111,918 research outputs found
Galaxy bimodality versus stellar mass and environment
We analyse a z<0.1 galaxy sample from the Sloan Digital Sky Survey focusing
on the variation of the galaxy colour bimodality with stellar mass and
projected neighbour density Sigma, and on measurements of the galaxy stellar
mass functions. The characteristic mass increases with environmental density
from about 10^10.6 Msun to 10^10.9 Msun (Kroupa IMF, H_0=70) for Sigma in the
range 0.1--10 per Mpc^2. The galaxy population naturally divides into a red and
blue sequence with the locus of the sequences in colour-mass and
colour-concentration index not varying strongly with environment. The fraction
of galaxies on the red sequence is determined in bins of 0.2 in log Sigma and
log mass (12 x 13 bins). The red fraction f_r generally increases continuously
in both Sigma and mass such that there is a unified relation: f_r =
F(Sigma,mass). Two simple functions are proposed which provide good fits to the
data. These data are compared with analogous quantities in semi-analytical
models based on the Millennium N-body simulation: the Bower et al. (2006) and
Croton et al. (2006) models that incorporate AGN feedback. Both models predict
a strong dependence of the red fraction on stellar mass and environment that is
qualitatively similar to the observations. However, a quantitative comparison
shows that the Bower et al. model is a significantly better match; this appears
to be due to the different treatment of feedback in central galaxies.Comment: 19 pages, 17 figures; accepted by MNRAS, minor change
A Unified tool to estimate Distances, Ages and Masses (UniDAM) from spectrophotometric data
Galactic archaeology - the study of the formation and evolution of the Milky
Way by reconstructing its past from its current constituents - requires precise
and accurate knowledge of stellar parameters for as many stars as possible. To
achieve this a number of large spectroscopic surveys have been undertaken and
are still ongoing. So far consortia carrying out the different spectroscopic
surveys have used different tools to determine stellar parameters of stars from
their derived effective temperatures (Teff), surface gravities (log g) and
metallicities ([Fe/H]) possibly combined with photometric, astrometric,
interferometric or asteroseismic information. Here we aim to homogenise the
stellar characterisation by applying a unified tool to a large set of publicly
available spectrophotometric data. We use spectroscopic data from a variety of
large surveys combined with infra-red photometry from 2MASS and AllWISE and
compare these in a Bayesian manner with PARSEC isochrones to derive probability
density functions (PDFs) for stellar masses, ages and distances. We treat PDFs
of pre-helium-core burning, helium-core burning and post helium-core burning
solutions as well as different peaks in multi-modal PDFs (i.e. each unimodal
sub-PDF) of the different evolutionary phases separately. For over 2.5 million
stars we report mass, age and distance estimate for each evolutionary phase and
unimodal sub-PDF. We report Gaussian, skewed Gaussian, truncated Gaussian,
modified truncated exponential distribution or truncated Student's
t-distribution functions to represent each sub-PDF, allowing to reconstruct
detailed PDFs. Comparisons with stellar parameter estimates from the literature
show good agreement within uncertainties. We present UniDAM - the unified tool
applicable to spectrophotometric data of different surveys to obtain a
homogenised set of stellar parameters
Sensitivity to Z-prime and non-standard neutrino interactions from ultra-low threshold neutrino-nucleus coherent scattering
We discuss prospects for probing Z-prime and non-standard neutrino
interactions using neutrino-nucleus coherent scattering with ultra-low energy
(~ 10 eV) threshold Si and Ge detectors. The analysis is performed in the
context of a specific and contemporary reactor-based experimental proposal,
developed in cooperation with the Nuclear Science Center at Texas A&M
University, and referencing available technology based upon economical and
scalable detector arrays. For expected exposures, we show that sensitivity to
the Z-prime mass is on the order of several TeV, and is complementary to the
LHC search with low mass detectors in the near term. This technology is also
shown to provide sensitivity to the neutrino magnetic moment, at a level that
surpasses terrestrial limits, and is competitive with more stringent
astrophysical bounds. We demonstrate the benefits of combining silicon and
germanium detectors for distinguishing between classes of models of new
physics, and for suppressing correlated systematic uncertainties.Comment: As published in PRD; 13 pages, 7 figure
COOOL: A Learning-To-Rank Approach for SQL Hint Recommendations
Query optimization is a pivotal part of every database management system
(DBMS) since it determines the efficiency of query execution. Numerous works
have introduced Machine Learning (ML) techniques to cost modeling, cardinality
estimation, and end-to-end learned optimizer, but few of them are proven
practical due to long training time, lack of interpretability, and integration
cost. A recent study provides a practical method to optimize queries by
recommending per-query hints but it suffers from two inherited problems. First,
it follows the regression framework to predict the absolute latency of each
query plan, which is very challenging because the latencies of query plans for
a certain query may span multiple orders of magnitude. Second, it requires
training a model for each dataset, which restricts the application of the
trained models in practice. In this paper, we propose COOOL to predict Cost
Orders of query plans to cOOperate with DBMS by Learning-To-Rank. Instead of
estimating absolute costs, COOOL uses ranking-based approaches to compute
relative ranking scores of the costs of query plans. We show that COOOL is
theoretically valid to distinguish query plans with different latencies. We
implement COOOL on PostgreSQL, and extensive experiments on
