1,697 research outputs found
Two phase galaxy formation: The Evolutionary Properties of Galaxies
We use our model for the formation and evolution of galaxies within a
two-phase galaxy formation scenario, showing that the high-redshift domain
typically supports the growth of spheroidal systems, whereas at low redshifts
the predominant baryonic growth mechanism is quiescent and may therefore
support the growth of a disc structure. Under this framework we investigate the
evolving galaxy population by comparing key observations at both low and
high-redshifts, finding generally good agreement. By analysing the evolutionary
properties of this model, we are able to recreate several features of the
evolving galaxy population with redshift, naturally reproducing number counts
of massive star-forming galaxies at high redshifts, along with the galaxy
scaling relations, star formation rate density and evolution of the stellar
mass function. Building upon these encouraging agreements, we make model
predictions that can be tested by future observations. In particular, we
present the expected evolution to z=2 of the super-massive black hole mass
function, and we show that the gas fraction in galaxies should decrease with
increasing redshift in a mass, with more and more evolution going to higher and
higher masses. Also, the characteristic transition mass from disc to bulge
dominated system should decrease with increasing redshift.Comment: 15 pages, 11 figures. Version polished for publication in MNRA
Phase-coherence threshold and vortex-glass state in diluted Josephson-junction arrays in a magnetic field
We study numerically the interplay of phase coherence and vortex-glass state
in two-dimensional Josephson-junction arrays with average rational values of
flux quantum per plaquette and random dilution of junctions. For ,
we find evidence of a phase coherence threshold value , below the
percolation concentration of diluted junctions , where the superconducting
transition vanishes. For the array behaves as a
zero-temperature vortex glass with nonzero linear resistance at finite
temperatures. The zero-temperature critical currents are insensitive to
variations in in the vortex glass region while they are strongly
dependent in the phase coherent region.Comment: 6 pages, 4 figures, to appear in Phys. Rev.
Numerical Studies of the Two Dimensional XY Model with Symmetry Breaking Fields
We present results of numerical studies of the two dimensional XY model with
four and eight fold symmetry breaking fields. This model has recently been
shown to describe hydrogen induced reconstruction on the W(100) surface. Based
on mean-field and renormalization group arguments,we first show how the
interplay between the anisotropy fields can give rise to different phase
transitions in the model. When the fields are compatible with each other there
is a continuous phase transition when the fourth order field is varied from
negative to positive values. This transition becomes discontinuous at low
temperatures. These two regimes are separated by a multicritical point. In the
case of competing four and eight fold fields, the first order transition at low
temperatures opens up into two Ising transitions. We then use numerical methods
to accurately locate the position of the multicritical point, and to verify the
nature of the transitions. The different techniques used include Monte Carlo
histogram methods combined with finite size scaling analysis, the real space
Monte Carlo Renormalization Group method, and the Monte Carlo Transfer Matrix
method. Our numerical results are in good agreement with the theoretical
arguments.Comment: 29 pages, HU-TFT-94-36, to appear in Phys. Rev. B, Vol 50, November
1, 1994. A LaTeX file with no figure
Nonlinear sliding friction of adsorbed overlayers on disordered substrates
We study the response of an adsorbed monolayer on a disordered substrate
under a driving force using Brownian molecular-dynamics simulation. We find
that the sharp longitudinal and transverse depinning transitions with
hysteresis still persist in the presence of weak disorder. However, the
transitions are smeared out in the strong disorder limit. The theoretical
results here provide a natural explanation for the recent data for the
depinning transition of Kr films on gold substrate.Comment: 8 pages, 8 figs, to appear in Phys. Rev.
