2,586 research outputs found
Study of stellar populations in the bulges of barred galaxies
We have obtained long-slit spectroscopy for a sample of 20 early-type barred
galaxies to study the influence of bars in the building of galaxy bulges. Line
strength indices were measured and used to derive age and metallicity gradients
in the bulge region by comparing with stellar population models. The same
analysis was also carried out with similar data of unbarred galaxies taken from
the literature. The bulges of barred galaxies seem to be more metal rich, at a
given velocity dispersion (sigma), than the bulges of unbarred galaxies, as
measured by some metallicity sensitive indices. There are indications that the
ratio of relative abundance of alpha-elements with respect to iron, [E/Fe],
derived for the bulges of barred galaxies tend to lie above the values of the
unbarred galaxies at a given sigma. The metallicity gradients for the majority
of the bulges are negative, less metal rich towards the end of the bulge. The
gradient values show a large scatter for galaxies with sigma below 150 km/s.
The age distribution is related to the presence of bulge substructure such as a
nuclear ring or an inner disk. The metallicity of both the bulge and the bar
are very well correlated indicating a close link between the enrichment
histories of both components. Bulges of barred early type galaxies might have
suffered a different chemical enrichment compared to the bulges of unbarred
galaxies of the same morphological type, same central velocity dispersion and
low inclination angles. The hinted stellar populations differences separating
the bulges of barred and unbarred galaxies and the strong link existing between
the metallicity of the bulge and the presence of a bar points to scenarios were
they both form simultaneously in processes leading to rapid and massive
episodes of star formation, possibly linked to the bar formation.Comment: Accepted for publication in Astronomy and Astrophysic
Near-Optimally Teaching the Crowd to Classify
How should we present training examples to learners to teach them
classification rules? This is a natural problem when training workers for
crowdsourcing labeling tasks, and is also motivated by challenges in
data-driven online education. We propose a natural stochastic model of the
learners, modeling them as randomly switching among hypotheses based on
observed feedback. We then develop STRICT, an efficient algorithm for selecting
examples to teach to workers. Our solution greedily maximizes a submodular
surrogate objective function in order to select examples to show to the
learners. We prove that our strategy is competitive with the optimal teaching
policy. Moreover, for the special case of linear separators, we prove that an
exponential reduction in error probability can be achieved. Our experiments on
simulated workers as well as three real image annotation tasks on Amazon
Mechanical Turk show the effectiveness of our teaching algorithm
Temperature Structure of the Intra-Cluster Medium from SPH and AMR simulations
Analyses of cosmological hydrodynamic simulations of galaxy clusters suggest
that X-ray masses can be underestimated by 10% to 30%. The largest bias
originates by both violation of hydrostatic equilibrium and an additional
temperature bias caused by inhomogeneities in the X-ray emitting intra-cluster
medium (ICM). To elucidate on this large dispersion among theoretical
predictions, we evaluate the degree of temperature structures in cluster sets
simulated either with smoothed-particle-hydrodynamics (SPH) and
adaptive-mesh-refinement (AMR) codes. We find that the SPH simulations produce
larger temperature variations connected to the persistence of both
substructures and their stripped cold gas. This difference is more evident in
no-radiative simulations, while it is reduced in the presence of radiative
cooling. We also find that the temperature variation in radiative cluster
simulations is generally in agreement with the observed one in the central
regions of clusters. Around R_500 the temperature inhomogeneities of the SPH
simulations can generate twice the typical hydrostatic-equilibrium mass bias of
the AMR sample. We emphasize that a detailed understanding of the physical
processes responsible for the complex thermal structure in ICM requires
improved resolution and high sensitivity observations in order to extend the
analysis to higher temperature systems and larger cluster-centric radii.Comment: 13 pages, 12 figures, 4 table
Microeconomic Effects of Capital Controls: The Chilean Experience During the 1990s.
This paper studies the experience with the use of capital controls in Chile during the 1990s. Rather than revisiting previous studies, it complements previous research by providing, for the first time, empirical evidence on some of the microeconomic effects of capital controls, in particular, the unremunerated reserve requirement (URR). By looking at financial statements for a group of 73 Chilean firms during 1986-2001, the paper attempts to identify the effects of the URR on the firmsâ costs and ways of financing. Chilean firms are grouped by economic sector, size and access to international capital markets. Results show that the effects of the URR are firm specific; forinstance, there are striking differences in the response to the URR among firms of different size and those with or without access to international capital markets.
Faint recombination lines in Galactic PNe with [WC] nucleus
We present spatially resolved high-resolution spectrophotometric data for the
planetary nebulae PB8, NGC2867, and PB6. We have analyzed two knots in NGC2867
and PB6 and one in PB8. The three nebulae are ionized by [WC] type nuclei:
early [WO] for PB6 and NGC2867 and [WC 5-6] in the case of PB8. Our aim is to
study the behavior of the abundance discrepancy problem (ADF) in this type of
PNe. We measured a large number of optical recombination (ORL) and
collisionally excited lines (CEL), from different ionization stages (many more
than in any previous work), thus, we were able to derive physical conditions
from many different diagnostic procedures. We determined ionic abundances from
the available collisionally excited lines and recombination lines. Based on
both sets of ionic abundances, we derived total chemical abundances in the
nebulae using suitable ionization correction factors. From CELs, we have found
abundances typical of Galactic disk planetary nebulae. Moderate ADF(O++) were
found for PB8 (2.57) and NGC2867 (1.63). For NGC2867, abundances from ORLs are
higher but still consistent with Galactic disk planetary nebulae. On the
contrary, PB8 presents a very high O/H ratio from ORLs. A high C/O was obtained
from ORLs for NGC2867; this ratio is similar to C/O obtained from CELs and with
the chemical composition of the wind of the central star, indicating that there
was no further C-enrichment in the star, relative to O, after the nebular
material ejection. On the contrary, we found C/O<1 in PB8. Interestingly, we
obtain (C/O)ORLs/(C/O)CELs < 1 in PB8 and NGC2867; this added to the similarity
between the heliocentric velocities measured in [OIII] and OII lines for our
three objects, argue against the presence of H-deficient metal-rich knots
coming from a late thermal pulse event.Comment: 25 pages, 13 Tables, 4 Figures Accepted for publication in A&A. First
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Spatially resolved spectroscopy of Coma cluster early-type galaxies IV. Completing the dataset
The long-slit spectra obtained along the minor axis, offset major axis and
diagonal axis are presented for 12 E and S0 galaxies of the Coma cluster drawn
from a magnitude-limited sample studied before. The rotation curves, velocity
dispersion profiles and the H_3 and H_4 coefficients of the Hermite
decomposition of the line of sight velocity distribution are derived. The
radial profiles of the Hbeta, Mg, and Fe line strength indices are measured
too. In addition, the surface photometry of the central regions of a subsample
of 4 galaxies recently obtained with Hubble Space Telescope is presented. The
data will be used to construct dynamical models of the galaxies and study their
stellar populations.Comment: 40 pages, 7 figures, 6 tables. Accepted for publication in ApJ
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