Graduate School in Physics and Astrophysics
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ANNUAL REPORT
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name: Arianna Gallo
email: a.gallo@unito.it
ciclo: XXXIV
year completed (1,2 or 3): 2
supervisor: Luisa Ostorero
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GRADUATE SCHOOL COURSES
(only completed courses, with examination passed in the year)
code: 17
title: Search and characterization for extrasolar planets
teacher: Alessandro Sozzetti
hours: 16 (4 CFU)
code: 08
title: Data Analysis Techniques
teacher: Livio Bianchi
hours: 24 (6 CFU)
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Research activity/Publications in the current year
(max characters 2500)
During my second year of PhD, I continued my study on hypervelocity stars (HVSs) as a tool to constrain the Milky Way (MW) dark matter (DM) halo shape.
I focused on the impact of the DM halo shape on the distributions of different HVS observables, such as their positions and velocities.
With the numerical simulations implemented during the first year of my PhD, I computed the trajectories of a sample of HVSs traveling across a MW ruled by Newtonian gravity and embedded in a DM halo modeled with different shapes (spheroidal and triaxial, with different axis ratios).
By means of the Kolmogorov-Smirnov (K-S) test, I performed a statistical analysis of the distributions of the HVS observables obtained in different DM halo models. I proved that the DM halo shape mostly affects the Galactocentric tangential velocity, which is the key HVS observable to discriminate a spherical from a non-spherical DM halo.
To constrain the halo’s axis ratio, I developed a statistical method that efficiently compares the properties of a synthetic “observed sample” of HVSs, properly constructed to mimic a true observational HVS sample, to the properties of a series of synthetic samples of HVSs (mock catalogs) that have traveled through halos of different shapes.
The full mock catalog contains the positions and velocities of a sample of about 800 HVSs that, for each of the analyzed halo shapes, was simulated with 5000 different sets of initial conditions for the trajectory.
My method is based on the property of uniform distribution of the p-value of the K-S test under the occurrence of the null hypothesis and recovers the correct halo model for the “observed sample” in 98% of the cases.
Since the available HVS sample is composed of about 20 HVSs only, I am studying the impact of the size of the "observed sample" on the efficiency of my method, in order to apply it to real data.
The above results are presented in the paper "Constraining the shape of the Milky Way dark matter halo through a statistical study of hypervelocity stars" by Gallo et al. (almost ready for submission).
This year, I also contributed to the review paper “Dark Matters on the Scale of Galaxies” (de Martino, Chakrabarty, Cesare, Gallo, Ostorero & Diaferio; Universe 2020, 6, 107), in which we illustrate the challenges faced by the ΛCDM cosmological model on the scale of galaxies, and present possible solutions both within ΛCDM and beyond it, with a focus on different DM natures and modifications of Newtonian gravity.
BE AWARE: visits and stages,research activity and pubs are not evaluated as didactic credits,
but are requested to trace the PhD students' career