286 research outputs found
Large-Scale Distributed Bayesian Matrix Factorization using Stochastic Gradient MCMC
Despite having various attractive qualities such as high prediction accuracy
and the ability to quantify uncertainty and avoid over-fitting, Bayesian Matrix
Factorization has not been widely adopted because of the prohibitive cost of
inference. In this paper, we propose a scalable distributed Bayesian matrix
factorization algorithm using stochastic gradient MCMC. Our algorithm, based on
Distributed Stochastic Gradient Langevin Dynamics, can not only match the
prediction accuracy of standard MCMC methods like Gibbs sampling, but at the
same time is as fast and simple as stochastic gradient descent. In our
experiments, we show that our algorithm can achieve the same level of
prediction accuracy as Gibbs sampling an order of magnitude faster. We also
show that our method reduces the prediction error as fast as distributed
stochastic gradient descent, achieving a 4.1% improvement in RMSE for the
Netflix dataset and an 1.8% for the Yahoo music dataset
Family Medicine needs assessment: Studying the clinical work of general practitioners in Ethiopia
Background and Objective: Some universities in sub-Saharan Africa have initiated Family Medicine (FM) residency programs. This study was conducted by FM colleagues at Addis Ababa University (AAU) in Ethiopia and the University of Toronto, Canada to inform the FM residency curriculum at AAU. It was designed to determine the clinical problems that family physicians in Ethiopia may encounter.Methods: We used a mixed methods approach: Modified time-motion study and brief interviews. We observed 46 general practitioners (GPs) across ten sites in Ethiopia. Trained observers recorded time-motion data while GPs conducted their daily work. This data was supplemented by brief interviews with the GPs.Findings: Clinical encounters occupied 82% of GP work. The common symptoms were digestive-abdominal pain (21% visits), respiratory-cough (16%), and general-fever and chills (16%). The common diagnoses were infectious (22% visits), genitourinary (12%), circulatory (10%), and endocrine (10%). Challenges identified were lack of clinical resources (57% of GPs), difficulties in communication (48%) and excessive workload (33%). Most common requests were for information technology (78%) and HIV (46%) training.Conclusion: The profile of common symptoms and diagnoses indicated the competencies family physicians in the regions should have. This information will be used to develop an appropriate FM curriculum at AAU
Evidence of major dry mergers at M* > 2 x 10^11 Msun from curvature in early-type galaxy scaling relations?
For early-type galaxies, the correlations between stellar mass and size,
velocity dispersion, surface brightness, color, axis ratio and color-gradient
all indicate that two mass scales, M* = 3 x 10^10 Msun and M* = 2 x 10^11 Msun,
are special. The smaller scale could mark the transition between wet and dry
mergers, or it could be related to the interplay between SN and AGN feedback,
although quantitative measures of this transition may be affected by
morphological contamination. At the more massive scale, mean axis ratios and
color gradients are maximal, and above it, the colors are redder, the sizes
larger and the velocity dispersions smaller than expected based on the scaling
at lower M*. In contrast, the color-sigma relation, and indeed, most scaling
relations with sigma, are not curved: they are well-described by a single power
law, or in some cases, are almost completely flat. When major dry mergers
change masses, sizes, axis ratios and color gradients, they are expected to
change the colors or velocity dispersions much less. Therefore, the fact that
scaling relations at sigma > 150 km/s show no features, whereas the size-M*,
b/a-M*, color-M* and color gradient-M* relations do, suggests that M* = 2 x
10^11 Msun is the scale above which major dry mergers dominate the assembly
histories of early-type galaxies.Comment: 5 pages, 3 figures. Accepted for publication in MNRA
On the evolution of environmental and mass properties of strong lens galaxies in COSMOS
Among the 100 strong lens candidates found in the COSMOS field, 20 with
redshifts in the range [0.34,1.13], feature multiple images of background
sources. Using the multi-wavelength coverage of the field and its spectroscopic
follow-up, we characterize the evolution with redshift of the environment and
of the dark-matter (DM) fraction of the lens galaxies. We present new redshift
of the strong lens candidates. The lens environment is characterized by the
projected 10 closest galaxies around each lens and by the number of galaxies
with a projected distance less than 1Mpc at the lens galaxy redshift. In both
cases, we perform similar measurements on a control sample of twin non-lens
early type galaxies (ETGs). In addition, we identify group members and field
galaxies in the X-ray and optical catalogs of galaxy groups. From those
