816 research outputs found
Finding Strong Gravitational Lenses in the Kilo Degree Survey with Convolutional Neural Networks
The volume of data that will be produced by new-generation surveys requires
automatic classification methods to select and analyze sources. Indeed, this is
the case for the search for strong gravitational lenses, where the population
of the detectable lensed sources is only a very small fraction of the full
source population. We apply for the first time a morphological classification
method based on a Convolutional Neural Network (CNN) for recognizing strong
gravitational lenses in square degrees of the Kilo Degree Survey (KiDS),
one of the current-generation optical wide surveys. The CNN is currently
optimized to recognize lenses with Einstein radii arcsec, about
twice the -band seeing in KiDS. In a sample of colour-magnitude
selected Luminous Red Galaxies (LRG), of which three are known lenses, the CNN
retrieves 761 strong-lens candidates and correctly classifies two out of three
of the known lenses. The misclassified lens has an Einstein radius below the
range on which the algorithm is trained. We down-select the most reliable 56
candidates by a joint visual inspection. This final sample is presented and
discussed. A conservative estimate based on our results shows that with our
proposed method it should be possible to find massive LRG-galaxy
lenses at z\lsim 0.4 in KiDS when completed. In the most optimistic scenario
this number can grow considerably (to maximally 2400 lenses), when
widening the colour-magnitude selection and training the CNN to recognize
smaller image-separation lens systems.Comment: 24 pages, 17 figures. Published in MNRA
Cosmic dance in the Shapley Concentration Core - I. A study of the radio emission of the BCGs and tailed radio galaxies
The Shapley Concentration () covers several degrees in the
Southern Hemisphere, and includes galaxy clusters in advanced evolutionary
stage, groups of clusters in the early stages of merger, fairly massive
clusters with ongoing accretion activity, and smaller groups located in
filaments in the regions between the main clusters. With the goal to
investigate the role of cluster mergers and accretion on the radio galaxy
population, we performed a multi-wavelength study of the BCGs and of the
galaxies showing extended radio emission in the cluster complexes of Abell 3528
and Abell 3558. Our study is based on a sample of 12 galaxies. We observed the
clusters with the GMRT at 235, 325 and 610 MHz, and with the VLA at 8.46 GHz.
We complemented our study with the TGSS at 150 MHz, the SUMSS at 843 MHz and
ATCA at 1380, 1400, 2380, and 4790 MHz data. Optical imaging with ESO-VST and
mid-IR coverage with WISE are also available for the host galaxies. We found
deep differences in the properties of the radio emission of the BCGs in the two
cluster complexes. The BCGs in the A3528 complex and in A3556, which are
relaxed cool-core objects, are powerful active radio galaxies. They also
present hints of restarted activity. On the contrary, the BCGs in A3558 and
A3562, which are well known merging systems, are very faint, or quiet, in the
radio band. The optical and IR properties of the galaxies are fairly similar in
the two complexes, showing all passive red galaxies. Our study shows remarkable
differences in the radio properties of the BGCs, which we relate to the
different dynamical state of the host cluster. On the contrary, the lack of
changes between such different environments in the optical band suggests that
the dynamical state of galaxy clusters does not affect the optical counterparts
of the radio galaxies, at least over the life-time of the radio emission.Comment: 24 pages, 11 figures, accepted for publication in Astronomy &
Astrophysic
Shapley Supercluster Survey: Construction of the photometric catalogues and i-band data release
The Shapley Supercluster Survey is a multi-wavelength survey covering an area of ∼23 deg² (∼260 Mpc² at z = 0.048) around the supercluster core, including nine Abell and two poor clusters, having redshifts in the range 0.045–0.050. The survey aims to investigate the role of the cluster-scale mass assembly on the evolution of galaxies, mapping the effects of the environment from the cores of the clusters to their outskirts and along the filaments. The optical (ugri) imaging acquired with OmegaCAM on the VLT Survey Telescope is essential to achieve the project goals providing accurate multi-band photometry for the galaxy population down to m∗ + 6. We describe the methodology adopted to construct the optical catalogues and to separate extended and point-like sources. The catalogues reach average 5σ limiting magnitudes within a 