei (AGNs) at high redshift. In addition to simulated clusters in
Xu et al. (2010, 2011), we present a new simulation with magnetic field
injections from multiple AGNs. We find that the cluster with multiple injection
sources is magnetized to a similar level as in previous simulations with a
single AGN. The RM profiles from simulated clusters, both and the
dispersion of RM (), are consistent at a first-order with the
radial distribution from observations. The correlations between the
and X-ray surface brightness from simulations are in a broad
agreement with the observations, although there is an indication that the
simulated clusters could be slightly over-dense and less magnetized with
respect to those in the observed sample. In addition, the simulated radio halos
agree with the observed correlations between the radio power versus the cluster
X-ray luminosity and between the radio power versus the radio halo size. These
studies show that the cluster wide magnetic fields that originate from AGNs and
are then amplified by the ICM turbulence (Xu et al. 2010) match observations of
magnetic fields in galaxy clusters.Comment: Accepted for publication in Ap
Studying the Warm-Hot Intergalactic Medium in Emission
We assess the possibility to detect the warm-hot intergalactic medium (WHIM)
in emission and to characterize its physical conditions and spatial
distribution through spatially resolved X-ray spectroscopy, in the framework of
the recently proposed DIOS, EDGE, Xenia, and ORIGIN missions, all of which are
equipped with microcalorimeter-based detectors. For this purpose we analyze a
large set of mock emission spectra, extracted from a cosmological
hydrodynamical simulation. These mock X-ray spectra are searched for emission
features showing both the OVII K alpha triplet and OVIII Ly alpha line, which
constitute a typical signature of the warm hot gas. Our analysis shows that 1
Ms long exposures and energy resolution of 2.5 eV will allow us to detect about
400 such features per deg^2 with a significance >5 sigma and reveals that these
emission systems are typically associated with density ~100 above the mean. The
temperature can be estimated from the line ratio with a precision of ~20%. The
combined effect of contamination from other lines, variation in the level of
the continuum, and degradation of the energy resolution reduces these
estimates. Yet, with an energy resolution of 7 eV and all these effects taken
into account, one still expects about 160 detections per deg^2. These line
systems are sufficient to trace the spatial distribution of the line-emitting
gas, which constitute an additional information, independent from line
statistics, to constrain the poorly known cosmic chemical enrichment history
and the stellar feedback processes.Comment: 19 pages, 10 figures, ApJ in press; revised version according to
revie
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