1,366 research outputs found
Where Are the Baryons? II: Feedback Effects
Numerical simulations of the intergalactic medium have shown that at the
present epoch a significant fraction (40-50%) of the baryonic component should
be found in the (T~10^6K) Warm-Hot Intergalactic Medium (WHIM) - with several
recent observational lines of evidence indicating the validity of the
prediction. We here recompute the evolution of the WHIM with the following
major improvements: (1) galactic superwind feedback processes from galaxy/star
formation are explicitly included; (2) major metal species (O V to O IX) are
computed explicitly in a non-equilibrium way; (3) mass and spatial dynamic
ranges are larger by a factor of 8 and 2, respectively, than in our previous
simulations. Here are the major findings: (1) galactic superwinds have dramatic
effects, increasing the WHIM mass fraction by about 20%, primarily through
heating up warm gas near galaxies with density 10^{1.5}-10^4 times the mean
density. (2) the fraction of baryons in WHIM is increased modestly from the
earlier work but is ~40-50%. (3) the gas density of the WHIM is broadly peaked
at a density 10-20 times the mean density, ranging from underdense regions to
regions that are overdense by 10^3-10^4. (4) the median metallicity of the WHIM
is 0.18 Zsun for oxygen with 50% and 90% intervals being (0.040,0.38) and
(0.0017,0.83).Comment: 44 pages, 17 figures, high res version at
http://www.astro.princeton.edu/~cen/baryonII.ps.g
Cosmological SPH simulations with four million particles: statistical properties of X-ray clusters in a low-density universe
We present results from a series of cosmological SPH (smoothed particle
hydrodynamics) simulations coupled with the P3M
(Particle-Particle-Particle-Mesh) solver for the gravitational force. The
simulations are designed to predict the statistical properties of X-ray
clusters of galaxies as well as to study the formation of galaxies. We have
seven simulation runs with different assumptions on the thermal state of the
intracluster gas. Following the recent work by Pearce et al., we modify our SPH
algorithm so as to phenomenologically incorporate the galaxy formation by
decoupling the cooled gas particles from the hot gas particles. All the
simulations employ 128^3 particles both for dark matter and for gas components,
and thus constitute the largest systematic catalogues of simulated clusters in
the SPH method performed so far. These enable us to compare the analytical
predictions on statistical properties of X-ray clusters against our direct
simulation results in an unbiased manner. We find that the luminosities of the
simulated clusters are quite sensitive to the thermal history and also to the
numerical resolution of the simulations, and thus are not reliable. On the
other hand, the mass-temperature relation for the simulated clusters is fairly
insensitive to the assumptions of the thermal state of the intracluster gas,
robust against the numerical resolution, and in fact agrees well with the
analytic prediction. Therefore the prediction for the X-ray temperature
function of clusters on the basis of the Press-Schechter mass function and the
virial equilibrium is fairly reliable.Comment: Accepted for publication in The Astrophysical Journal. 18 pages with
7 embedded figure
What Powers Lyman alpha Blobs?
Lyman alpha blobs (LABs) are spatially extended lyman alpha nebulae seen at
high redshift. The origin of Lyman alpha emission in the LABs is still unclear
and under debate. To study their heating mechanism(s), we present Australia
Telescope Compact Array (ATCA) observations of the 20 cm radio emission and
Herschel PACS and SPIRE measurements of the far-infrared (FIR) emission towards
the four LABs in the protocluster J2143-4423 at z=2.38. Among the four LABs, B6
and B7 are detected in the radio with fluxes of 67+/-17 microJy and 77+/-16
microJy, respectively, and B5 is marginally detected at 3 sigma (51+/-16
microJy). For all detected sources, their radio positions are consistent with
the central positions of the LABs. B6 and B7 are obviously also detected in the
FIR. By fitting the data with different templates, we obtained redshifts of
2.20 for B6 and 2.20 for B7 which are
consistent with the redshift of the lyman alpha emission within uncertainties,
indicating that both FIR sources are likely associated with the LABs. The
associated FIR emission in B6 and B7 and high star formation rates strongly
favor star formation in galaxies as an important powering source for the lyman
alpha emission in both LABs. However, the other two, B1 and B5, are
predominantly driven by the active galactic nuclei or other sources of energy
still to be specified, but not mainly by star formation. In general, the LABs
are powered by quite diverse sources of energy.Comment: 7 pages and 3 figurs, accepted by A&
Time Evolution of Galaxy Formation and Bias in Cosmological Simulations
The clustering of galaxies relative to the mass distribution declines with
time because: first, nonlinear peaks become less rare events; second, the