3 arcsec diameter aperture of ugri = [24.4,24.6,24.1,23.3] and are 93 per cent complete down to ugri = [23.8,23.8,23.5,22.0] mag, corresponding to ∼m∗ r + 8.5. The data are highly uniform in terms of observing conditions and all acquired with seeing less than 1.1 arcsec full width at half-maximum. The median seeing in r band is 0.6 arcsec, corresponding to 0.56 kpc h⁻¹ 70 at z = 0.048. While the observations in the u, g and r bands are still ongoing, the i-band observations have been completed, and we present the i-band catalogue over the whole survey area. The latter is released and it will be regularly updated, through the use of the Virtual Observatory tools. This includes 734 319 sources down to i = 22.0 mag and it is the first optical homogeneous catalogue at such a depth, covering the central region of the Shapley supercluster
Earth-Moon Lagrangian points as a testbed for general relativity and effective field theories of gravity
We first analyse the restricted four-body problem consisting of the Earth, the Moon and the Sun as the primaries and a spacecraft as the planetoid. This scheme allows us to take into account the solar perturbation in the description of the motion of a spacecraft in the vicinity of the stable Earth-Moon libration points L4 and L5 both in the classical regime and in the context of effective field theories of gravity. A vehicle initially placed at L4 or L5 will not remain near the respective points. In particular, in the classical case the vehicle moves on a trajectory about the libration points for at least 700 days before escaping away. We show that this is true also if the modified long-distance Newtonian potential of effective gravity is employed. We also evaluate the impulse required to cancel out the perturbing force due to the Sun in order to force the spacecraft to stay precisely at L4 or L5. It turns out that this value is slightly modified with respect to the corresponding Newtonian one. In the second part of the paper, we first evaluate the location of all Lagrangian points in the Earth-Moon system within the framework of general relativity. For the points L4 and L5, the corrections of coordinates are of order a few millimeters and describe a tiny departure from the equilateral triangle. After that, we set up a scheme where the theory which is quantum corrected has as its classical counterpart the Einstein theory, instead of the Newtonian one. In other words, we deal with a theory involving quantum corrections to Einstein gravity, rather than to Newtonian gravity. By virtue of the effective-gravity correction to the long-distance form of the potential among two point masses, all terms involving the ratio between the gravitational radius of the primary and its separation from the planetoid get modified. Within this framework, for the Lagrangian points of stable equilibrium, we find quantum corrections of order two millimeters, whereas for Lagrangian points of unstable equilibrium we find quantum corrections below a millimeter. In the latter case, for the point L1, general relativity corrects Newtonian theory by 7.61 meters, comparable, as an order of magnitude, with the lunar geodesic precession of about 3 meters per orbit. The latter is a cumulative effect accurately measured at the centimeter level through the lunar laser ranging positioning technique. Thus, it is possible to study a new laser ranging test of general relativity to measure the 7.61-meter correction to the L1 Lagrangian point, an observable never used before in the Sun-Earth-Moon system. Performing such an experiment requires controlling the propulsion to precisely reach L1, an instrumental accuracy comparable to the measurement of the lunar geodesic precession, understanding systematic effects resulting from thermal radiation and multi-body gravitational perturbations. This will then be the basis to consider a second-generation experiment to study deviations of effective field theories of gravity from general relativity in the Sun-Earth-Moon system
Photometric redshifts for the Kilo-Degree Survey. Machine-learning analysis with artificial neural networks
We present a machine-learning photometric redshift analysis of the
Kilo-Degree Survey Data Release 3, using two neural-network based techniques:
ANNz2 and MLPQNA. Despite limited coverage of spectroscopic training sets,
these ML codes provide photo-zs of quality comparable to, if not better than,
those from the BPZ code, at least up to zphot<0.9 and r<23.5. At the bright end
of r<20, where very complete spectroscopic data overlapping with KiDS are
available, the performance of the ML photo-zs clearly surpasses that of BPZ,
currently the primary photo-z method for KiDS.
