150 research outputs found
Unifying static analysis of gravitational structures with a scale-dependent scalar field gravity as an alternative to dark matter
Aims. We investigated the gravitational effects of a scalar field within
scalar-tensor gravity as an alternative to dark matter. Motivated by chameleon,
symmetron and f(R)-gravity models, we studied a phenomenological scenario where
the scalar field has both a mass (i.e. interaction length) and a coupling
constant to the ordinary matter which scale with the local properties of the
considered astrophysical system. Methods. We analysed the feasibility of this
scenario using the modified gravitational potential obtained in its context and
applied it to the galactic and hot gas/stellar dynamics in galaxy clusters and
elliptical/spiral galaxies respectively. This is intended to be a first step in
assessing the viability of this new approach in the context of "alternative
gravity" models. Results. The main results are: 1. the velocity dispersion of
elliptical galaxies can be fitted remarkably well by the suggested scalar
field, with model significance similar to a classical Navarro-Frenk-White dark
halo profile; 2. the analysis of the stellar dynamics and the gas equilibrium
in elliptical galaxies has shown that the scalar field can couple with ordinary
matter with different strengths (different coupling constants) producing and/or
depending on the different clustering state of matter components; 3. elliptical
and spiral galaxies, combined with clusters of galaxies, show evident
correlations among theory parameters which suggest the general validity of our
results at all scales and a way toward a possible unification of the theory for
all types of gravitational systems we considered. All these results demonstrate
that the proposed scalar field scenario can work fairly well as an alternative
to dark matter.Comment: 23 pages, 15 figures, 5 tables, accepted for publication on Astronomy
& Astrophysic
The central dark matter content of early-type galaxies: scaling relations and connections with star formation histories
We examine correlations between masses, sizes and star formation histories for a large sample of low-redshift early-type galaxies, using a simple suite of dynamical and stellar population models. We confirm an anticorrelation between the size and stellar age and go on to survey for trends with the central content of dark matter (DM). An average relation between the central DM density and galaxy size of ăÏDMăâRâ2eff provides the first clear indication of cuspy DM haloes in these galaxies - akin to standard Î cold dark matter haloes that have undergone adiabatic contraction. The DM density scales with galaxy mass as expected, deviating from suggestions of a universal halo profile for dwarf and late-type galaxies. We introduce a new fundamental constraint on galaxy formation by finding that the central DM fraction decreases with stellar age. This result is only partially explained by the size-age dependencies, and the residual trend is in the opposite direction to basic DM halo expectations. Therefore, we suggest that there may be a connection between age and halo contraction and that galaxies forming earlier had stronger baryonic feedback, which expanded their haloes, or lumpier baryonic accretion, which avoided halo contraction. An alternative explanation is a lighter initial mass function for older stellar population
VEGAS: A VST Early-type GAlaxy Survey. III. Mapping the galaxy structure, interactions and intragroup light in the NGC 5018 group
Most of the galaxies in the Universe at present day are in groups, which are
key to understanding the galaxy evolution. In this work we present a new deep
mosaic of 1.2 x 1.0 square degrees of the group of galaxies centered on NGC
5018, acquired at the ESO VLT Survey Telescope. We use u, g, r images to
analyse the structure of the group members and to estimate the intra-group
light. Taking advantage of the deep and multiband photometry and of the large
field of view of the VST telescope, we studied the structure of the galaxy
members and the faint features into the intra-group space and we give an
estimate of the intragroup diffuse light in the NGC 5018 group of galaxies. We
found that ~ 41% of the total g-band luminosity of the group is in the form of
intragroup light (IGL). The IGL has a (g - r) color consistent with those of
other galaxies in the group, indicating that the stripping leading to the
formation of IGL is ongoing. From the study of this group we can infer that
there are at least two different interactions involving the group members: one
between NGC 5018 and NGC 5022, which generates the tails and ring-like
structures detected in the light, and another between NGC 5022 and
MCG-03-34-013 that have produced the HI tail. A minor merging event also
happened in the formation history of NGC 5018 that have perturbed the inner
structure of this galaxy.Comment: 21 pages, 15 figures. Accepted for publication in Ap
Statistical analysis of probability density functions for photometric redshifts through the KiDS-ESO-DR3 galaxies
Despite the high accuracy of photometric redshifts (zphot) derived using
Machine Learning (ML) methods, the quantification of errors through reliable
and accurate Probability Density Functions (PDFs) is still an open problem.
