4,793 research outputs found

### Lessons from cosmic history: The case for a linear star formation -- H2 relation

Observations show that star formation in galaxies is closely correlated with
the abundance of molecular hydrogen. Modeling this empirical relation from
first principles proves challenging, however, and many questions regarding its
properties remain open. For instance, the exact functional form of the relation
is still debated and it is also unknown whether it applies at z>4, where CO
observations are sparse. Here, we analyze how the shape of the star formation
-- gas relation affects the cosmic star formation history and global galaxy
properties using an analytic model that follows the average evolution of
galaxies in dark matter halos across cosmic time. We show that a linear
relation with an H2 depletion time of ~2.5 Gyr, as found in studies of nearby
galaxies, results in good agreement with current observations of galaxies at
both low and high redshift. These observations include the evolution of the
cosmic star formation rate density, the z~4-9 UV luminosity function, the
evolution of the mass -- metallicity relation, the relation between stellar and
halo mass, and the gas-to-stellar mass ratios of galaxies. In contrast, the
short depletion times that result from adopting a highly super-linear star
formation -- gas relation lead to large star formation rates, substantial metal
enrichment (~0.1 solar), and low gas-to-stellar mass ratios already at z~10, in
disagreement with observations. These results can be understood in terms of an
equilibrium picture of galaxy evolution in which gas inflows, outflows, and
star formation drive the metallicities and gas fractions toward equilibrium
values that are determined by the ratio of the accretion time to the gas
depletion time. In this picture, the cosmic modulation of the accretion rate is
the primary process that drives the evolution of stellar masses, gas masses,
and metallicities of galaxies from high redshift until today.Comment: 22 pages, 13 figures, minor revision after referee repor

### Random Magnetic Interactions and Spin Glass Order Competing with Superconductivity: Interference of the Quantum Parisi Phase

We analyse the competition between spin glass (SG) order and local pairing
superconductivity (SC) in the fermionic Ising spin glass with frustrated
fermionic spin interaction and nonrandom attractive interaction. The phase
diagram is presented for all temperatures T and chemical potentials \mu. SC-SG
transitions are derived for the relevant ratios between attractive and
frustrated-magnetic interaction. Characteristic features of pairbreaking caused
by random magnetic interaction and/or by spin glass proximity are found. The
existence of low-energy excitations, arising from replica permutation symmetry
breaking (RPSB) in the Quantum Parisi Phase, is shown to be relevant for the
SC-SG phase boundary. Complete 1-step RPSB-calculations for the SG-phase are
presented together with a few results for infinity-step breaking. Suppression
of reentrant SG - SC - SG transitions due to RPSB is found and discussed in
context of ferromagnet - SG boundaries. The relative positioning of the SC and
SG phases presents a theoretical landmark for comparison with experiments in
heavy fermion systems and high T_c superconductors. We find a crossover line
traversing the SG-phase with (\mu=0,T=0) as its quantum critical (end)point in
complete RPSB, and scaling is proposed for its vicinity. We argue that this
line indicates a random field instability and suggest Dotsenko-Mezard vector
replica symmetry breaking to occur at low temperatures beyond.Comment: 24 pages, 14 figures replaced by published versio

### Pseudogaps and Charge Band in the Parisi Solution of Insulating and Superconducting Electronic Spin Glasses at Arbitrary Fillings

We report progress in understanding the fermionic Ising spin glass with
arbitrary filling. A crossover from a magnetically disordered single band phase
via two intermediate bands just below the freezing temperature to a 3-band
structure at still lower temperatures - beyond an almost random field
instability - is shown to emerge in the magnetic phase. An attempt is made to
explain the exact solution in terms of a quantum Parisi phase. A central
nonmagnetic band is found and seen to become sharply separated at T=0 by gaps
from upper and lower magnetic bands. The gap sizes tend towards zero as the
number of replica symmetry breaking steps increases towards infinity. In an
extended model, the competition between local pairing superconductivity and
spin glass order is discussed.Comment: 3 pages, contribution to "ECRYS-99

