8,617 research outputs found
Double lenses
The analysis of the shear induced by a single cluster on the images of a
large number of background galaxies is all centered around the curl-free
character of a well-known vector field that can be derived from the data. Such
basic property breaks down when the source galaxies happen to be observed
through two clusters at different redshifts, partially aligned along the line
of sight. In this paper we address the study of double lenses and obtain five
main results. (i) First we generalize the procedure to extract the available
information, contained in the observed shear field, from the case of a single
lens to that of a double lens. (ii) Then we evaluate the possibility of
detecting the signature of double lensing given the known properties of the
distribution of clusters of galaxies. (iii) As a different astrophysical
application, we demonstrate how the method can be used to detect the presence
of a dark cluster that might happen to be partially aligned with a bright
cluster studied in terms of statistical lensing. (iv) In addition, we show that
the redshift distribution of the source galaxies, which in principle might also
contribute to break the curl-free character of the shear field, actually
produces systematic effects typically two orders of magnitude smaller than the
double lensing effects we are focusing on. (v) Remarkably, a discussion of
relevant contributions to the noise of the shear measurement has brought up an
intrinsic limitation of weak lensing analyses, since one specific contribution,
associated with the presence of a non-vanishing two-galaxy correlation
function, turns out not to decrease with the density of source galaxies (and
thus with the depth of the observations).Comment: 40 pages, 15 figures. Accepted for publication in ApJ main journa
Hipparcos distances of Ophiuchus and Lupus cloud complexes
We combine extinction maps from the Two Micron All Sky Survey (2MASS) with
Hipparcos and Tycho parallaxes to obtain reliable and high-precision estimates
of the distance to the Ophiuchus and Lupus dark complexes. Our analysis, based
on a rigorous maximum-likelihood approach, shows that the rho-Ophiuchi cloud is
located at (119 +/- 6) pc and the Lupus complex is located at (155 +/- 8) pc;
in addition, we are able to put constraints on the thickness of the clouds and
on their orientation on the sky (both these effects are not included in the
error estimate quoted above). For Ophiuchus, we find some evidence that the
streamers are closer to us than the core. The method applied in this paper is
currently limited to nearby molecular clouds, but it will find many natural
applications in the GAIA-era, when it will be possible to pin down the distance
and three-dimensional structure of virtually every molecular cloud in the
Galaxy.Comment: A&A in press - Corrected typo (Lupus distance) in the electronic
abstrac
Strong Lensing Reconstruction
We present a general linear algorithm for measuring the surface mass density
1-\kappa from the observable reduced shear g=\gamma/(1-\kappa) in the strong
lensing regime. We show that in general, the observed polarization field can be
decomposed into ``electric'' and ``magnetic'' components, which have
independent and redundant solutions, but perfectly orthogonal noise properties.
By combining these solutions, one can increase the signal-to-noise ratio by
\sqrt{2}. The solutions allow dynamic optimization of signal and noise, both in
real and Fourier space (using arbitrary smoothing windows). Boundary conditions
have no effect on the reconstructions, apart from its effect on the
signal-to-noise. Many existing reconstruction techniques are recovered as
special cases of this framework. The magnetic solution has the added benefit of
yielding the global and local parity of the reconstruction in a single step.Comment: final accepted version for ApJ
Larson's third law and the universality of molecular cloud structure
Larson (1981) first noted a scaling relation between masses and sizes in
molecular clouds that implies that these objects have approximately constant
column densities. This original claim, based upon millimeter observations of
carbon monoxide lines, has been challenged by many theorists, arguing that the
apparent constant column density observed is merely the result of the limited
dynamic range of observations, and that in reality clouds have column density
variations over two orders of magnitudes. In this letter we investigate a set
of nearby molecular clouds with near-infrared excess methods, which guarantee
very large dynamic ranges and robust column density measurements, to test the
validity of Larson's third law. We verify that different clouds have almost
identical average column densities above a given extinction threshold; this
holds regardless of the extinction threshold, but the actual average surface
mass density is a function of the specific threshold used. We show that a
second version of Larson's third law, involving the mass-radius relation for
single clouds and cores, does not hold in our sample, indicating that
individual clouds are not objects that can be described by constant column
density. Our results instead indicate that molecular clouds are characterized
by a universal structure. Finally we point out that this universal structure
can be linked to the log-normal nature of cloud column density distributions.Comment: 5 pages, 4 figures, A&A in press (letter
Scattering induced dynamical entanglement and the quantum-classical correspondence
The generation of entanglement produced by a local potential interaction in a
bipartite system is investigated. The degree of entanglement is contrasted with
the underlying classical dynamics for a Rydberg molecule (a charged particle
colliding on a kicked top). Entanglement is seen to depend on the structure of
classical phase-space rather than on the global dynamical regime. As a
consequence regular classical dynamics can in certain circumstances be
associated with higher entanglement generation than chaotic dynamics. In
addition quantum effects also come into play: for example partial revivals,
which are expected to persist in the semiclassical limit, affect the long time
behaviour of the reduced linear entropy. These results suggest that
entanglement may not be a pertinent universal signature of chaos.Comment: Published versio
HST/ACS weak lensing analysis of the galaxy cluster RDCS 1252.9-2927 at z=1.24
We present a weak lensing analysis of one of the most distant massive galaxy
cluster known, RDCS 1252.9-2927 at z=1.24, using deep images from the Advanced
Camera for Survey (ACS) on board the Hubble Space Telescope (HST). By taking
advantage of the depth and of the angular resolution of the ACS images, we