Using the Galaxy And Mass Assembly (GAMA) spectroscopic survey as
calibration, we furthermore study how photo-zs improve for bright sources when
photometric parameters additional to magnitudes are included in the photo-z
derivation, as well as when VIKING and WISE infrared bands are added. While the
fiducial four-band ugri setup gives a photo-z bias and scatter
at mean z = 0.23, combining magnitudes, colours, and galaxy
sizes reduces the scatter by ~7% and the bias by an order of magnitude. Once
the ugri and IR magnitudes are joined into 12-band photometry spanning up to 12
, the scatter decreases by more than 10% over the fiducial case. Finally,
using the 12 bands together with optical colours and linear sizes gives and .
This paper also serves as a reference for two public photo-z catalogues
accompanying KiDS DR3, both obtained using the ANNz2 code. The first one, of
general purpose, includes all the 39 million KiDS sources with four-band ugri
measurements in DR3. The second dataset, optimized for low-redshift studies
such as galaxy-galaxy lensing, is limited to r<20, and provides photo-zs of
much better quality than in the full-depth case thanks to incorporating optical
magnitudes, colours, and sizes in the GAMA-calibrated photo-z derivation.Comment: A&A, in press. Data available from the KiDS website
http://kids.strw.leidenuniv.nl/DR3/ml-photoz.php#annz
Complete roughness and conductivity corrections for the recent Casimir force measurement
We consider detailed roughness and conductivity corrections to the Casimir
force in the recent Casimir force measurement employing an Atomic Force
Microscope. The roughness of the test bodies-a metal plate and a sphere- was
investigated with the Atomic Force Microscope and the Scanning Electron
Microscope respectively. It consists of separate crystals of different heights
and a stochastic background. The amplitude of roughness relative to the zero
roughness level was determined and the corrections to the Casimir force were
calculated up to the fourth order in a small parameter (which is this amplitude
divided by the distance between the two test bodies). Also the corrections due
to finite conductivity were found up to the fourth order in relative
penetration depth of electromagnetic zero point oscillations into the metal.
The theoretical result for the configuration of a sphere above a plate taking
into account both corrections is in excellent agreement with the measured
Casimir force
The first sample of spectroscopically confirmed ultra-compact massive galaxies in the Kilo Degree Survey
We present results from an ongoing investigation using the Kilo Degree Survey
(KiDS) on the VLT Survey Telescope (VST) to provide a census of ultra-compact
massive galaxies (UCMGs), defined as galaxies with stellar masses and effective radii . UCMGs, which are expected to have undergone very few merger
events, provide a unique view on the accretion history of the most massive
galaxies in the Universe. Over an effective sky area of nearly 330 square
degrees, we select UCMG candidates from KiDS multi-colour images, which provide
high quality structural parameters, photometric redshifts and stellar masses.
Our sample of photometrically selected UCMGs at
represents the largest sample of UCMG candidates assembled to date over the
largest sky area. In this paper we present the first effort to obtain their
redshifts using different facilities, starting with first results for 28
candidates with redshifts , obtained at NTT and TNG telescopes. We
confirmed, as bona fide UCMGs, 19 out of the 28 candidates with new redshifts.
A further 46 UCMG candidates are confirmed with literature spectroscopic
redshifts (35 at ), bringing the final cumulative sample of
spectroscopically-confirmed lower-z UCMGs to 54 galaxies, which is the largest
sample at redshifts below . We use these spectroscopic redshifts to
quantify systematic errors in our photometric selection, and use these to
correct our UCMG number counts. We finally compare the results to independent
datasets and simulations.Comment: Accepted for publication on MNRAS, 27 pages, 13 figures, 7 tables.
