764 research outputs found
Separating inner and outer contributions in gravitational lenses using the perturbative method
This paper presents a reconstruction of the gravitational lens SL2S02176-0513
using the singular perturbative method presented in Alard 2007, MNRAS Letters,
382, 58 and Alard, C., 2008, MNRAS, 388, 375. The ability of the perturbative
method to separate the inner and outer contributions of the potential in
gravitational lenses is tested using SL2S02176-0513. In this lens, the
gravitational field of the central galaxy is dominated by a nearby group of
galaxies located at a distance of a few critical radius. The perturbative
functionals are re-constructed using local polynomials. The polynomial
interpolation is smoothed using Fourier series, and numerically fitted to HST
data using a non-linear minimization procedure. The potential inside and
outside the critical circle is derived from the reconstruction of the
perturbative fields. The inner and outer potential contours are very
different.The inner contours are consistent with the central galaxy, while the
outer contours are fully consistent with the perturbation introduced by the
group of galaxies. The ability of the perturbative method to separate the inner
and outer contribution is confirmed, and indicates that in the perturbative
approach the field of the central deflector can be separated from outer
perturbations. The separation of the inner and outer contribution is especially
important for the study of the shape of dark matter halo's as well as for the
statistical analysis of the effect of dark matter substructures
The baryonic self similarity of dark matter
The cosmological simulations indicates that the dark matter haloes have
specific self similar properties. However the halo similarity is affected by
the baryonic feedback, the momentum injected by the supernovae re-shape the
dark matter core and transform it to a flat density core, with a scale length
imposed by the baryonic feedback. Additionally the baryon feedback impose also
an equilibrium condition, which when coupled with the imposed baryonic scale
length induce a new type of similarity. The new self similar solution implies
that the acceleration generated by dark matter is scale free, which in turns
implies that the baryonic acceleration at a reference radius is also scale
free. Constant dark matter and baryonic accelerations at a reference radius
have effectively been observed for a large class of different galaxies, which
is in support of this approach. The new self similar properties implies that
the total acceleration at larger distances is scale free, the transition
between the dark matter and baryons dominated regime occurs at a constant
acceleration, and the maximum of the velocity curve which defines the amplitude
of the velocity curve at larger distances is proportional to .
These results demonstrates that in this self similar model, cold dark matter is
consistent with the basics of MOND phenomenology for the galaxies. In agreement
with the observation the coincidence between the self similar model and MOND is
expected to break at the scale of clusters of galaxies. Some numerical
experiments shows that the behavior of the density near the origin is closely
approximated by a Einasto profile.Comment: Last versio
Unbiased reconstruction of the mass function using microlensing survey data
The large number of microlensing events discovered towards the Galactic Bulge
bears the promise to reconstruct the stellar mass function. The more
interesting issue concerning the mass function is certainly to probe its low
mass end. However due to the source confusion, even if the distribution and the
kinematics of the lenses are known, the estimation of the mass function is
extremely biased at low masses. The blending due to the source confusion biases
the duration of the event, which in turn dramatically biases the estimation of
the mass of the lens. To overcome this problem we propose to use differential
photometry of the microlensing events obtained using the image subtraction
method. Differential photometry is free of any bias due to blending, however
the drawback of differential photometry is that the baseline flux is unknown.
In this paper we will show that even without knowing the baseline flux, purely
differential photometry allow to estimate the mass function without any biases.
The basis of the method is that taking the scalar product of the microlensing
light curves with a given function and taking its sum over all the microlensing
events is equivalent to project the mass function on another function. This
method demonstrates that there is a direct correspondancy between the space of
the observations and the space of the mass function. Concerning the function to
use in order to project the observations, we show that the principal components
of the light curves are an optimal set. To illustrate the method we simulate
sets consistent with the microlensing experiments. By using 1000 of these
simulations, we show that for instance the exponent of the mass function can be
reconstructed without any biases.Comment: 8 pages, 4 Figure, submitted to MNRA
Reconstructing the cosmic Horseshoe gravitational lens using the singular perturbative approach
The cosmic horseshoe gravitational lens is analyzed using the perturbative
approach. The two first order perturbative fields are expanded in Fourier
series. The source is reconstructed using a fine adaptive grid. The expansion
of the fields at order 2 produces a higher value of the chi-square. Expanding
at order 3 provides a very significant improvement, while order 4 does not
bring a significant improvement over order 3. The presence of the order 3 terms
is not a consequence of limiting the perturbative expansion to the first order.
The amplitude and signs of the third order terms are recovered by including the
contribution of the other group members. This analysis demonstrates that the
fine details of the potential of the lens could be recovered independently of
any assumptions by using the perturbative approach.Comment: 22 pages 11 figure
Gravitational arcs as a perturbation of the perfect ring
The image of a point situated at the center of a circularly symmetric
potential is a perfect circle. The perturbative effect of non-symmetrical
potential terms is to displace and break the perfect circle. These 2 effects,
displacement and breaking are directly related to the Taylor expansion of the
perturbation at first order on the circle. The numerical accuracy of this
perturbative approach is tested in the case of an elliptical potential with a
core radius. The contour of the images and the caustics lines are well
re-produced by the perturbative approach. These results suggests that the
modeling of arcs, and in particular of tangential arcs may be simplified by
using a general perturbative representation on the circle. An interesting
feature of the perturbative approach, is that the equation of the caustic line
depends only on the values on the circle of the lens displacement field along
the direction.Comment: 9 pages, 2 figure
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