8,838 research outputs found
Introducing the AASLD president: Anna S.F. Lok
Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/136432/1/hep29073_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/136432/2/hep29073.pd
Large phenotype jumps in biomolecular evolution
By defining the phenotype of a biopolymer by its active three-dimensional
shape, and its genotype by its primary sequence, we propose a model that
predicts and characterizes the statistical distribution of a population of
biopolymers with a specific phenotype, that originated from a given genotypic
sequence by a single mutational event. Depending on the ratio g0 that
characterizes the spread of potential energies of the mutated population with
respect to temperature, three different statistical regimes have been
identified. We suggest that biopolymers found in nature are in a critical
regime with g0 in the range 1-6, corresponding to a broad, but not too broad,
phenotypic distribution resembling a truncated Levy flight. Thus the biopolymer
phenotype can be considerably modified in just a few mutations. The proposed
model is in good agreement with the experimental distribution of activities
determined for a population of single mutants of a group I ribozyme.Comment: to appear in Phys. Rev. E; 7 pages, 6 figures; longer discussion in
VII, new fig.
The B-Band Luminosity Function of Red and Blue Galaxies up to z=3.5
We have explored the redshift evolution of the luminosity function of red and
blue galaxies up to . This was possible joining a deep I band composite
galaxy sample, which includes the spectroscopic K20 sample and the HDFs
samples, with the deep and samples derived from the
deep NIR images of the Hubble Deep Fields North and South, respectively. About
30% of the sample has spectroscopic redshifts and the remaining fraction
well-calibrated photometric redshifts. This allowed to select and measure
galaxies in the rest-frame blue magnitude up to and to derive the
redshift evolution of the B-band luminosity function of galaxies separated by
their rest-frame color or specific (i.e. per unit mass) star-formation
rate. The class separation was derived from passive evolutionary tracks or from
their observed bimodal distributions. Both distributions appear bimodal at
least up to and the locus of red/early galaxies is clearly identified
up to these high redshifts. Both luminosity and density evolutions are needed
to describe the cosmological behaviour of the red/early and blue/late
populations. The density evolution is greater for the early population with a
decrease by one order of magnitude at with respect to the value at
. The luminosity densities of the early and late type galaxies with
. Indeed while star-forming
galaxies slightly increase or keep constant their luminosity density, "early"
galaxies decrease in their luminosity density by a factor from
to . A comparison with one of the latest versions of
the hierarchical CDM models shows a broad agreement with the observed number
and luminosity density evolutions of both populations.Comment: 41 pages, 14 figures, accepted for publication in Ap
The Population of Damped Lyman-alpha and Lyman Limit Systems in the Cold Dark Matter Model
Lyman limit and damped Lyman-alpha absorption systems probe the distribution
of collapsed, cold gas at high redshift. Numerical simulations that incorporate
gravity and gas dynamics can predict the abundance of such absorbers in
cosmological models. We develop a semi-analytical method to correct the
numerical predictions for the contribution of unresolved low mass halos, and we
apply this method to the Katz et al. (1996) simulation of the standard cold
dark matter model (, , , ). Using
this simulation and higher resolution simulations of individual low mass
systems, we determine the relation between a halo's circular velocity and
its cross section for producing Lyman limit or damped absorption. We combine
this relation with the Press-Schechter formula for the abundance of halos to
compute the number of absorbers per unit redshift. The resolution correction
increases the predicted abundances by about a factor of two at z=2, 3, and 4,
bringing the predicted number of damped absorbers into quite good agreement
with observations. Roughly half of the systems reside in halos with circular
velocities v_c>100\kms and half in halos with 35\kms. Halos
with v_c>150\kms typically harbor two or more systems capable of producing
damped absorption. Even with the resolution correction, the predicted abundance
of Lyman limit systems is a factor of three below observational estimates,
signifying either a failure of standard CDM or a failure of these simulations
to resolve the systems responsible for most Lyman limit absorption. By
comparing simulations with and without star formation, we find that depletion
of the gas supply by star formation affects absorption line statistics at