densest regions stop forming new galaxies because gas there becomes too hot to
cool and collapse; third, after galaxies form, they are gravitationally
``debiased'' because their velocity field is the same as the dark matter. To
show these effects, we perform a hydrodynamic cosmological simulation and
examine the density field of recently formed galaxies as a function of
redshift. We find the bias b_* of recently formed galaxies (the ratio of the
rms fluctuations of these galaxies and mass), evolves from 4.5 at z=3 to around
1 at z=0, on 8 h^{-1} Mpc comoving scales. The correlation coefficient r_*
between recently formed galaxies and mass evolves from 0.9 at z=3 to 0.25 at
z=0. As gas in the universe heats up and prevents star formation, star-forming
galaxies become poorer tracers of the mass density field. After galaxies form,
the linear continuity equation is a good approximation to the gravitational
debiasing, even on nonlinear scales. The most interesting observational
consequence of the simulations is that the linear regression of the
star-formation density field on the galaxy density field evolves from about 0.9
at z=1 to 0.35 at z=0. These effects also provide a possible explanation for
the Butcher-Oemler effect, the excess of blue galaxies in clusters at redshift
z ~ 0.5. Finally, we examine cluster mass-to-light ratio estimates of Omega,
finding that while Omega(z) increases with z, one's estimate Omega_est(z)
decreases. (Abridged)Comment: 31 pages of text and figures; submitted to Ap
Dual-Frequency Observations of 140 Compact, Flat-Spectrum Active Galactic Nuclei for Scintillation-Induced Variability
The 4.9 GHz Micro-Arcsecond Scintillation-Induced Variability (MASIV) Survey
detected a drop in Interstellar Scintillation (ISS) for sources at redshifts z
> 2, indicating an apparent increase in angular diameter or a decrease in flux
density of the most compact components of these sources, relative to their
extended emission. This can result from intrinsic source size effects or
scatter broadening in the Intergalactic Medium (IGM), in excess of the expected
(1+z)^0.5 angular diameter scaling of brightness temperature limited sources
due to cosmological expansion. We report here 4.9 GHz and 8.4 GHz observations
and data analysis for a sample of 140 compact, flat-spectrum sources which may
allow us to determine the origin of this angular diameter-redshift relation by
exploiting their different wavelength dependences. In addition to using ISS as
a cosmological probe, the observations provide additional insight into source
morphologies and the characteristics of ISS. As in the MASIV Survey, the
variability of the sources is found to be significantly correlated with
line-of-sight H-alpha intensities, confirming its link with ISS. For 25
sources, time delays of about 0.15 to 3 days are observed between the
scintillation patterns at both frequencies, interpreted as being caused by a
shift in core positions when probed at different optical depths. Significant
correlation is found between ISS amplitudes and source spectral index; in
particular, a large drop in ISS amplitudes is observed at spectral indices of <
-0.4 confirming that steep spectrum sources scintillate less. We detect a
weakened redshift dependence of ISS at 8.4 GHz over that at 4.9 GHz, with the
mean variance at 4-day timescales reduced by a factor of 1.8 in the z > 2
sources relative to the z < 2 sources, as opposed to the factor of 3 decrease
observed at 4.9 GHz. This suggests scatter broadening in the IGM.Comment: 30 pages, 14 figures, accepted for publication in the Astronomical
Journa
X-ray Preionisation Powered by Accretion on the First Black Holes. II: Cosmological Simulations and Observational Signatures
We use cosmological simulations to study the X-ray ionisation and heating of
the intergalactic medium by an early population of accreting black holes. By
considering observational constraints from the X-ray background, we find an
upper limit for the optical depth to Thompson scattering tau_e~0.17. The
redshifted soft X-ray background from these early sources produces:(i) fully
ionised atomic hydrogen in the low density intergalactic medium before redshift
z~7 (consequently stellar reionisation is characterised by an instantaneous
overlap phase of HII regions),(ii) a second HeII reionisation at z~3 and (iii)
heats the intergalactic medium to near 10000 K at low redshifts. The typical
luminosity in the soft X-ray band of the galaxies hosting the black holes is
about one order of magnitude below the sensitivity limit of the Chandra deep
field. About a thousand of these sources may be present per square arcmin of
the sky, producing detectable fluctuations. Few rarer objects could be luminous
enough to be visible in the Chandra deep field. XEUS and Con-X satellites will
be able to detect more of these sources that, if radio loud, could be used to
study the 21cm forest in absorption.A signature of an early X-ray preionisation
is the production of secondary CMB anisotropies on small angular scales. We