### Short Time Cycles of Purely Quantum Refrigerators

Four stroke Otto refrigerator cycles with no classical analogue are studied.
Extremely short cycle times with respect to the internal time scale of the
working medium characterize these refrigerators. Therefore these cycles are
termed sudden. The sudden cycles are characterized by the stable limit cycle
which is the invariant of the global cycle propagator. During their operation
the state of the working medium possesses significant coherence which is not
erased in the equilibration segments due to the very short time allocated. This
characteristic is reflected in a difference between the energy entropy and the
Von Neumann entropy of the working medium. A classification scheme for sudden
refrigerators is developed allowing simple approximations for the cooling power
and coefficient of performance.Comment: 20 pages, 12 figures. Among the figures there are 6 figures which are
double, namely with two parts, Top and Botto

### How to evaluate ground-state landscapes of disordered systems thermodynamical correctly

Ground states of three-dimensional EA Ising spin glasses are calculated for
sizes up to 14^3 using a combination of a genetic algorithm and cluster-exact
approximation. For each realization several independent ground states are
obtained. Then, by applying ballistic search and T=0 Monte-Carlo simulations,
it is ensured that each ground state appears with the same probability.
Consequently, the results represent the true T=0 thermodynamic behavior. The
distribution P(|q|) of overlaps is evaluated. For increasing size the width of
P(|q|) and the fraction of the distribution below q_0=0.5 converge to zero.
This indicates that for the infinite system P(|q|) is a delta function, in
contrast to previous results. Thus, the ground-state behavior is dominated by
few large clusters of similar ground states.Comment: 7 pages revtex, 6 figures, 27 reference

### The Hubble Sequence in Groups: The Birth of the Early-Type Galaxies

The physical mechanisms and timescales that determine the morphological
signatures and the quenching of star formation of typical (~L*) elliptical
galaxies are not well understood. To address this issue, we have simulated the
formation of a group of galaxies with sufficient resolution to track the
evolution of gas and stars inside about a dozen galaxy group members over
cosmic history. Galaxy groups, which harbor many elliptical galaxies in the
universe, are a particularly promising environment to investigate morphological
transformation and star formation quenching, due to their high galaxy density,
their relatively low velocity dispersion, and the presence of a hot intragroup
medium. Our simulation reproduces galaxies with different Hubble morphologies
and, consequently, enables us to study when and where the morphological
transformation of galaxies takes place. The simulation does not include
feedback from active galactic nuclei showing that it is not an essential
ingredient for producing quiescent, red elliptical galaxies in galaxy groups.
Ellipticals form, as suspected, through galaxy mergers. In contrast with what
has often been speculated, however, these mergers occur at z>1, before the
merging progenitors enter the virial radius of the group and before the group
is fully assembled. The simulation also shows that quenching of star formation
in the still star-forming elliptical galaxies lags behind their morphological
transformation, but, once started, is taking less than a billion years to
complete. As long envisaged the star formation quenching happens as the
galaxies approach and enter the finally assembled group, due to quenching of
gas accretion and (to a lesser degree) stripping. A similar sort is followed by
unmerged, disk galaxies, which, as they join the group, are turned into the
red-and-dead disks that abound in these environments.Comment: 12 pages, 12 Figures, 1 Table, accepted for publication in AP

### Semi-fermionic representation of SU(N) Hamiltonians

We represent the generators of the SU(N) algebra as bilinear combinations of
Fermi operators with imaginary chemical potential. The distribution function,
consisting of a minimal set of discrete imaginary chemical potentials, is found
for arbitrary N. This representation leads to the conventional temperature
diagram technique with standard Feynman codex, except that the Matsubara
frequencies are determined by neither integer nor half-integer numbers. The
real-time Schwinger-Keldysh formalism is formulated in the framework of complex
distribution functions. We discuss the continuous large N and SU(2) large spin
limits. We illustrate the application of this technique for magnetic and
spin-liquid states of the Heisenberg model.Comment: 11 pages, 7 EPS figures included, extended versio

### Ground State Entropy in Potts Antiferromagnets and Chromatic Polynomials

We discuss recent results on ground state entropy in Potts antiferromagnets
and connections with chromatic polynomials. These include rigorous lower and
upper bounds, Monte Carlo measurements, large--$q$ series, exact solutions, and
studies of analytic properties. Some related results on Fisher zeros of Potts
models are also mentioned.Comment: LATTICE98(spin) 3 pages, Late

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