join-order-benchmark and TPC-H data demonstrate that COOOL outperforms
PostgreSQL and state-of-the-art methods on single-dataset tasks as well as a
unified model for multiple-dataset tasks. Our experiments also shed some light
on why COOOL outperforms regression approaches from the representation learning
perspective, which may guide future research
Environment of 1 ≤ z ≤ 2 MIR selected obscured and unobscured AGNs in the Extended Chandra Deep Field South
Context. In unified models, different types of active galaxy nuclei (AGN) correspond to a single class of objects, where their observed differences are solely due to the different orientations of the obscuring material around the central inner regions. Recent studies also show that this obscuring material can even extend at galactic scales due to debris from galaxy interactions and/or mergers. In standard unified models the different AGN types are expected to show similar galaxy environments. Aims. We aim to investigate properties and environment of obscured and unobscured AGNs selected from mid-infrared (MIR) bands from the Multiwavelength Survey by Yale-Chile (MUSYC), in order to test the unified model and evolutionary scenarios. Methods. The sample of AGNs was selected from images obtained with the Infrared Array Camera (IRAC) mounted on the Spitzer Space Telescope, based on their MIR colors centered at wavelengths [3.6], [4.5], [5.8] and [8.0] microns. We selected two samples of AGNs with redshifts in the range 1 ≤ z ≤ 2 and rest-frame absolute magnitudes Mv ≤ -21: obscured and unobscured AGNs by means of a simple optical-MIR color cut criterion (R - [4:5] = 3:05.) Results. We find that obscured AGNs are intrinsically optically faint in the R band, suggesting that luminous IR-selected AGNs have a significant dust extinction. From a cross-correlation with several X-ray surveys, we find that the majority of the AGNs in our sample have X-ray luminosities similar to those found in Seyfert-like galaxies. We study the properties of galaxies surrounding these two samples. Neighbouring galaxies located close to (~200 kpc) obscured AGNs tend to have redder colors, compared to the local environment of unobscured AGNs. Results obtained from a KS test show that the two color distributions are different at ~95% confidence level. We find that obscured AGNs are located in denser local galaxy environments compared to the unobscured AGN sample. Conclusions. Our results suggest that AGN obscuration can occur at galactic scales, possibly due to galaxy interactions or mergers, and that the simple unified model based solely on the local torus orientation may not be sufficient to explain all the observations.Fil: Bornancini, Carlos Guillermo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Astronomía Teórica y Experimental. Universidad Nacional de Córdoba. Observatorio Astronómico de Córdoba. Instituto de Astronomía Teórica y Experimental; ArgentinaFil: Taormina, Mónica Silvia. Polish Academy of Sciences. Nicolaus Copernicus Astronomical Center; Polonia. Universidad Nacional de Cordoba. Observatorio Astronomico de Cordoba; ArgentinaFil: Garcia Lambas, Diego Rodolfo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Astronomía Teórica y Experimental. Universidad Nacional de Córdoba. Observatorio Astronómico de Córdoba. Instituto de Astronomía Teórica y Experimental; Argentin
Large-scale structure in mimetic Horndeski gravity
In this paper, we propose to use the mimetic Horndeski model as a model for
the dark universe. Both cold dark matter (CDM) and dark energy (DE) phenomena
are described by a single component, the mimetic field. In linear theory, we
show that this component effectively behaves like a perfect fluid with zero
sound speed and clusters on all scales. For the simpler mimetic cubic Horndeski
model, if the background expansion history is chosen to be identical to a
perfect fluid DE (PFDE) then the mimetic model predicts the same power spectrum
of the Newtonian potential as the PFDE model with zero sound speed. In
particular, if the background is chosen to be the same as that of LCDM, then
also in this case the power spectrum of the Newtonian potential in the mimetic
model becomes indistinguishable from the power spectrum in LCDM on linear
scales. A different conclusion may be found in the case of non-adiabatic
perturbations. We also discuss the distinguishability, using power spectrum
measurements from LCDM N-body simulations as a proxy for future observations,
between these mimetic models and other popular models of DE. For instance, we
find that if the background has an equation of state equal to -0.95 then we
will be able to distinguish the mimetic model from the PFDE model with unity
sound speed. On the other hand, it will be hard to do this distinction with
respect to the LCDM model.Comment: v1: 17 pages, 6 figures. v2: minor changes, comments added, replaced
to match published versio
Time variation of fundamental couplings and dynamical dark energy
Scalar field dynamics may give rise to a nonzero cosmological variation of
fundamental constants. Within different scenarios based on the unification of
gauge couplings, the various claimed observations and bounds may be combined in
order to trace or restrict the time history of the couplings and masses. If the
scalar field is responsible for a dynamical dark energy or quintessence,
cosmological information becomes available for its time evolution. Combining
this information with the time variation of couplings, one can determine the
interaction strength between the scalar and atoms, which may be observed by
tests of the Weak Equivalence Principle. We compute bounds on the present rate
of coupling variation from experiments testing the differential accelerations
for bodies with equal mass and different composition and compare the
sensitivity of various methods. In particular, we discuss two specific models
of scalar evolution: crossover quintessence and growing neutrino models.Comment: 26 pages, 2 figures; minor typos & added references, to be published
in JCA
- …