only for column densities exceeding .Comment: AASlatex, 17 pages w/ 3 embedded ps figures. Submitted to Ap
Combined effects due to phase, intensity and contrast in electrooptic modulation. Application to ferroelectric materials
The combination of phase, intensity, and contrast effects during electrooptic
modulation is theoretically and exper- imentally investigated. One consequence
of this combination is the modification of the amplitude of the
single-frequency signals which are commonly used as working points for
electrooptic mod- ulators and for the measurements of the electrooptic
coefficients. Another consequence of direct intensity modulation is to shift
the double-frequency points of the transfer function from the positions they
normally occupy at the intensity extrema. They can even make them disappear if
the direct intensity modulation is stronger than the phase modulation. Such
phenomena are expected with any ferroelectric material in which a significant
part of the incident light is deflected or scattered by domain walls or grain
boundaries. They can lead to considerable mistakes in the determination of the
electrooptic coefficients. Appropriate procedures to extract the different
contributions are explained. Experimental results in rubidium hydrogen selenate
are given, and consequences of the working of electrooptic modulators are
discussed
Non-equilibrium phase transitions in biomolecular signal transduction
We study a mechanism for reliable switching in biomolecular
signal-transduction cascades. Steady bistable states are created by system-size
cooperative effects in populations of proteins, in spite of the fact that the
phosphorylation-state transitions of any molecule, by means of which the switch
is implemented, are highly stochastic. The emergence of switching is a
nonequilibrium phase transition in an energetically driven, dissipative system
described by a master equation. We use operator and functional integral methods
from reaction-diffusion theory to solve for the phase structure, noise
spectrum, and escape trajectories and first-passage times of a class of minimal
models of switches, showing how all critical properties for switch behavior can
be computed within a unified framework
The assembly of "normal" galaxies at z=7 probed by ALMA
We report new deep ALMA observations aimed at investigating the [CII]158um
line and continuum emission in three spectroscopically confirmed Lyman Break
Galaxies at 6.8<z<7.1, i.e. well within the re-ionization epoch. With Star
Formation Rates of SFR ~ 5-15 Msun/yr these systems are much more
representative of the high-z galaxy population than other systems targeted in
the past by millimeter observations. For the galaxy with the deepest
observation we detect [CII] emission at redshift z=7.107, fully consistent with
the Lyalpha redshift, but spatially offset by 0.7" (4 kpc) from the optical
emission. At the location of the optical emission, tracing both the Lyalpha
line and the far-UV continuum, no [CII] emission is detected in any of the
three galaxies, with 3sigma upper limits significantly lower than the [CII]
emission observed in lower reshift galaxies. These results suggest that
molecular clouds in the central parts of primordial galaxies are rapidly
disrupted by stellar feedback. As a result, [CII] emission mostly arises from
more external accreting/satellite clumps of neutral gas. These findings are in
agreement with recent models of galaxy formation. Thermal far-infrared
continuum is not detected in any of the three galaxies. However, the upper
limits on the infrared-to-UV emission ratio do not exceed those derived in
metal- and dust-poor galaxies.Comment: 15 pages, 9 figures, MNRAS in press, replaced with accepted versio
A flash in the dark: UVES/VLT high resolution spectroscopy of GRB afterglows
We present the first high resolution (R=20000--45000, corresponding to 14
km/s at 4200A to 6.6 km/s at 9000A) observations of the optical afterglow of
Gamma Ray Bursts. GRB020813 and GRB021004 were observed by UVES@VLT 22.19 hours
and 13.52 hours after the trigger, respectively. These spectra show that the
inter--stellar matter of the GRB host galaxies is complex, with many components
contributing to each main absorption system, and spanning a total velocity
range of up to about 3000 km/s. Several narrow components are resolved down to
a width of a few tens of km/s. In the case of GRB021004 we detected both low
and high ionization lines. Combined with photoionization results obtained with
CLOUDY, the ionization parameters of the various systems are consistent with a
remarkably narrow range with no clear trend with system velocity. This can be
interpreted as due to density fluctuations on top of a regular R^-2 wind
density profile.Comment: Most figure improved, a few typos corrected, added a new subsection.
ApJ in pres
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