First, because it is difficult to accurately assess the contribution from
different sources of errors, namely internal to the method itself and from the
photometric features defining the available parameter space. Second, because
the problem of defining a robust statistical method, always able to quantify
and qualify the PDF estimation validity, is still an open issue. We present a
comparison among PDFs obtained using three different methods on the same data
set: two ML techniques, METAPHOR (Machine-learning Estimation Tool for Accurate
PHOtometric Redshifts) and ANNz2, plus the spectral energy distribution
template fitting method, BPZ. The photometric data were extracted from the KiDS
(Kilo Degree Survey) ESO Data Release 3, while the spectroscopy was obtained
from the GAMA (Galaxy and Mass Assembly) Data Release 2. The statistical
evaluation of both individual and stacked PDFs was done through quantitative
and qualitative estimators, including a dummy PDF, useful to verify whether
different statistical estimators can correctly assess PDF quality. We conclude
that, in order to quantify the reliability and accuracy of any zphot PDF
method, a combined set of statistical estimators is required.Comment: Accepted for publication by MNRAS, 20 pages, 14 figure
Total and dark mass from observations of galaxy centers with Machine Learning
The galaxy total mass inside the effective radius encode important
information on the dark matter and galaxy evolution model. Total "central"
masses can be inferred via galaxy dynamics or with gravitational lensing, but
these methods have limitations. We propose a novel approach, based on Random
Forest, to make predictions on the total and dark matter content of galaxies
using simple observables from imaging and spectroscopic surveys. We use
catalogs of multi-band photometry, sizes, stellar mass, kinematic
"measurements" (features) and dark matter (targets) of simulated galaxies, from
Illustris-TNG100 hydrodynamical simulation, to train a Mass Estimate machine
Learning Algorithm (Mela). We separate the simulated sample in passive
early-type galaxies (ETGs), both "normal" and "dwarf", and active late-type
galaxies (LTGs) and show that the mass estimator can accurately predict the
galaxy dark masses inside the effective radius in all samples. We finally test
the mass estimator against the central mass estimates of a series of low
redshift (z0.1) datasets, including SPIDER, MaNGA/DynPop and SAMI dwarf
galaxies, derived with standard dynamical methods based on Jeans equations.
Dynamical masses are reproduced within 0.30 dex (), with a limited
fraction of outliers and almost no bias. This is independent of the
sophistication of the kinematical data collected (fiber vs. 3D spectroscopy)
and the dynamical analysis adopted (radial vs. axisymmetric Jeans equations,
virial theorem). This makes Mela a powerful alternative to predict the mass of
galaxies of massive stage-IV surveys' datasets
LEMON:LEns MOdelling with Neural networks - I. Automated modelling of strong gravitational lenses with Bayesian Neural Networks
The unprecedented number of gravitational lenses expected from new-generation facilities such as the ESA Euclid telescope and the Vera Rubin Observatory makes it crucial to rethink our classical approach to lens-modelling. In this paper, we present LEMON (Lens Modelling with Neural networks): a new machine-learning algorithm able to analyse hundreds of thousands of gravitational lenses in a reasonable amount of time. The algorithm is based on a Bayesian Neural Network: a new generation of neural networks able to associate a reliable confidence interval to each predicted parameter. We train the algorithm to predict the three main parameters of the singular isothermal ellipsoid model (the Einstein radius and the two components of the ellipticity) by employing two simulated data sets built to resemble the imaging capabilities of the Hubble Space Telescope and the forthcoming Euclid satellite. In this work, we assess the accuracy of the algorithm and the reliability of the estimated uncertainties by applying the network to several simulated data sets of 104 images each. We obtain accuracies comparable to previous studies present in the current literature and an average modelling time of just âŒ0.5 s per lens. Finally, we apply the LEMON algorithm to a pilot data set of real lenses observed with HST during the SLACS program, obtaining unbiased estimates of their SIE parameters. The code is publicly available on GitHub (https://github.com/fab-gentile/LEMON).</p
