32,588 research outputs found
Blocking and Persistence in the Zero-Temperature Dynamics of Homogeneous and Disordered Ising Models
A ``persistence'' exponent theta has been extensively used to describe the
nonequilibrium dynamics of spin systems following a deep quench: for
zero-temperature homogeneous Ising models on the d-dimensional cubic lattice,
the fraction p(t) of spins not flipped by time t decays to zero like
t^[-theta(d)] for low d; for high d, p(t) may decay to p(infinity)>0, because
of ``blocking'' (but perhaps still like a power). What are the effects of
disorder or changes of lattice? We show that these can quite generally lead to
blocking (and convergence to a metastable configuration) even for low d, and
then present two examples --- one disordered and one homogeneous --- where p(t)
decays exponentially to p(infinity).Comment: 8 pages (LaTeX); to appear in Physical Review Letter
Velocity Dispersions and Dynamical Masses for a Large Sample of Quiescent Galaxies at z > 1: Improved Measures of the Growth in Mass and Size
We present Keck LRIS spectroscopy for a sample of 103 massive galaxies with
redshifts 0.9 < z < 1.6. Of these, 56 are quiescent with high signal-to-noise
absorption line spectra, enabling us to determine robust stellar velocity
dispersions for the largest sample yet available beyond a redshift of 1.
Together with effective radii measured from deep Hubble Space Telescope images,
we calculate dynamical masses and address key questions relating to the
puzzling size growth of quiescent galaxies over 0 < z < 2. We examine the
relationship between stellar and dynamical masses at high redshift, finding
that it closely follows that determined locally. We also confirm the utility of
the locally-established empirical calibration which enables high-redshift
velocity dispersions to be estimated photometrically, and we determine its
accuracy to be 35%. To address recent suggestions that progenitor bias - the
continued arrival of recently-quenched larger galaxies - can largely explain
the size evolution of quiescent galaxies, we examine the growth at fixed
velocity dispersion assuming this quantity is largely unaffected by the merger
history. We demonstrate that significant size and mass growth have clearly
occurred in individual systems. Parameterizing the relation between mass and
size growth over 0 < z < 1.6 as R \propto M^alpha, we find alpha = 1.6 +- 0.3,
in agreement with theoretical expectations from simulations of minor mergers.
Relaxing the assumption that the velocity dispersion is unchanging, we examine
growth assuming a constant ranking in galaxy velocity dispersion. This approach
is applicable only to the large-dispersion tail of the distribution, but yields
a consistent growth rate of alpha = 1.4 +- 0.2. Both methods confirm that
progenitor bias alone is insufficient to explain our new observations and that
quiescent galaxies have grown in both size and stellar mass over 0 < z < 1.6.Comment: Updated to match the published versio
MOSFIRE Spectroscopy of Quiescent Galaxies at 1.5 < z < 2.5. II - Star Formation Histories and Galaxy Quenching
We investigate the stellar populations for a sample of 24 quiescent galaxies
at 1.5 < z < 2.5 using deep rest-frame optical spectra obtained with Keck
MOSFIRE. By fitting templates simultaneously to the spectroscopic and
photometric data, and exploring a variety of star formation histories, we
obtain robust measurements of median stellar ages and residual levels of star
formation. After subtracting the stellar templates, the stacked spectrum
reveals the Halpha and [NII] emission lines, providing an upper limit on the
ongoing star formation rate of 0.9 +/- 0.1 Msun/yr. By combining the MOSFIRE
data to our sample of Keck LRIS spectra at lower redshift, we analyze in a
consistent manner the quiescent population at 1 < z < 2.5. We find a tight
relation (with a scatter of 0.13 dex) between the stellar age and the
rest-frame U-V and V-J colors, which can be used to estimate the age of
quiescent galaxies given their colors. Applying this age--color relation to
large, photometric samples, we are able to model the number density evolution
for quiescent galaxies of various ages. We find evidence for two distinct
quenching paths: a fast quenching that produces compact post-starburst systems,
and a slow quenching of larger galaxies. Fast quenching accounts for about a
fifth of the growth of the red sequence at z~1.4, and half at z~2.2. We
conclude that fast quenching is triggered by dramatic events such as gas-rich
mergers, while slow quenching is likely caused by a different physical
mechanism.Comment: 28 pages, 15 figures, accepted in Ap
MOSFIRE Spectroscopy of Quiescent Galaxies at 1.5 < z < 2.5. I - Evolution of Structural and Dynamical Properties
We present deep near-infrared spectra for a sample of 24 quiescent galaxies
in the redshift range 1.5 < z < 2.5 obtained with the MOSFIRE spectrograph at
the W. M. Keck Observatory. In conjunction with a similar dataset we obtained
in the range 1 < z < 1.5 with the LRIS spectrograph, we analyze the kinematic
and structural properties for 80 quiescent galaxies, the largest
homogeneously-selected sample to date spanning 3 Gyr of early cosmic history.
