8,281 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
Smooth maps from clumpy data: Covariance analysis
Interpolation techniques play a central role in Astronomy, where one often
needs to smooth irregularly sampled data into a smooth map. In a previous
article (Lombardi & Schneider 2001), we have considered a widely used smoothing
technique and we have evaluated the expectation value of the smoothed map under
a number of natural hypotheses. Here we proceed further on this analysis and
consider the variance of the smoothed map, represented by a two-point
correlation function. We show that two main sources of noise contribute to the
total error budget and we show several interesting properties of these two
noise terms. The expressions obtained are also specialized to the limiting
cases of low and high densities of measurements. A number of examples are used
to show in practice some of the results obtained.Comment: 23 pages, 10 figures, final version, A&A in pres
Variational Estimates using a Discrete Variable Representation
The advantage of using a Discrete Variable Representation (DVR) is that the
Hamiltonian of two interacting particles can be constructed in a very simple
form. However the DVR Hamiltonian is approximate and, as a consequence, the
results cannot be considered as variational ones. We will show that the
variational character of the results can be restored by performing a reduced
number of integrals.
In practice, for a variational description of the lowest n bound states only
n(n+1)/2 integrals are necessary whereas D(D+1)/2 integrals are enough for the
scattering states (D is the dimension of the S matrix). Applications of the
method to the study of dimers of He, Ne and Ar, for both bound and scattering
states, are presented.Comment: 30 pages, 7 figures. Minor changes (title modified, typos corrected,
1 reference added). To be published in PR
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
Smooth maps from clumpy data: generalizations
In a series of papers (Lombardi & Schneider 2001, 2002) we studied in detail
the statistical properties of an interpolation technique widely used in
astronomy. In particular, we considered the average interpolated map and its
covariance under the hypotheses that the map is obtained by smoothing unbiased
measurements of an unknown field, and that the measurements are uniformly
distributed on the sky. In this paper we generalize the results obtained to the
case of observations carried out only on a finite field and distributed on the
field with a non-uniform density. These generalizations, which are required in
many astronomically relevant cases, still allow an exact, analytical solution
of the problem. We also consider a number of properties of the interpolated
map, and provide asymptotic expressions for the average map and the two-point
correlation function which are valid at high densities.Comment: 9 pages, 3 figures, A&A in pres
Smooth maps from clumpy data
We study an estimator for smoothing irregularly sampled data into a smooth map. The estimator has been widely used in astronomy, owing to its low level of noise; it involves a weight function -- or smoothing kernel -- w(\theta). We show that this estimator is not unbiased, in the sense that the expectation value of the smoothed map is not the underlying process convolved with , but a convolution with a modified kernel w_eff(\theta). We show how to calculate w_eff for a given kernel w and investigate its properties. In particular, it is found that (1) w_eff is normalized, (2) has a shape `similar' to the original kernel w, (3) converges to w in the limit of high number density of data points, and (4) reduces to a top-hat filter in the limit of very small number density of data points. Hence, although the estimator is biased, the bias is well understood analytically, and since w_eff has all the desired properties of a smoothing kernel, the estimator is in fact very useful. We present explicit examples for several filter functions which are commonly used, and provide a series expression valid in the limit of large density of data points
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
El Roque de Los Muchachos Site Characteristics. II. Analysis of Wind, Relative Humidity and Air Pressure
In this paper we present an analysis of wind speed, wind direction, relative
humidity and air pressure taken at TNG, CAMC and NOT at Observatorio del Roque
de Los Muchachos, in the Canary Islands. Data are compared in order to check
local variations and both long term and short term trends of the microclimate.
Furthermore, influence of wind speed on the astronomical seeing is estimated to
the aim to better understand the influence of wide scale parameters on local
meteorological data. The three telescopes show different prevailing wind
direction, wind speed, relative humidity and air pressure confirming
differences in local microclimate. We found that seeing deteriorates when wind
speed is lower than 3.3 m/s. Comparison in terms of wind speed and high
relative humidity (> 90%) shows that TNG seems to have optimal observational
conditions with respect to CAMC and NOT. Air pressure analysis shows that ORM
is dominated by high pressure. Short time variations of pressure anticipate
temperature variations tipically by 2-3 hours, this property vanishes in time
scales higher than some hours and disappear in longer time scales.Comment: 10 pages, 9 figures, accepted for publication in PAS
Mapping Global Star Formation in the Interacting Galaxy Pair Arp32
A multi-wavelength set of photometric data including UV (GALEX), optical, near-IR, infrared (Spitzer) and radio (VLA 20cm) images and spectroscopic observations are used to map the dust-obscured and unobscured star formation in the galaxy pair Arp 32. The system consists of an actively starforming galaxy and another one with depressed star formation. The most active galaxy has disrupted morphology and different sites of star formation. Spectroscopic data show hints of nuclear activity in its core, intense star formation in limited regions of the galaxy as well as an underlying population of stars witnessing a past episode of star formation. Current star formation rates are estimated from UV and bolometric IR luminosities
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