16,006 research outputs found
Connectivity-Driven Coherence in Complex Networks
We study the emergence of coherence in complex networks of mutually coupled
non-identical elements. We uncover the precise dependence of the dynamical
coherence on the network connectivity, on the isolated dynamics of the elements
and the coupling function. These findings predict that in random graphs, the
enhancement of coherence is proportional to the mean degree. In locally
connected networks, coherence is no longer controlled by the mean degree, but
rather on how the mean degree scales with the network size. In these networks,
even when the coherence is absent, adding a fraction s of random connections
leads to an enhancement of coherence proportional to s. Our results provide a
way to control the emergent properties by the manipulation of the dynamics of
the elements and the network connectivity.Comment: 4 pages, 2 figure
Continuous Hawking-Page transitions in Einstein-scalar gravity
We investigate continuous Hawking-Page transitions in Einstein's gravity
coupled to a scalar field with an arbitrary potential in the weak gravity
limit. We show that this is only possible in a singular limit where the
black-hole horizon marginally traps a curvature singularity. Depending on the
subleading terms in the potential, a rich variety of continuous phase
transitions arise. Our examples include second and higher order, including the
Berezinskii-Kosterlitz-Thouless type. In the case when the scalar is dilaton,
the condition for a continuous phase transition lead to (asymptotically)
linear-dilaton background. We obtain the scaling laws of thermodynamic
functions, as well as the viscosity coefficients near the transition. In the
limit of weak gravitational interactions, the bulk viscosity asymptotes to a
universal constant, independent of the details of the scalar potential. As a
byproduct of our analysis we obtain a one-parameter family of kink solutions in
arbitrary dimension d that interpolate between AdS near the boundary and
linear-dilaton background in the deep interior. The continuous Hawking-Page
transitions found here serve as holographic models for normal-to superfluid
transitions.Comment: 35 pages + appendice
Stellar mass functions of galaxies at 4<z<7 from an IRAC-selected sample in COSMOS/UltraVISTA: limits on the abundance of very massive galaxies
We build a Spitzer IRAC complete catalog of objects, obtained by
complementing the -band selected UltraVISTA catalog with objects
detected in IRAC only. With the aim of identifying massive (i.e.,
) galaxies at , we consider the systematic effects
on the measured photometric redshifts from the introduction of an old and dusty
SED template and from the introduction of a bayesian prior taking into account
the brightness of the objects, as well as the systematic effects from different
star formation histories (SFHs) and from nebular emission lines in the recovery
of stellar population parameters. We show that our results are most affected by
the bayesian luminosity prior, while nebular emission lines and SFHs only
introduce a small dispersion in the measurements. Specifically, the number of
galaxies ranges from 52 to 382 depending on the adopted configuration.
Using these results we investigate, for the first time, the evolution of the
massive end of the stellar mass functions (SMFs) at . Given the rarity
of very massive galaxies in the early universe, major contributions to the
total error budget come from cosmic variance and poisson noise. The SMF
obtained without the introduction of the bayesian luminosity prior does not
show any evolution from to , implying that massive
galaxies could already be present when the Universe was ~Gyr old.
However, the introduction of the bayesian luminosity prior reduces the number
of galaxies with best fit masses by 83%, implying
a rapid growth of very massive galaxies in the first 1.5 Gyr of cosmic history.
From the stellar-mass complete sample, we identify one candidate of a very
massive (), quiescent galaxy at , with
MIPS m detection suggesting the presence of a powerful obscured AGN.Comment: 23 pages, 18 figures. ApJ accepte
Critical behavior of a one-dimensional monomer-dimer reaction model with lateral interactions
A monomer-dimer reaction lattice model with lateral repulsion among the same
species is studied using a mean-field analysis and Monte Carlo simulations. For
weak repulsions, the model exhibits a first-order irreversible phase transition
between two absorbing states saturated by each different species. Increasing
the repulsion, a reactive stationary state appears in addition to the saturated
states. The irreversible phase transitions from the reactive phase to any of
the saturated states are continuous and belong to the directed percolation
universality class. However, a different critical behavior is found at the
point where the directed percolation phase boundaries meet. The values of the
critical exponents calculated at the bicritical point are in good agreement
with the exponents corresponding to the parity-conserving universality class.
