335 research outputs found
Ultracold quantum gases in triangular optical lattices
Over the last years the exciting developments in the field of ultracold atoms
confined in optical lattices have led to numerous theoretical proposals devoted
to the quantum simulation of problems e.g. known from condensed matter physics.
Many of those ideas demand for experimental environments with non-cubic lattice
geometries. In this paper we report on the implementation of a versatile
three-beam lattice allowing for the generation of triangular as well as
hexagonal optical lattices. As an important step the superfluid-Mott insulator
(SF-MI) quantum phase transition has been observed and investigated in detail
in this lattice geometry for the first time. In addition to this we study the
physics of spinor Bose-Einstein condensates (BEC) in the presence of the
triangular optical lattice potential, especially spin changing dynamics across
the SF-MI transition. Our results suggest that below the SF-MI phase
transition, a well-established mean-field model describes the observed data
when renormalizing the spin-dependent interaction. Interestingly this opens new
perspectives for a lattice driven tuning of a spin dynamics resonance occurring
through the interplay of quadratic Zeeman effect and spin-dependent
interaction. We finally discuss further lattice configurations which can be
realized with our setup.Comment: 19 pages, 7 figure
Constraints on Warm Dark Matter from Cosmological Reionization
We study the constraints that high-redshift structure formation in the
universe places on warm dark matter (WDM) dominated cosmological models. We
modify the extended Press-Schechter formalism to derive the halo mass function
in WDM models. We show that our predictions agree with recent numerical
simulations at low redshift over the halo masses of interest. Applying our
model to galaxy formation at high redshift, we find that the loss of power on
small scales, together with the delayed collapse of low-mass objects, results
in strong limits on the root-mean-square velocity dispersion v_rms of the WDM
particles at z=0. For fermions decoupling while relativistic, these limits are
equivalent to constraints on the mass m_X of the particles. The presence of a 4
billion solar mass black hole at z=5.8, believed to power the quasar SDSS
1044-1215, implies m_X > 0.5 keV (or v_rms < 0.10 km/s), assuming that the
quasar is unlensed and radiating at or below the Eddington limit. Reionization
by z=5.8 also implies a limit on m_X. If high-redshift galaxies produce
ionizing photons with an efficiency similar to their z=3 counterparts, we find
m_X > 1.2 keV (or v_rms < 0.03 km/s). However, given the uncertainties in
current measurements from the proximity effect of the ionizing background at
redshift 3, values of m_X as low as 0.75 keV (v_rms = 0.06 km/s) are not ruled
out. The limit weakens further if, instead, the ionizing-photon production
efficiency is greater at high z, but this limit will tighten considerably if
reionization is shown in the future to have occurred at higher redshifts. WDM
models with m_X 0.04 km/s) produce a low-luminosity cutoff in
the high-redshift galaxy luminosity function which is directly detectable with
the Next Generation Space Telescope (abridged).Comment: 38 pages, 10 figures, to appear in ApJ. One figure added, some
discussion revise
The Most Massive Black Holes in the Universe: Effects of Mergers in Massive Galaxy Clusters
Recent observations support the idea that nuclear black holes grew by gas
accretion while shining as luminous quasars at high redshift, and they
establish a relation of the black hole mass with the host galaxy's spheroidal
stellar system. We develop an analytic model to calculate the expected impact
of mergers on the masses of black holes in massive clusters of galaxies. We use
the extended Press-Schechter formalism to generate Monte Carlo merger histories
of halos with a mass 10^{15} h^{-1} Msun. We assume that the black hole mass
function at z=2 is similar to that inferred from observations at z=0 (since
quasar activity declines markedly at z<2), and we assign black holes to the
progenitor halos assuming a monotonic relation between halo mass and black hole
mass. We follow the dynamical evolution of subhalos within larger halos,
allowing for tidal stripping, the loss of orbital energy by dynamical friction,
and random orbital perturbations in gravitational encounters with subhalos, and
we assume that mergers of subhalos are followed by mergers of their central
black holes. Our analytic model reproduces numerical estimates of the subhalo
mass function. We find that the most massive black holes in massive clusters
typically grow by a factor ~ 2 by mergers after gas accretion has stopped. In
our ten realizations of 10^{15} h^{-1} Msun clusters, the highest initial (z=2)
black hole masses are 5-7 x 10^9 Msun, but four of the clusters contain black
holes in the range 1-1.5 x 10^{10} Msun at z=0. Satellite galaxies may host
black holes whose mass is comparable to, or even greater than, that of the
central galaxy. Thus, black hole mergers can significantly extend the very high
end of the black hole mass function.Comment: 13 pages, 7 figures, accepted for publication in The Astrophysical
Journa
Scaling Laws for Advection Dominated Flows: Applications to Low Luminosity Galactic Nuclei
We present analytical scaling laws for self-similar advection dominated
flows. The spectra from these systems range from 10 - 10 Hz, and
are determined by considering cooling of electrons through synchrotron,
bremsstrahlung, and Compton processes. We show that the spectra can be quite
accurately reproduced without detailed numerical calculations, and that there
is a strong testable correlation between the radio and X-ray fluxes from these
systems. We describe how different regions of the spectrum scale with the mass
of the accreting black hole, , the accretion rate of the gas, , and
the equilibrium temperature of the electrons, . We show that the universal
radio spectral index of 1/3 observed in most elliptical galaxies (Slee et al.