Star Formation History and Extinction in the central kpc of M82-like Starbursts
We report on the star formation histories and extinction in the central kpc
region of a sample of starburst galaxies that have similar far infrared (FIR),
10 micron and K-band luminosities as those of the archetype starburst M82. Our
study is based on new optical spectra and previously published K-band
photometric data, both sampling the same area around the nucleus. Model
starburst spectra were synthesized as a combination of stellar populations of
distinct ages formed over the Hubble time, and were fitted to the observed
optical spectra and K-band flux. The model is able to reproduce simultaneously
the equivalent widths of emission and absorption lines, the continuum fluxes
between 3500-7000 Ang, the K-band and the FIR flux. We require a minimum of 3
populations -- (1) a young population of age < 8 Myr, with its corresponding
nebular emission, (2) an intermediate-age population (age < 500 Myr), and (3)
an old population that forms part of the underlying disk or/and bulge
population. The contribution of the old population to the K-band luminosity
depends on the birthrate parameter and remains above 60% in the majority of the
sample galaxies. Even in the blue band, the intermediate age and old
populations contribute more than 40% of the total flux in all the cases. A
relatively high contribution from the old stars to the K-band nuclear flux is
also apparent from the strength of the 4000 Ang break and the CaII K line. The
extinction of the old population is found to be around half of that of the
young population. The contribution to the continuum from the relatively old
stars has the effect of diluting the emission equivalent widths below the
values expected for young bursts. The mean dilution factors are found to be 5
and 3 for the Halpha and Hbeta lines respectively.Comment: 20 pages, uses emulateapj.cls. Scheduled to appear in ApJ Jan 1, 200
Cotunneling Transport and Quantum Phase Transitions in Coupled Josephson-Junction Chains with Charge Frustration
We investigate the quantum phase transitions in two capacitively coupled
chains of ultra-small Josephson-junctions, with emphasis on the external charge
effects. The particle-hole symmetry of the system is broken by the gate voltage
applied to each superconducting island, and the resulting induced charge
introduces frustration to the system. Near the maximal-frustration line, where
the system is transformed into a spin-1/2 Heisenberg antiferromagnetic chain,
cotunneling of the particles along the two chains is shown to play a major role
in the transport and to drive a quantum phase transition out of the
charge-density wave insulator, as the Josephson-coupling energy is increased.
We also argue briefly that slightly off the symmetry line, the universality
class of the transition remains the same as that right on the line, still being
driven by the particle-hole pairs.Comment: Final version accepted to Phys. Rev. Lett. (Longer version is
available from http://ctp.snu.ac.kr/~choims/
The Role of the Dust in Primeval Galaxies: A Simple Physical Model for Lyman Break Galaxies and Lyman Alpha Emitters
We explore the onset of star formation in the early Universe, exploiting the
observations of high-redshift Lyman-break galaxies (LBGs) and Lyman alpha
emitters (LAEs), in the framework of the galaxy formation scenario elaborated
by Granato et al. (2004) already successfully tested against the wealth of data
on later evolutionary stages. Complementing the model with a simple, physically
plausible, recipe for the evolution of dust attenuation in metal poor galaxies
we reproduce the luminosity functions (LFs) of LBGs and of LAEs at different
redshifts. This recipe yields a much faster increase with galactic age of
attenuation in more massive galaxies, endowed with higher star formation rates.
These objects have therefore shorter lifetimes in the LAE and LBG phases, and
are more easily detected in the dusty submillimeter bright phase. The short UV
bright lifetimes of massive objects strongly mitigate the effect of the fast
increase of the massive halo density with decreasing redshift, thus accounting
for the weaker evolution of the LBG LF, compared to that of the halo mass
function, and the even weaker evolution between z~6 and z~3 of the LAE LF. LAEs
are on the average expected to be younger, with lower stellar masses, and
associated to less massive halos than LBGs. Finally, we show that the
intergalactic medium can be completely reionized at redshift z~6-7 by massive
stars shining in protogalactic spheroids with halo masses from a few 10^10 to a
few 10^11 M_sun, showing up as faint LBGs with magnitude in the range
-17<M_1350<-20, without resorting to any special stellar initial mass function.Comment: 13 pages, 8 figures, uses REVTeX 4 + emulateapj.cls and apjfonts.sty.
Title changed and text revised following referee's comments. Accepted by Ap
Resistivity scaling and critical dynamics of fully frustrated Josephson-junction arrays with on-site dissipation
We study the scaling behavior and critical dynamics of the resistive
transition in Josephson-junction arrays, at f=1/2 flux quantum per plaquette,
by numerical simulation of an on-site dissipation model for the dynamics. The
results are compared with recent simulations using the
resistively-shunted-junction model. For both models, we find that the
resistivity scaling and critical dynamics of the phases are well described by
the same critical temperature as for the chiral (vortex-lattice) transition,
with a power-law divergent correlation length. The behavior is consistent with
the single transition scenario, where phase and chiral variables order at the
same temperature, but with different dynamic exponents z for phase coherence
and chiral order.Comment: 17 pages, 13 figures, to appear in Phys. Rev.
Model Energy Landscapes of Low-Temperature Fluids: Dipolar Hard Spheres
An analytical model of non-Gaussian energy landscape of low-temperature
fluids is developed based on the thermodynamics of the fluid of dipolar hard
spheres. The entire excitation profile of the liquid, from the high
temperatures to the point of ideal-glass transition, has been obtained from the
Monte Carlo simulations. The fluid of dipolar hard spheres loses stability when
reaching the point of ideal-glass transition transforming via a first-order
transition into a columnar liquid phase of dipolar chains locally arranged in a
body-centered tetragonal order.Comment: 4 pages, 3 figure
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