Analysis of our Keck spectra together with measurements derived from associated
HST images reveals increasingly larger stellar velocity dispersions and smaller
sizes to redshifts beyond z~2. By classifying our sample according to Sersic
indices, we find that among disk-like systems the flatter ones show a higher
dynamical to stellar mass ratio compared to their rounder counterparts which we
interpret as evidence for a significant contribution of rotational motion. For
this subset of disk-like systems, we estimate that V/sigma, the ratio of the
circular velocity to the intrinsic velocity dispersion, is a factor of two
larger than for present-day disky quiescent galaxies. We use the velocity
dispersion measurements also to explore the redshift evolution of the dynamical
to stellar mass ratio, and to measure for the first time the physical size
growth rate of individual systems over two distinct redshift ranges, finding a
faster evolution at earlier times. We discuss the physical origin of this
time-dependent growth in size in the context of the associated reduction of the
systematic rotation.Comment: Updated to match the published versio
Discovery of a Strongly Lensed Massive Quiescent Galaxy at z=2.636: Spatially Resolved Spectroscopy and Indications of Rotation
We report the discovery of RG1M0150, a massive, recently quenched galaxy at
z=2.636 that is multiply imaged by the cluster MACSJ0150.3-1005. We derive a
stellar mass of log M_*=11.49+0.10-0.16 and a half-light radius of R_e,maj
=1.8+-0.4 kpc. Taking advantage of the lensing magnification, we are able to
spatially resolve a remarkably massive yet compact quiescent galaxy at z>2 in
ground-based near-infrared spectroscopic observations using Magellan/FIRE and
Keck/MOSFIRE. We find no gradient in the strength of the Balmer absorption
lines over 0.6 R_e - 1.6 R_e, which are consistent with an age of 760 Myr. Gas
emission in [NII] broadly traces the spatial distribution of the stars and is
coupled with weak Halpha emission (log [NII]/Halpha = 0.6+-0.2), indicating
that OB stars are not the primary ionizing source. The velocity dispersion
within the effective radius is sigma_e = 271+-41 km/s. We detect rotation in
the stellar absorption lines for the first time beyond z~1. Using a
two-integral Jeans model that accounts for observational effects, we measure a
dynamical mass of log M_dyn =11.24+-0.14 and V/sigma=0.70+-0.21. This is a high
degree of rotation considering the modest observed ellipticity of 0.12+-0.08,
but it is consistent with predictions from dissipational merger simulations
that produce compact remnants. The mass of RG1M0150 implies that it is likely
to become a slowly rotating elliptical. If it is typical, this suggests that
the progenitors of massive ellipticals retain significant net angular momentum
after quenching which later declines, perhaps through accretion of satellites.Comment: Accepted to ApJ Letters; updated to include revisions from the
referee process, including an improved Fig.