detect for the first time at z>1 a clear weak lensing signal in both the i
(F775W) and z (F850LP) filters. We measure a 5-\sigma signal in the i band and
a 3-\sigma signal in the shallower z band image. The two radial mass profiles
are found to be in very good agreement with each other, and provide a
measurement of the total mass of the cluster inside a 1Mpc radius of M(<1Mpc) =
(8.0 +/- 1.3) x 10^14 M_\odot in the current cosmological concordance model h
=0.70, \Omega_m=0.3, \Omega_\Lambda=0.7, assuming a redshift distribution of
background galaxies as inferred from the Hubble Deep Fields surveys. A weak
lensing signal is detected out to the boundary of our field (3' radius,
corresponding to 1.5Mpc at the cluster redshift). We detect a small offset
between the centroid of the weak lensing mass map and the brightest cluster
galaxy, and we discuss the possible origin of this discrepancy. The cumulative
weak lensing radial mass profile is found to be in good agreement with the
X-ray mass estimate based on Chandr and XMM-Newton observations, at least out
to R_500=0.5Mpc.Comment: 38 pages, ApJ in press. Full resolution images available at
http://www.eso.org/~prosati/RDCS1252/Lombardi_etal_accepted.pd
Tachoastrometry: astrometry with radial velocities
Spectra of composite systems (e.g., spectroscopic binaries) contain spatial
information that can be retrieved by measuring the radial velocities (i.e.,
Doppler shifts) of the components in four observations with the slit rotated by
90 degrees in the sky. By using basic concepts of slit spectroscopy we show
that the geometry of composite systems can be reliably retrieved by measuring
only radial velocity differences taken with different slit angles. The spatial
resolution is determined by the precision with which differential radial
velocities can be measured. We use the UVES spectrograph at the VLT to observe
the known spectroscopic binary star HD 188088 (HIP 97944), which has a maximum
expected separation of 23 milli-arcseconds. We measure an astrometric signal in
radial velocity of 276 \ms, which corresponds to a separation between the two
components at the time of the observations of 18 milli-arcseconds. The
stars were aligned east-west. We describe a simple optical device to
simultaneously record pairs of spectra rotated by 180 degrees, thus reducing
systematic effects. We compute and provide the function expressing the shift of
the centroid of a seeing-limited image in the presence of a narrow slit.The
proposed technique is simple to use and our test shows that it is amenable for
deriving astrometry with milli-arcsecond accuracy or better, beyond the
diffraction limit of the telescope. The technique can be further improved by
using simple devices to simultaneously record the spectra with 180 degrees
angles.With tachoastrometry, radial velocities and astrometric positions can be
measured simultaneously for many double line system binaries in an easy way.
The method is not limited to binary stars, but can be applied to any
astrophysical configuration in which spectral lines are generated by separate
(non-rotational symmetric) regions.Comment: Accepted for publication in A&
The mid-infrared extinction law in the darkest cores of the Pipe Nebula
Context. The properties of dust grains, in particular their size
distribution, are expected to differ from the interstellar medium to the
high-density regions within molecular clouds. Aims. We measure the mid-infrared
extinction law produced by dense material in molecular cloud cores. Since the
extinction at these wavelengths is caused by dust, the extinction law in cores
should depart from that found in low-density environments if the dust grains
have different properties. Methods. We use the unbiased LINES method to measure
the slope of the reddening vectors in color-color diagrams. We derive the
mid-infrared extinction law toward the dense cores B59 and FeSt 1-457 in the
Pipe Nebula over a range of visual extinction between 10 and 50 magnitudes,
using a combination of Spitzer/IRAC, and ESO NTT/VLT data. Results. The
mid-infrared extinction law in both cores departs significantly from a
power-law between 3.6 and 8 micron, suggesting that these cores contain dust
with a considerable fraction of large dust grains. We find no evidence for a
dependence of the extinction law with column density up to 50 magnitudes of
visual extinction in these cores, and no evidence for a variation between our
result and those for other clouds at lower column densities reported elsewhere
in the literature. This suggests that either large grains are present even in
low column density regions, or that the existing dust models need to be revised
at mid-infrared wavelengths. We find a small but significant difference in the
extinction law of the two cores, that we tentatively associate with the onset
of star formation in B59.Comment: 8 pages, 6 figures. Accepted to A&
The noise of cluster mass reconstructions from a source redshift distribution
The parameter-free reconstruction of the surface-mass density of clusters of
galaxies is one of the principal applications of weak gravitational lensing.
From the observable ellipticities of images of background galaxies, the tidal
gravitational field (shear) of the mass distribution is estimated, and the
corresponding surface mass density is constructed. The noise of the resulting
mass map is investigated here, generalizing previous work which included mainly
the noise due to the intrinsic galaxy ellipticities. Whereas this dominates the
noise budget if the lens is very weak, other sources of noise become important,
or even dominant, for the medium-strong lensing regime close to the center of
clusters. In particular, shot noise due to a Poisson distribution of galaxy
images, and increased shot noise owing to the correlation of galaxies in
angular position and redshift, can yield significantly larger levels of noise
than that from the intrinsic ellipticities only. We estimate the contributions
from these various effects for two widely used smoothing operations, showing
that one of them effectively removes the Poisson and the correlation noises
related to angular positions of galaxies. Noise sources due to the spread in
redshift of galaxies are still present in the optimized estimator and are shown
to be relevant in many cases. We show how (even approximate) redshift
information can be profitably used to reduce the noise in the mass map. The
dependence of the various noise terms on the relevant parameters (lens
redshift, strength, smoothing length, redshift distribution of background
galaxies) are explicitly calculated and simple estimates are provided.Comment: 18 pages, A&A in pres
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