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