This revised and improved version presents different updates. In particular,
systematics and uncertainties in the measurement of the effective radii are
now better discussed, and new plots are adde
Identification of strontium in the merger of two neutron stars.
Half of all of the elements in the Universe that are heavier than iron were created by rapid neutron capture. The theory underlying this astrophysical r-process was worked out six decades ago, and requires an enormous neutron flux to make the bulk of the elements1. Where this happens is still debated2. A key piece of evidence would be the discovery of freshly synthesized r-process elements in an astrophysical site. Existing models3-5 and circumstantial evidence6 point to neutron-star mergers as a probable r-process site; the optical/infrared transient known as a 'kilonova' that emerges in the days after a merger is a likely place to detect the spectral signatures of newly created neutron-capture elements7-9. The kilonova AT2017gfo-which was found following the discovery of the neutron-star merger GW170817 by gravitational-wave detectors10-was the first kilonova for which detailed spectra were recorded. When these spectra were first reported11,12, it was argued that they were broadly consistent with an outflow of radioactive heavy elements; however, there was no robust identification of any one element. Here we report the identification of the neutron-capture element strontium in a reanalysis of these spectra. The detection of a neutron-capture element associated with the collision of two extreme-density stars establishes the origin of r-process elements in neutron-star mergers, and shows that neutron stars are made of neutron-rich matter13
Physical properties of galaxies and their evolution in the VIMOS VLT Deep Survey. I. The evolution of the mass-metallicity relation up to z~0.9
We derive the mass-metallicity relation of star-forming galaxies up to
, using data from the VIMOS VLT Deep Survey. Automatic measurement of
emission-line fluxes and equivalent widths have been performed on the full
spectroscopic sample. This sample is divided into two sub-samples depending on
the apparent magnitude selection: wide () and deep
). These two samples span two different ranges of stellar
masses. Emission-line galaxies have been separated into star-forming galaxies
and active galactic nuclei using emission line ratios. For the star-forming
galaxies the emission line ratios have also been used to estimate gas-phase
oxygen abundance, using empirical calibrations renormalized in order to give
consistent results at low and high redshifts. The stellar masses have been
estimated by fitting the whole spectral energy distributions with a set of
stellar population synthesis models. We assume at first order that the shape of
the mass-metallicity relation remains constant with redshift. Then we find a
stronger metallicity evolution in the wide sample as compared to the deep
sample. We thus conclude that the mass-metallicity relation is flatter at
higher redshift. The observed flattening of the mass-metallicity relation at
high redshift is analyzed as an evidence in favor of the open-closed model.Comment: 21 pages, revised version submitted to A&
First narrow-band search for continuous gravitational waves from known pulsars in advanced detector data
Spinning neutron stars asymmetric with respect to their rotation axis are potential sources of
continuous gravitational waves for ground-based interferometric detectors. In the case of known pulsars a
fully coherent search, based on matched filtering, which uses the position and rotational parameters
obtained from electromagnetic observations, can be carried out. Matched filtering maximizes the signalto-
noise (SNR) ratio, but a large sensitivity loss is expected in case of even a very small mismatch
between the assumed and the true signal parameters. For this reason, narrow-band analysis methods have
been developed, allowing a fully coherent search for gravitational waves from known pulsars over a
fraction of a hertz and several spin-down values. In this paper we describe a narrow-band search of
11 pulsars using data from Advanced LIGO’s first observing run. Although we have found several initial
outliers, further studies show no significant evidence for the presence of a gravitational wave signal.
Finally, we have placed upper limits on the signal strain amplitude lower than the spin-down limit for 5 of
the 11 targets over the bands searched; in the case of J1813-1749 the spin-down limit has been beaten for
the first time. For an additional 3 targets, the median upper limit across the search bands is below the
spin-down limit. This is the most sensitive narrow-band search for continuous gravitational waves carried
out so far
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