Stellar populations from spectroscopy of a large sample of quiescent galaxies at z > 1: Measuring the contribution of progenitor bias to early size growth
We analyze the stellar populations of a sample of 62 massive (log Mstar/Msun
> 10.7) galaxies in the redshift range 1 < z < 1.6, with the main goal of
investigating the role of recent quenching in the size growth of quiescent
galaxies. We demonstrate that our sample is not biased toward bright, compact,
or young galaxies, and thus is representative of the overall quiescent
population. Our high signal-to-noise ratio Keck LRIS spectra probe the
rest-frame Balmer break region which contains important absorption line
diagnostics of recent star formation activity. We obtain improved measures of
the various stellar population parameters, including the star-formation
timescale tau, age and dust extinction, by fitting templates jointly to both
our spectroscopic and broad-band photometric data. We identify which quiescent
galaxies were recently quenched and backtrack their individual evolving
trajectories on the UVJ color-color plane finding evidence for two distinct
quenching routes. By using sizes measured in the previous paper of this series,
we confirm that the largest galaxies are indeed among the youngest at a given
redshift. This is consistent with some contribution to the apparent growth from
recent arrivals, an effect often called progenitor bias. However, we calculate
that recently-quenched objects can only be responsible for about half the
increase in average size of quiescent galaxies over a 1.5 Gyr period,
corresponding to the redshift interval 1.25 < z < 2. The remainder of the
observed size evolution arises from a genuine growth of long-standing quiescent
galaxies.Comment: Accepted for publication in the Astrophysical Journal, 14 pages, 11
figure
Behaviour of spin-1/2 particle around a charged black hole
Dirac equation is separable in curved space-time and its solution was found
for both spherically and axially symmetric geometry. But most of the works were
done without considering the charge of the black hole. Here we consider the
spherically symmetric charged black hole background namely Reissner-Nordstrom
black hole. Due to presence of the charge of black-hole charge-charge
interaction will be important for the cases of incoming charged particle (e.g.
electron, proton etc.). Therefore both gravitational and electromagnetic gauge
fields should be introduced. Naturally behaviour of the particle will be
changed from that in Schwarzschild geometry. We compare both the solutions. In
the case of Reissner-Nordstrom black hole there is a possibility of
super-radiance unlike Schwarzschild case. We also check this branch of the
solution.Comment: 8 Latex pages and 4 Figures; RevTex.style; Accepted for Publication
in Classical and Quantum Gravit
Sign-time distributions for interface growth
We apply the recently introduced distribution of sign-times (DST) to
non-equilibrium interface growth dynamics. We are able to treat within a
unified picture the persistence properties of a large class of relaxational and
noisy linear growth processes, and prove the existence of a non-trivial scaling
relation. A new critical dimension is found, relating to the persistence
properties of these systems. We also illustrate, by means of numerical
simulations, the different types of DST to be expected in both linear and
non-linear growth mechanisms.Comment: 4 pages, 5 ps figs, replaced misprint in authors nam
Measurement of the interaction strength in a Bose-Fermi mixture with 87Rb and 40K
A quantum degenerate, dilute gas mixture of bosonic and fermionic atoms was
produced using 87Rb and 40K. The onset of degeneracy was confirmed by observing
the spatial distribution of the gases after time-of-flight expansion. Further,
the magnitude of the interspecies scattering length between the doubly spin
polarized states of 87Rb and 40K, |a_RbK|, was determined from
cross-dimensional thermal relaxation. The uncertainty in this collision
measurement was greatly reduced by taking the ratio of interspecies and
intraspecies relaxation rates, yielding |a_RbK| = 250 +/- 30 a_0, which is a
lower value than what was reported in [M. Modugno et al., Phys. Rev. A 68,
043626 (2003)]. Using the value for |a_RbK| reported here, current T=0 theory
would predict a threshold for mechanical instability that is inconsistent with
the experimentally observed onset for sudden loss of fermions in [G. Modugno et
al., Science 297, 2240 (2002)].Comment: RevTeX4 + 4 eps figures; Replaced with published versio
Deep Inelastic Lepton-Nucleon Scattering at HERA
Data from the HERA collider experiments, H1 and ZEUS, have been fundamental
to the rapid recent development of our understanding of the partonic
composition of the proton and of QCD. This report focuses on inclusive
measurements of neutral and charged current cross sections at HERA, using the
full available data taken to date. The present precision on the proton parton
densities and the further requirements for future measurements at the Tevatron
and LHC are explored. Emphasis is also placed on the region of very low
Bjorken-x and Q^2. In this region, the `confinement' transition takes place
from partons to hadrons as the relevant degrees of freedom and novel or exotic
QCD effects associated with large parton densities are most likely to be
observed. Finally, prospects for the second phase of HERA running are
discussed.Comment: 13 pages, 15 figures, to appear in Proceedings of the XXI
International Symposium on lepton and Photon Interactions at High Energies,
Fermilab, August 200
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