9,628 research outputs found
HI Structure Observations of Reionization and Dark Energy
This proceeding concentrates on the BAO signature of dark energy, and how the
SKA dark energy case has been complicated by the emergence of HI structure
experiments modeled after the Epoch of Reionization observatories. The purpose
of the conference talk was to review the current status of the Murchison
Widefield Array (MWA), and show the applications of HI structure observations
for both reionization and dark energy measurements. Since the status of the MWA
is changing weekly, please see the website www.haystack.mit.edu/ast/arrays/mwa/
for the current status. This proceedings will instead concentrate on HI
structure observations, their applicability to reionization and cosmography,
and the implications for the SKA and future HI structure observations of dark
energy.Comment: Proceeding from The Evolution of Galaxies through the Neutral
Hydrogen Window, Arecibo, 2008, AIP Conference Proceedings, Volume 103
Selective advantage for multicellular replicative strategies: A two-cell example
This paper develops a quasispecies model where cells can adopt a two-cell
survival strategy. Within this strategy, pairs of cells join together, at which
point one of the cells sacrifices its own replicative ability for the sake of
the other cell. We develop a simplified model for the evolutionary dynamics of
this process, allowing us to solve for the steady-state using standard
approaches from quasispecies theory. We find that our model exhibits two
distinct regimes of behavior: At low concentrations of limiting resource, the
two-cell strategy outcompetes the single-cell survival strategy, while at high
concentrations of limiting resource, the single-cell survival strategy
dominates. Associated with the two solution regimes of our model is a
localization to delocalization transition over the portion of the genome coding
for the multicell strategy, analogous to the error catastrophe in standard
quasispecies models. The existence of such a transition indicates that
multicellularity can emerge because natural selection does not act on specific
cells, but rather on replicative strategies. Within this framework, individual
cells become the means by which replicative strategies are propagated. Such a
framework is therefore consistent with the concept that natural selection does
not act on individuals, but rather on populations.Comment: 4 pages, 2 figures, to be submitted to Physical Review Letter
An Examination of Spinozaâs Moral Philosophy
Spinoza's moral philosopher represents his most concerted attempt to come to
terms with the great philosophical questions of the existence and identity of God, the
nature and origin of the human mind concerning God, the origin and nature of
emotions, the power of emotions as they restrict freedom of choice. His ethics is
derived from his metaphysics and psychology. His belief that everything emanates
from a perfect and infinite God made him conclude that evil does not exist. Further, he
argues that anything that happens could have happened otherwise since it emanated
from the unchangeable laws of nature. The surest part of happiness according to
Spinoza is the study of philosophy and meditation. Arising from the foregoing, this
discourse views Spinoza's doctrine as running contrary to human nature. For
maintaining that everything is fated and determined including human disposition
implies that all human actions can, therefore, be said to be amoral. The corollary of
the above is that institutions such as law court, police, prisons, and judiciary,
Christianity and Islam are superfluous, irrational and serving no purpose.
Consequently, his postulates smack of a moral lacun
Prediction of pressure drop in multiphase horizontal pipe flow
Empirical correlations were tested against reliable two phase pipe flow data for the prediction of pressure drop. Correlations are recommended for the prediction with stratified and annular type flows. When these correlations were adapted to three phase gaswater-oil pipe flow in general they predicted for intermittent slug type flows. Momentum balance models could not be successfully adapted to the prediction of pipe three phase pressure drop
Chemical enrichment of Damped Lyman Alpha systems as a direct constraint on Population III star formation
Damped Ly-alpha absorbers (DLAs) can be used to measure gas-phase
metallicities at large cosmological lookback times with high precision.
Relative abundances can still be measured accurately deep into the reionization
epoch (z > 6) using transitions redward of Ly-alpha. Here we study chemical
evolution of DLAs using a constrained model for evolution of galaxies and IGM
to determine the degree to which DLA abundance measurements can probe
Population III enrichment. We find that if the critical metallicity of
Population III to II transition is < 10^-4 Zsun, the cosmic Population III SFR
is zero for z<8. Nevertheless, at high redshift (z ~ 6) Population III chemical
signatures are retained in low-mass galaxies (halo mass < 10^9 Msun). This is
because photoionization feedback suppresses star formation in these galaxies
until relatively low redshift (z ~ 10), and the chemical record of early
Population III star formation is retained. We model DLAs as these galaxies by
assigning to them a mass-dependent H I absorption cross section and predict
distribution of DLA abundance ratios. We find that these distributions are
anchored towards abundance ratios set by Population II yields, but exhibit a
tail that depends on the Population III IMF for z > 5. Thus, a sample of DLA
abundance measurements at high redshift holds the promise to constrain
Population III IMF. A sample of just 10 DLAs with relative abundances measured
to an accuracy of 0.1 dex is sufficient to constrain the Population III IMF at
4-sigma. These constraints may prove stronger than other probes such as
metal-poor stars and individual DLAs. Our results provide a global picture of
the cosmic thermal, ionization, and chemical evolution, and can rule out
certain Population III scenarios.Comment: 21 pages, 12 figures; this version accepted in Ap
Chemical constraints on the contribution of Population III stars to cosmic reionization
Recent studies have highlighted that galaxies at z = 6-8 fall short of
producing enough ionizing photons to reionize the IGM, and suggest that
Population III stars could resolve this tension, because their harder spectra
can produce ~10x more ionizing photons than Population II. But this argument
depends critically on the duration of the Population III era, and because
Population III stars form from pristine gas, in turn depends on the rate of
galactic enrichment. We use a semi-analytic model of galaxy formation which
tracks galactic chemical evolution, to gauge the impact of Population III stars
on reionization. Population III SNe produce distinct metal abundances, and we
argue that the duration of the Population III era can be constrained by precise
relative abundance measurements in high-z damped Ly{\alpha} absorbers (DLAs),
which provide a chemical record of past star-formation. We find that a single
generation of Population III stars can self-enrich galaxies above the critical
metallicity Zcrit=10^-4 Zsun for the Population III-to-II transition, on a very
short timescale of ~10^6 yr, owing to the large metal yields and short
lifetimes of Population III stars. This subsequently terminates the Population
III era, hence they contribute >~ 50% of the ionizing photons only for z >~ 30,
and at z=10 contribute <1%. The Population III contribution can be increased by
delaying metal mixing into the ISM. However comparing the resulting metal
abundance pattern to existing measurements in z <~ 6 DLAs, we show that the
fractional contribution of high-mass Population III stars to the ionization
rate must be <~ 10% at z = 10. Future abundance measurements of z~7-8 QSOs and
GRBs should probe the era when the chemical vestiges of Population III star
formation become detectable.Comment: 12 pages, 5 figures; Submitted to ApJ; Comments welcom
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