1,535 research outputs found
Ionizing radiation fluctuations and large-scale structure in the Lyman-alpha forest
We investigate the large-scale inhomogeneities of the hydrogen ionizing
radiation field in the Universe at redshift z=3. Using a raytracing algorithm,
we simulate a model in which quasars are the dominant sources of radiation. We
make use of large scale N-body simulations of a LambdaCDM universe, and include
such effects as finite quasar lifetimes and output on the lightcone, which
affects the shape of quasar light echoes. We create Lya forest spectra that
would be generated in the presence of such a fluctuating radiation field,
finding that the power spectrum of the Lya forest can be suppressed by as much
as 15 % for modes with k=0.05-1 Mpc/h. This relatively small effect may have
consequences for high precision measurements of the Lya power spectrum on
larger scales than have yet been published. We also investigate another
radiation field probe, the cross-correlation of quasar positions and the Lya
forest. For both quasar lifetimes which we simulate (10^7 yr and 10^8 yr), we
expect to see a strong decrease in the Lya absorption close to other quasars
(the ``foreground'' proximity effect). We then use data from the Sloan Digital
Sky Survey First Data Release to make an observational determination of this
statistic. We find no sign of our predicted lack of absorption, but instead
increased absorption close to quasars. If the bursts of radiation from quasars
last on average < 10^6 yr, then we would not expect to be able to see the
foreground effect. However, the strength of the absorption itself seems to be
indicative of rare objects, and hence much longer total times of emission per
quasar. Variability of quasars in bursts with timescales > 10^4yr and < 10^6 yr
could reconcile these two facts.Comment: Submitted to ApJ, 21 pages, 17 postscript figures, emulateapj.st
Constraining quasar host halo masses with the strength of nearby Lyman-alpha forest absorption
Using cosmological hydrodynamic simulations we measure the mean transmitted
flux in the Lyman alpha forest for quasar sightlines that pass near a
foreground quasar. We find that the trend of absorption with pixel-quasar
separation distance can be fitted using a simple power law form including the
usual correlation function parameters r_{0} and \gamma so that ( = \sum
exp(-tau_eff*(1+(r/r_{0})^(-\gamma)))). From the simulations we find the
relation between r_{0} and quasar mass and formulate this as a way to estimate
quasar host dark matter halo masses, quantifying uncertainties due to
cosmological and IGM parameters, and redshift errors. With this method, we
examine data for ~3000 quasars from the Sloan Digital Sky Survey (SDSS) Data
Release 3, assuming that the effect of ionizing radiation from quasars (the
so-called transverse proximity effect) is unimportant (no evidence for it is
seen in the data.) We find that the best fit host halo mass for SDSS quasars
with mean redshift z=3 and absolute G band magnitude -27.5 is log10(M/M_sun) =
12.48^{+0.53}_{-0.89}. We also use the Lyman-Break Galaxy (LBG) and Lyman alpha
forest data of Adelberger et al in a similar fashion to constrain the halo mass
of LBGs to be log10(M/M_sun) = 11.13^{+0.39}_{-0.55}, a factor of ~20 lower
than the bright quasars. In addition, we study the redshift distortions of the
Lyman alpha forest around quasars, using the simulations. We use the quadrupole
to monopole ratio of the quasar-Lyman alpha forest correlation function as a
measure of the squashing effect. We find that this does not have a measurable
dependence on halo mass, but may be useful for constraining cosmic geometry.Comment: 10 pages, 11 figures, submitted to MNRA
Artificial Brains and Hybrid Minds
The paper develops two related thought experiments exploring variations on an ‘animat’ theme. Animats are hybrid devices with both artificial and biological components. Traditionally, ‘components’ have been construed in concrete terms, as physical parts or constituent material structures. Many fascinating issues arise within this context of hybrid physical organization. However, within the context of functional/computational theories of mentality, demarcations based purely on material structure are unduly narrow. It is abstract functional structure which does the key work in characterizing the respective ‘components’ of thinking systems, while the ‘stuff’ of material implementation is of secondary importance. Thus the paper extends the received animat paradigm, and investigates some intriguing consequences of expanding the conception of bio-machine hybrids to include abstract functional and semantic structure. In particular, the thought experiments consider cases of mind-machine merger where there is no physical Brain-Machine Interface: indeed, the material human body and brain have been removed from the picture altogether. The first experiment illustrates some intrinsic theoretical difficulties in attempting to replicate the human mind in an alternative material medium, while the second reveals some deep conceptual problems in attempting to create a form of truly Artificial General Intelligence
Enhanced radiation damage tolerance of amorphous interphase and grain boundary complexions in Cu-Ta