1994) is a natural consequence of self-absorbed synchrotron radiation from
these flows. We also give expressions for the total luminosity of these flows,
and the critical accretion rate, , above which the advection
solutions cease to exist. We find that for most cases of interest the
equilibrium electron temperature is fairly insensitive to , , and
parameters in the model. We apply these results to low luminosity black holes
in galactic nuclei. We show that the problem posed by Fabian & Canizares (1988)
of whether bright elliptical galaxies host dead quasars is resolved, as pointed
out recently by Fabian & Rees (1995), by considering advection-dominated flows.Comment: 30 pages, 5 postscript files. Accepted to ApJ. Also available
http://cfa-www.harvard.edu/~rohan/publications.htm
Measuring Feedback Using the Intergalactic Medium State and Evolution Inferred from the Soft X-ray Background
We explore the intergalactic medium (IGM) as a potential source of the
unresolved soft X-ray background (XRB) and the feasibility to extract the IGM
state and evolution from XRB observations. We build two analytical models, the
continuum field model and the halo model, to calculate the IGM XRB mean flux,
angular auto correlation and cross correlation with galaxies. Our results
suggest that the IGM may contribute a significant fraction to the unresolved
soft XRB flux and correlations. We calibrated non-Gaussian errors estimated
against our moving mesh hydro simulation and estimate that the ROSAT
all sky survey plus Sloan galaxy photometric redshift survey would allow a
accuracy in the IGM XRB-galaxy cross correlation power spectrum
measurement for and a accuracy in the redshift resolved
X-ray emissivity-galaxy cross correlation power spectrum measurement for
. At small scales, non-gravitational heating, e.g. feedback,
dominates over gravity and leaves unique signatures in the IGM XRB, which
allows a comparable accuracy in the measurement of the amount of
non-gravitational heating and the length scales where non-gravitational energy
balances gravity.Comment: 17 pages, 5 figures. Will appear on ApJ May issu
Algorithmic fairness and bias mitigation for clinical machine learning with deep reinforcement learning
As models based on machine learning continue to be developed for healthcare applications, greater effort is needed to ensure that these technologies do not reflect or exacerbate any unwanted or discriminatory biases that may be present in the data. Here we introduce a reinforcement learning framework capable of mitigating biases that may have been acquired during data collection. In particular, we evaluated our model for the task of rapidly predicting COVID-19 for patients presenting to hospital emergency departments and aimed to mitigate any site (hospital)-specific and ethnicity-based biases present in the data. Using a specialized reward function and training procedure, we show that our method achieves clinically effective screening performances, while significantly improving outcome fairness compared with current benchmarks and state-of-the-art machine learning methods. We performed external validation across three independent hospitals, and additionally tested our method on a patient intensive care unit discharge status task, demonstrating model generalizability
Constraints from Gravitational Recoil on the Growth of Supermassive Black Holes at High Redshift
Recent studies have shown that during their coalescence, binary supermassive
black holes (SMBHs) experience a gravitational recoil with velocities of 100
km/s < v(kick) < 600 km/s. These velocities exceed the escape velocity v(esc)
from typical dark matter (DM) halos at high-redshift (z>6), and therefore put
constraints on scenarios in which early SMBHs grow at the centers of DM halos.
Here we quantify these constraints for the most distant known SMBHs, with
inferred masses in excess of 10^9 M(sun), powering the bright quasars
discovered in the Sloan Digital Sky Survey at z>6. We assume that these SMBHs
grew via a combination of accretion and mergers between pre-existing seed BHs
in individual progenitor halos, and that mergers between progenitors with
v(esc) < v(kick) disrupt the BH growth process. Our results suggest that under
these assumptions, the z=6 SMBHs had a phase during which gained mass
significantly more rapidly than under an Eddington-limited exponential growth
rate.Comment: submitted to ApJ Letters, 5 emulateapj pages with 1 figur
Cosmological constraints from the cluster contribution to the power spectrum of the soft X-ray background. New evidence for a low sigma_8
We use the X-ray power spectrum of the ROSAT all-sky survey in the R6 band
(approximately 0.9-1.3 keV) to set an upper limit on the galaxy cluster power
spectrum. The cluster power spectrum is modelled with a minimum number of
robust assumptions regarding the structure of the clusters. The power spectrum
of ROSAT sets an upper limit on the Omega_m-sigma_8 plane which excludes all
the models with sigma_8 above sigma_8 = 0.5/(Omega_m^0.38) in a flat LCDM
universe. We discuss the possible sources of systematic errors in our
conclusions, mainly dominated by the assumed L_x-T relation. Alternatively,
this relation could be constrained by using the X-ray power spectrum, if the
cosmological model is known. Our conclusions suggest that only models with a
low value of sigma_8 (sigma_8 < 0.8 for Omega_m = 0.3) may be compatible with
our upper limit. We also find that models predicting lower luminosities in
galaxy clusters are favoured. Reconciling our cosmological constraints with
these arising by other methods might require either a high entropy floor or
wide-spread presence of cooling flows in the low-redshift clusters.Comment: 14 pages, 19 plots (2 as gif files). MNRAS submitte
Constraining the Lifetime of Quasars from their Spatial Clustering
The lifetime t_Q of the luminous phase of quasars is constrained by current
observations to be between 10^6 and 10^8 years, but is otherwise unkown. We
model the quasar luminosity function in detail in the optical and X-ray bands
using the Press-Schechter formalism, and show that the expected clustering of
quasars depends strongly on their assumed lifetime. We quantify this
dependence, and find that existing measurements of the correlation length of
quasars are consistent with the range 10^6 < t_Q < 10^8 years. We then show
that future measurements of the power spectrum of quasars out to z=3, from the
2dF or Sloan Digital Sky Survey, can significantly improve this constraint, and
in principle allow a precise determination of t_Q. We estimate the systematic
errors introduced by uncertainties in the modeling of the quasar-halo
relationship, as well as by the possible existence of obscured quasars.Comment: ApJ, in press (emulateapj
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