PhD Tesis defense
On the 10th of September, at 10:30 , in the classroom of the mathematics faculty, Burjassot, will take place the PhD thesis defense of Raquel Forés Toribio, that has been supervised by José Antonio Muñoz Lozano.
Abstract:
The phenomenon of gravitational lensing occurs when a massive object is placed between the observer and an emitting source. As described by general relativity, the massive object curves the spacetime and bends the light rays coming from the source. The deflection can produce an increase of the brightness and multiple images of the source. Hence, gravitational lensing has become a powerful tool to study distant sources but also the deflector object (or lens) as the light deflection will depend on its mass distribution. This thesis is focused on the scenario where a distant quasar is lensed by a galaxy or a galaxy cluster. Quasars are observed as point-like time-varying luminous sources which are the result of the material accreted around a supermassive black hole in the center of their host galaxy. The physical conditions and structures formed through this process are still a subject of research and gravitational lensing can be used as a probe to access the unresolved structure of quasars.
In this thesis, we present a new mass model of the lens system SDSS J1004+4112 where the deflector is a galaxy cluster with a background quasar and three other groups of background galaxies. Using gravitational lensing observables (image positions, magnification ratios and time delays) and other measured parameters, the inner slope of the dark matter halo of the cluster has been constrained along with the other parameters of the model. A small offset between the Brightest Cluster Galaxy and the dark matter halo is found as well as a certain degree of alignment in the direction of the ellipticity of both components. Also, an estimate for the fourth unmeasured time delay is inferred from the model.
Given the compactness of the quasar accretion disk, stellar mass objects in the main lens deflect the light rays on the order of microarcseconds producing changes in the brightness of the source. This variability is independent in each quasar image and by subtracting the common (delayed) quasar intrinsic variability, the microlensing signature can be extracted. We study this signature for SDSS J1004+4112 and we infer the quasar accretion disk size in r-band and the stellar mass fractions of the intracluster medium at the four quasar image positions. The stellar fraction at the brightest image is slightly beyond the expectations and may indicate the presence of an undetected stellar component.
The deflections due to the microlenses also deviate the centroid of the image by several microarcseconds. In this work, we propose and study the feasibility to measure these shifts in a single epoch of observations by selecting as a reference a large quasar component not affected by microlensing. We estimate the astrometric shifts of 79 quasar images and select the best suited candidates for observations with the next generation of instrumentation such as HARMONI at the ELT.