CASCO: Cosmological and AStrophysical parameters from Cosmological simulations and Observations -- I. Constraining physical processes in local star-forming galaxies
We compare the structural properties and dark matter content of star-forming
galaxies taken from the CAMELS cosmological simulations to the observed trends
derived from the SPARC sample in the stellar mass range , to provide constraints on the value of
cosmological and astrophysical (SN- and AGN-related) parameters. We consider
the size-, internal DM fraction-, internal DM mass- and total-stellar mass
relations for all the 1065 simulations from the IllustrisTNG, SIMBA and ASTRID
suites of CAMELS, and search for the parameters that minimize the
with respect to the observations. For the IllustrisTNG suite, we find the
following constraints for the cosmological parameters: , and , which are consistent within with the results
from the nine-year WMAP observations. SN feedback-related astrophysical
parameters, which describe the departure of outflow wind energy per unit star
formation rate and wind velocity from the reference IllustrisTNG simulations,
assume the following values: and
, respectively. Therefore, simulations
with a lower value of outflow wind energy per unit star formation rate with
respect to the reference illustrisTNG simulation better reproduce the
observations. Simulations based on SIMBA and ASTRID suites predict central dark
matter masses substantially larger than those observed in real galaxies, which
can be reconciled with observations only by requiring values of
inconsistent with cosmological constraints for SIMBA, or
simulations characterized by unrealistic galaxy mass distributions for ASTRID.Comment: 24 pages, 10 figures, 9 tables. Accepted by MNRAS for publication;
Added a reference to sec. 4.
A Dearth of Dark Matter in Ordinary Elliptical Galaxies
The kinematics of the outer parts of three intermediate-luminosity elliptical
galaxies have been studied using the Planetary Nebula Spectrograph. The
galaxies' velocity dispersion profiles are found to decline with radius;
dynamical modeling of the data indicates the presence of little if any dark
matter in these galaxies' halos. This surprising result conflicts with findings
in other galaxy types, and poses a challenge to current galaxy formation
theories.Comment: Science, 19 Sep 03, in press, 15 pp., 6 figs (4 color), supporting
online material integrated as appendix, uses scicite.sty. See high-res
version at http://astro.nottingham.ac.uk/~romanow/res.html and Science
Express online at http://www.sciencemag.org/cgi/content/abstract/1087441v
Rejection criteria based on outliers in the KiDS photometric redshifts and PDF distributions derived by machine learning
The Probability Density Function (PDF) provides an estimate of the
photometric redshift (zphot) prediction error. It is crucial for current and
future sky surveys, characterized by strict requirements on the zphot
precision, reliability and completeness. The present work stands on the
assumption that properly defined rejection criteria, capable of identifying and
rejecting potential outliers, can increase the precision of zphot estimates and
of their cumulative PDF, without sacrificing much in terms of completeness of
the sample. We provide a way to assess rejection through proper cuts on the
shape descriptors of a PDF, such as the width and the height of the maximum
PDF's peak. In this work we tested these rejection criteria to galaxies with
photometry extracted from the Kilo Degree Survey (KiDS) ESO Data Release 4,
proving that such approach could lead to significant improvements to the zphot
quality: e.g., for the clipped sample showing the best trade-off between
precision and completeness, we achieve a reduction in outliers fraction of
and an improvement of for NMAD, with respect to the
original data set, preserving the of its content.Comment: Preprint version of the manuscript to appear in the Volume
"Intelligent Astrophysics" of the series "Emergence, Complexity and
Computation", Book eds. I. Zelinka, D. Baron, M. Brescia, Springer Nature
Switzerland, ISSN: 2194-728
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