Since the adsorption-reaction processes does not lead to a non-trivial local
parity-conserving dynamics, this result confirms that the twofold symmetry
between absorbing states plays a relevant role in determining the universality
class. The value of the exponent , which characterizes the
fluctuations of an interface at the bicritical point, supports the
Bassler-Brown's conjecture which states that this is a new exponent in the
parity-conserving universality class.Comment: 19 pages, 22 figures, to be published in Phys. Rev
Star formation in galaxies at z~4-5 from the SMUVS survey: a clear starburst/main-sequence bimodality for Halpha emitters on the SFR-M* plane
We study a large galaxy sample from the Spitzer Matching Survey of the
UltraVISTA ultra-deep Stripes (SMUVS) to search for sources with enhanced 3.6
micron fluxes indicative of strong Halpha emission at z=3.9-4.9. We find that
the percentage of "Halpha excess" sources reaches 37-40% for galaxies with
stellar masses log10(M*/Msun) ~ 9-10, and decreases to <20% at log10(M*/Msun) ~
10.7. At higher stellar masses, however, the trend reverses, although this is
likely due to AGN contamination. We derive star formation rates (SFR) and
specific SFR (sSFR) from the inferred Halpha equivalent widths (EW) of our
"Halpha excess" galaxies. We show, for the first time, that the "Halpha excess"
galaxies clearly have a bimodal distribution on the SFR-M* plane: they lie on
the main sequence of star formation (with log10(sSFR/yr^{-1})<-8.05) or in a
starburst cloud (with log10(sSFR/yr^{-1}) >-7.60). The latter contains ~15% of
all the objects in our sample and accounts for >50% of the cosmic SFR density
at z=3.9-4.9, for which we derive a robust lower limit of 0.066 Msun yr^{-1}
Mpc^{-3}. Finally, we identify an unusual >50sigma overdensity of z=3.9-4.9
galaxies within a 0.20 x 0.20 sq. arcmin region. We conclude that the SMUVS
unique combination of area and depth at mid-IR wavelengths provides an
unprecedented level of statistics and dynamic range which are fundamental to
reveal new aspects of galaxy evolution in the young Universe.Comment: 18 pages, 11 figures, 1 table. Re-submitted to the ApJ, after
addressing referee report. Main changes with respect to v1: a new section and
a new appendix have been added to investigate further the origin and
robustness of the sSFR bimodality. No conclusion change
Restricted random walk model as a new testing ground for the applicability of q-statistics
We present exact results obtained from Master Equations for the probability
function P(y,T) of sums of the positions x_t of a discrete
random walker restricted to the set of integers between -L and L. We study the
asymptotic properties for large values of L and T. For a set of position
dependent transition probabilities the functional form of P(y,T) is with very
high precision represented by q-Gaussians when T assumes a certain value
. The domain of y values for which the q-Gaussian apply
diverges with L. The fit to a q-Gaussian remains of very high quality even when
the exponent of the transition probability g(x)=|x/L|^a+p with 0<p<<1 is
different from 1, all though weak, but essential, deviation from the q-Gaussian
does occur for . To assess the role of correlations we compare the T
dependence of P(y,T) for the restricted random walker case with the equivalent
dependence for a sum y of uncorrelated variables x each distributed according
to 1/g(x).Comment: 5 pages, 7 figs, EPL (2011), in pres
Exact Periodic Solutions of Shells Models of Turbulence
We derive exact analytical solutions of the GOY shell model of turbulence. In
the absence of forcing and viscosity we obtain closed form solutions in terms
of Jacobi elliptic functions. With three shells the model is integrable. In the
case of many shells, we derive exact recursion relations for the amplitudes of
the Jacobi functions relating the different shells and we obtain a Kolmogorov
solution in the limit of infinitely many shells. For the special case of six
and nine shells, these recursions relations are solved giving specific analytic
solutions. Some of these solutions are stable whereas others are unstable. All
our predictions are substantiated by numerical simulations of the GOY shell
model. From these simulations we also identify cases where the models exhibits
transitions to chaotic states lying on strange attractors or ergodic energy
surfaces.Comment: 25 pages, 7 figure
Diffusive Ionization of Relativistic Hydrogen-Like Atom
Stochastic ionization of highly excited relativistic hydrogenlike atom in the
monochromatic field is investigated. A theoretical analisis of chaotic dynamics
of the relativistic electron based on Chirikov criterion is given for the cases
of one- and three-dimensional atoms. Critical value of the external field is
evaluated analitically. The diffusion coefficient and ionization time are
calculated.Comment: 13 pages, latex, no figures, submitted to PR
Schwarzschild black hole with global monopole charge
We derive the metric for a Schwarzschild black hole with global monopole
charge by relaxing asymptotic flatness of the Schwarzschild field. We then
study the effect of global monopole charge on particle orbits and the Hawking
radiation. It turns out that existence, boundedness and stability of circular
orbits scale up by , and the perihelion shift and the
light bending by , while the Hawking temperature scales
down by the Schwarzschild values. Here is the
global charge.Comment: 12 pages, LaTeX versio
Macroscopic control parameter for avalanche models for bursty transport
Similarity analysis is used to identify the control parameter RA for the subset of avalanching systems that can exhibit self-organized criticality (SOC). This parameter expresses the ratio of driving to dissipation. The transition to SOC, when the number of excited degrees of freedom is maximal, is found to occur when RA-->0. This is in the opposite sense to (Kolmogorov) turbulence, thus identifying a deep distinction between turbulence and SOC and suggesting an observable property that could distinguish them. A corollary of this similarity analysis is that SOC phenomenology, that is, power law scaling of avalanches, can persist for finite RA with the same RA-->0 exponent if the system supports a sufficiently large range of lengthscales, necessary for SOC to be a candidate for physical (RA finite) systems
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