catalogs, we measure the external shear contribution at the lens galaxy
positions. The systems are then modeled using a SIE plus the external shear due
to the groups. We observe that the average stellar mass of lens galaxies
increases with z and that the environment of lens galaxies is compatible with
that of the twins. During the lens modeling, we notice that, when let free, the
external shear points in a direction which is the mean direction of the
external shear due to groups and of the closest galaxy to the lens. We notice
that the DM fraction of the lens galaxies within the Einstein radius decreases
as the redshift increases. Given these, we conclude that, while the environment
of lens galaxies is compatible with that of non-lens ETGS, their mass
properties evolves significantly with redshift: it is still not clear whether
this advocates in favor of a stronger lensing bias toward massive objects at
high redshift or is simply representative of the high proportion of massive and
high stellar density galaxies at high redshift.Comment: Accepted for publication in A&A. Significant modifications in the
paper but similar conclusion
Galaxy luminosities, stellar masses, sizes, velocity dispersions as a function of morphological type
We provide fits to the distribution of galaxy luminosity, size, velocity
dispersion and stellar mass as a function of concentration index C_r and
morphological type in the SDSS. We also quantify how estimates of the fraction
of `early' or `late' type galaxies depend on whether the samples were cut in
color, concentration or light profile shape, and compare with similar estimates
based on morphology. Our fits show that Es account for about 20% of the r-band
luminosity density, rho_Lr, and 25% of the stellar mass density, rho_*;
including S0s and Sas increases these numbers to 33% and 40%, and 50% and 60%,
respectively. Summed over all galaxy types, we find rho_* ~ 3 * 10^8 M_Sun
Mpc^{-3} at z ~ 0. This is in good agreement with expectations based on
integrating the star formation history. However, compared to most previous
work, we find an excess of objects at large masses, up to a factor of ~ 10 at
M_* ~ 5*10^{11} M_Sun. The stellar mass density further increases at large
masses if we assume different IMFs for Es and spiral galaxies, as suggested by
some recent chemical evolution models, and results in a better agreement with
the dynamical mass function. We also show that the trend for ellipticity to
decrease with luminosity is primarily because the E/S0 ratio increases at large
L. However, the most massive galaxies, M_* > 5 * 10^{11} M_Sun, are less
concentrated and not as round as expected if one extrapolates from lower L, and
they are not well-fit by pure deVaucouleur laws. This suggests formation
histories with recent radial mergers. Finally, we show that the age-size
relation is flat for Es of fixed dynamical mass, but, at fixed M_dyn, S0s and
Sas with large sizes tend to be younger. Explaining this difference between E
and S0 formation is a new challenge for models of early-type galaxy formation.Comment: 42 pages, 34 figures, 9 tables. Accepted for publication in MNRA
Clues from nearby galaxies to a better theory of cosmic evolution
The great advances in the network of cosmological tests show that the
relativistic Big Bang theory is a good description of our expanding universe.
But the properties of nearby galaxies that can be observed in greatest detail
suggest a still better theory would more rapidly gather matter into galaxies
and groups of galaxies. This happens in theoretical ideas now under discussion.Comment: published in Natur
Sizes and ages of SDSS ellipticals: Comparison with hierarchical galaxy formation models
In a sample of about 45,700 early-type galaxies extracted from SDSS, we find
that the shape, normalization, and dispersion around the mean size-stellar mass
relation is the same for young and old systems, provided the stellar mass is
greater than 3*10^10 Msun. This is difficult to reproduce in pure passive
evolution models, which generically predict older galaxies to be much more
compact than younger ones of the same stellar mass. However, this aspect of our
measurements is well reproduced by hierarchical models of galaxy formation.
Whereas the models predict more compact galaxies at high redshifts, subsequent
minor, dry mergers increase the sizes of the more massive objects, resulting in
a flat size-age relation at the present time. At lower masses, the models
predict that mergers are less frequent, so that the expected anti-correlation
between age and size is not completely erased. This is in good agreement with
our data: below 3*10^10 Msun, the effective radius R_e is a factor of ~2 lower
for older galaxies. These successes of the models are offset by the fact that
the predicted sizes have other serious problems, which we discuss.Comment: 13 pages, 9 Figures, 1 Table. Accepted by MNRA
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