find that in these models the power spectrum of temperature fluctuations
increases with decreasing angular scale (dT~16 muK at 1arcsec scales), while
for stellar reionisation scenarios the power decreases on smaller scales. We
also show that the redshifted 21 cm radiation from neutral hydrogen can be
marginally detected in emission at redshifts 7<z<12. At a redshift of about
z~30 a stronger and narrower (in redshift space) signal in absorption against
the CMB, that is peculiar to these models,could be detectable.[abridged]Comment: 14 pages including 14 figures and 2 tables. Accepted for publication
in MNRA
Gamma-Ray Background from Structure Formation in the Intergalactic Medium
The universe is filled with a diffuse and isotropic extragalactic background
of gamma-ray radiation, containing roughly equal energy flux per decade in
photon energy between 3 MeV-100 GeV. The origin of this background is one of
the unsolved puzzles in cosmology. Less than a quarter of the gamma-ray flux
can be attributed to unresolved discrete sources, but the remainder appears to
constitute a truly diffuse background whose origin has hitherto been
mysterious. Here we show that the shock waves induced by gravity during the
formation of large-scale structure in the intergalactic medium, produce a
population of highly-relativistic electrons with a maximum Lorentz factor above
10^7. These electrons scatter a small fraction of the microwave background
photons in the present-day universe up to gamma-ray energies, thereby providing
the gamma-ray background. The predicted diffuse flux agrees with the observed
background over more than four decades in photon energy, and implies a mean
cosmological density of baryons which is consistent with Big-Bang
nucleosynthesis.Comment: 7 pages, 1 figure. Accepted for publication in Nature. (Press embargo
until published.
Ultrafast geometric manipulation of electron spin and detection of the geometric phase via Faraday rotation spectroscopy
Time-resolved Faraday rotation spectroscopy is currently exploited as a
powerful technique to probe spin dynamics in semiconductors. We propose here an
all-optical approach to geometrically manipulate electron spin and to detect
the geometric phase by this type of extremely sensitive experiment. The global
nature of the geometric phase can make the quantum manipulation more stable,
which may find interesting application in quantum devices.Comment: 4 pages, 3 figures, to appear in Phys. Rev.
A wide-field spectroscopic survey of the cluster of galaxies Cl0024+1654: I. The catalogue
We present the catalogue of a wide-field CFHT/WHT spectroscopic survey of the
lensing cluster Cl0024+1654 at z=0.395. This catalogue contains 618 new
spectra, of which 581 have identified redshifts. Adding redshifts available
from the literature, the final catalogue contains data for 687 objects with
redshifts identified for 650 of them. 295 galaxies have redshifts in the range
0.37<z<0.41, i. e. are cluster members or lie in the immediate neighbourhood of
the cluster. The area covered by the survey is 21x25 arcmin2 in size,
corresponding to 4x4.8 h^-2 Mpc2 at the cluster redshift. The survey is 45%
complete down to V=22 over the whole field covered; within 3 arcmin of the
cluster centre the completeness exceeds 80% at the same magnitude. A detailed
completeness analysis is presented. The catalogue gives astrometric position,
redshift, V magnitude and V-I colour, as well as the equivalent widths for a
number of lines. Apart from the cluster Cl0024+1654 itself, three other
structures are identified in redshift space: a group of galaxies at z=0.38,
just in front of Cl0024+1654 and probably interacting with it, a close pair of
groups of galaxies at z~0.495 and an overdensity of galaxies at z~0.18 with no
obvious centre. The spectroscopic catalogue will be used to trace the
three-dimensional structure of the cluster Cl0024+1654 as well as study the
physical properties of the galaxies in the cluster and in its environment.Comment: 14 pages - figures included - A&A (re)submitted versio
Anchoring Bias in Online Voting
Voting online with explicit ratings could largely reflect people's
preferences and objects' qualities, but ratings are always irrational, because
they may be affected by many unpredictable factors like mood, weather, as well
as other people's votes. By analyzing two real systems, this paper reveals a
systematic bias embedding in the individual decision-making processes, namely
people tend to give a low rating after a low rating, as well as a high rating
following a high rating. This so-called \emph{anchoring bias} is validated via
extensive comparisons with null models, and numerically speaking, the extent of
bias decays with interval voting number in a logarithmic form. Our findings
could be applied in the design of recommender systems and considered as
important complementary materials to previous knowledge about anchoring effects
on financial trades, performance judgements, auctions, and so on.Comment: 5 pages, 4 tables, 5 figure
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