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Scientific Report 2007-2009<br />
Astronomy & Astrophysics<br />
A7. Gravitational lensing and its cosmological applications<br />
Gravitational lensing uses the property of light to be<br />
deflected by the gravitational potential. The banding of<br />
light ray by a gravitational force is a direct consequence<br />
of the principle of equivalence but the amplitude of this<br />
effect can be exactly computed only using general relativity,<br />
in which the deflection is described by geodetic<br />
lines following the curvature of the space-time. The measuremt<br />
of a deflection of 1.75” for the stars behind the<br />
Sun during the 1919 solar eclipse was one of the main observational<br />
test that confirmed Einstein theory. Nevertheless<br />
it was only at the end the last century that gravitational<br />
lensing started to be an important reasearch<br />
subject in astrophysics and during the last ten years it<br />
became a fundamental tool for modern cosmology. The<br />
reason for this delay is mainly the very high quality images<br />
needed to observe this phoenomenon in most of the<br />
relevant cases: in what is called the “weak regime” the<br />
only observable consequence of light deflection is a tiny<br />
distortion in the shape of the lensed image.<br />
In an international context were gravitational lensing is<br />
a leading research subject, Italy started with a serious<br />
delay comparing with the other countries. The group<br />
of Rome, the result of a close collaboration between the<br />
University “La <strong>Sapienza</strong>” and the astronomical observatory<br />
(OAR), is working very activily with Naples and<br />
Bologna to compensate this gap. The main interests of<br />
our group are the following:<br />
Measurement of the cosmic shear: Cosmic shear<br />
is the gravitational distortion of the shape of background<br />
galaxies by the large scale structure of the universe. It is<br />
considered one of the most promising probe to determine<br />
the distribution of the dark matter in the universe.<br />
Our group participated to the analysis of the Canada<br />
France Hawaii Legacy Survey (CFHTLS) data, up to<br />
now the most sensitive measurement of cosmic shear [1],<br />
that allowed to place tight constraints on two important<br />
cosmological parameters: the matter density Ω m and the<br />
normalization of the matter power spectrum σ 8 .<br />
σ8<br />
1.2<br />
1.1<br />
1.0<br />
0.9<br />
0.8<br />
0.7<br />
0.6<br />
0.5<br />
Aperture−mass<br />
85’−230’<br />
0.2 0.4 0.6 0.8 1.0<br />
Ω m<br />
Figure 1: Comparison (1, 2σ) between WMAP3 (green contours)<br />
and CFHTLS results (purple). The combined contours of WMAP3<br />
and CFHTLS are shown in orange.<br />
Dark Matter and Gravity using two independent cosmological<br />
probes: cosmic shear and baryonic acoustic<br />
oscillations. For this purpose, Euclid will measure the<br />
shape and spectra of galaxies over the entire extragalactic<br />
sky in the visible and NIR, out to redshift 2, thus<br />
covering the period over which dark energy accelerated<br />
the universe expansion. Our group participate to the<br />
development of the mission concept as a member of the<br />
Euclid Imaging Consortium [2].<br />
Mass determination of galaxy clusters: The<br />
only direct method to estimate cluster mass, regardless<br />
of its composition or dynamical behavior, is therefore<br />
via measuring the distortion (shear) of the shapes of<br />
background galaxies that are weakly lensed by the gravitational<br />
potential of the cluster. Our group performed<br />
a weak lensing analysis of the z = 0.288 cluster Abell<br />
611 on g-band data obtained at the Large Binacular<br />
Telescope (LBT) in order to estimate the cluster mass.<br />
The combination of the large aperture of the telescope<br />
and the wide field of view allowed us to map a region<br />
well beyond the expected virial radius of the cluster and<br />
to get a high surface density of background galaxies.<br />
This made possible to estimate an accurate mass for<br />
Abell 611, demonstrating that LBC is a powerful<br />
instrument for weak gravitational lensing studies. This<br />
project was completed performing a comparative study<br />
of the A611 mass results obtained with strong lensing<br />
and X-ray data.<br />
Figure 2: Projected mass map obtained from a weak lensing<br />
analysis of CCD images of the cluster Abell 611. The contour<br />
levels (σ min = 3.5, σ max = 5) are overplotted on a g-band greyscale<br />
image (∼ 4 ′ ) of the field of the cluster.<br />
References<br />
1. L. Fu, et al., A. & A. 479, 9 (2008).<br />
2. A. Refregier, et al., Exp. Astronomy 23, 17 (2009).<br />
Authors<br />
R. Maoli, A. Romano, Scaramella 5 R., Mainini 5 R.,<br />
Giordano 5 F.<br />
Participation to the space mission EUCLID:<br />
Euclid is a space mission selected for study within the<br />
ESA’s Cosmic Vision framework. Euclid primary goal is<br />
to to place high accuracy constraints on Dark Energy,<br />
<strong>Sapienza</strong> Università di Roma 154 Dipartimento di Fisica