Amorphous interfacial complexions are particularly resistant to radiation
damage and have been primarily studied in alloys with good glass-forming
ability, yet recent reports suggest that these features can form even in
immiscible alloys such as Cu-Ta under irradiation. In this study, the
mechanisms of damage production and annihilation due to primary knock-on atom
collisions are investigated for amorphous interphase and grain boundaries in a
Cu-Ta alloy using atomistic simulations. Amorphous complexions, in particular
amorphous interphase complexions that separate Cu and Ta grains, result in less
residual defect damage than their ordered counterparts. Stemming from the
nanophase chemical separation in this alloy, the amorphous complexions exhibit
a highly heterogeneous distribution of atomic excess volume, as compared to a
good glass former like Cu-Zr. Complexion thickness, a tunable structural
descriptor, plays a vital role in damage resistance. Thicker interfacial films
are more damage-tolerant because they alter the defect production rate due to
differences in intrinsic displacement threshold energies during the collision
cascade. Overall, the findings of this work highlight the importance of
interfacial engineering in enhancing the properties of materials operating in
radiation-prone environments and the promise of amorphous complexions as
particularly radiation damage-tolerant microstructural features
Weak lensing surveys and the intrinsic correlation of galaxy ellipticities
We explore the possibility that an intrinsic correlation between galaxy
ellipticities arising during the galaxy formation process may account for part
of the shear signal recently reported by several groups engaged in weak lensing
surveys. Using high resolution N-body simulations we measure the projected
ellipticities of dark matter halos and their correlations as a function of pair
separation. With this simplifying, but not necessarily realistic assumption
(halo shapes as a proxy for galaxy shapes), we find a positive detection of
correlations up to scales of at least 20 h^-1mpc (limited by the box size). The
signal is not strongly affected by variations in the halo finding technique, or
by the resolution of the simulations. We translate our 3d results into angular
measurements of ellipticity correlation functions and shear variance which can
be directly compared to observations. We also measure similar results from
simulated angular surveys made by projecting our simulation boxes onto the
plane of the sky and applying a radial selection function. Interestingly, the
shear variance we measure is a small, but not entirely negligible fraction
(from ~10-20 %) of that seen by the observational groups, and the ellipticity
correlation functions approximately mimic the functional form expected to be
caused by weak lensing. The amplitude depends on the width in redshift of the
galaxy distribution. If photometric redshifts are used to pick out a screen of
background galaxies with a small width, then the intrinsic correlation may
become comparable to the weak lensing signal. Although we are dealing with
simulated dark matter halos, whether there is a signal from real galaxies could
be checked with a nearby sample with known redshifts.Comment: 12 pages, 11 ps figures, emulateapj.sty, submitted to Ap
Non-standard antineutrino interactions at Daya Bay
We study the prospects of pinning down the effects of non-standard
antineutrino interactions in the source and in the detector at the Daya Bay
neutrino facility. It is well known that if the non-standard interactions in
the detection process are of the same type as those in the production, their
net effect can be subsumed into a mere shift in the measured value of the
leptonic mixing angle theta_13. Relaxing this assumption, the ratio of the
antineutrino spectra measured by the Daya Bay far and near detectors is
distorted in a characteristic way, and good fits based on the standard
oscillation hypothesis are no longer viable. We show that, under certain
conditions, three years of Daya Bay running can be sufficient to provide a
clear hint of non-standard neutrino physics.Comment: 31 pages, 12 figures; a brief discussion of systematics added in v2,
published versio
Raman spectroscopy - A powerful tool for in situ planetary science
This paper introduces Raman spectroscopy and discusses various scenarios where it might be applied to in situ planetary missions. We demonstrate the extensive capabilities of Raman spectroscopy for planetary investigations and argue that this technique is essential for future planetary missions
Crystallographic structure of ultrathin Fe films on Cu(100)
We report bcc-like crystal structures in 2-4 ML Fe films grown on fcc Cu(100)
using scanning tunneling microscopy. The local bcc structure provides a
straightforward explanation for their frequently reported outstanding magnetic
properties, i.e., ferromagnetic ordering in all layers with a Curie temperature
above 300 K. The non-pseudomorphic structure, which becomes pseudomorphic above
4 ML film thickness is unexpected in terms of conventional rules of thin film
growth and stresses the importance of finite thickness effects in ferromagnetic
ultrathin films.Comment: 4 pages, 3 figures, RevTeX/LaTeX2.0
- …