PhD::SPACE Thesis

List of the 2 PhD::SPACE Thesis completed by the fellows of the Program:


2. PhD Thesis: Star-Forming Galaxies’ Structural Evolution Across Cosmic Time and Environment

Author: Ana S. P. Afonso (fellow 2014/B003)

Abstract: From early times we find galaxies undergo major transformations which encode the physics of competing processes that shape the galaxy evolution (gas accretion, star formation, mergers, feedback processes, among others).

In this thesis, I study the galaxy structural evolution across cosmic time and environment. By linking the distant and local Universe, I show that the star-forming galaxies (SFGs, selected by their Ha emission) have typical disk-like morphologies and grow by a factor of 2-3 from z~2. I also attempt to measure the impact of cosmological dimming on the perceived galaxy evolution and find that it has little-to-no impact on the measured structural parameters.

By tracing Lya-selected galaxies (LAEs), I find that these galaxy population show little-to-no evolution in size (~1 kpc) across a ~3 Gyr period in the early Universe (2<z<6). However, when compared to other SFGs, this population deviates in morphological properties only at z<4. I hypothesize that LAEs and other SFGs trace the dominant galaxy population at earlier times while, later, LAEs only trace the smaller, younger, and less massive star-forming population.

By targeting a region in and around a superstructure at z~0.8 in the COSMOS field, I find that both stellar mass and environment impact the galaxy evolution. The environment has a pivotal role in the observed star formation activity in galaxies since one can witness an increase of the quiescent fraction of galaxies towards high-density regions. In the intermediate density regions, there is an increase of the star formation which can be linked with the change of the galaxy morphology from disks to ellipticals from the low- to high-density regions. The electron density decreases from low to high densities for galaxies up to 10^{10.75} M_sun. However, massive galaxies tend to point towards a different and opposite relation for which the possible ionizing mechanisms remain unclear.

Publications: Paper 1 | Paper 2 | Paper 3

Advisors: David Sobral (IA U.Lisboa), José Afonso (DF/FCUL & IA U.Lisboa)

Fellowship: PD/00040/2012 – SFRH/BD/52706/2014 - FCT

University: Lisboa

Finish date: 2019/01/07


1. PhD Thesis: Cosmic Paleontology: Searching for Superstrings

Author: Ivan Rybak (fellow 2014/B002)

Abstract: Cosmic strings as hypothetical objects passed through ups and downs of scientific community attention. They were predicted in the 1970s and reborn in the 2000s due to superstring theory implementation to the description of the early universe. The connection with superstring theory led to the new properties and a new name – cosmic superstrings, which remains the subject of theoretical and observational researchers.

This thesis is aimed at clarifying questions of the cosmic (super)string network evolution and subsequent observational predictions. The first chapter reviews the main principles and motivations to study cosmic (super)strings.

The second chapter is dedicated to the description of a semi-analytic velocity-depend one scale (VOS) model in order to study the evolution of topological defect networks. This chapter provides a thorough comparison between numerical simulations and the semi-analytic approaches. We investigate a broad range of regimes, conditions and possible errors to test the VOS model validity.

In the third chapter we develop our approach to go beyond the standard cosmic strings and include possible extensions to the strings dynamics. The developed general treatment applies to wiggly strings and strings with chiral currents. The latter is generalized for an arbitrary Lagrangian, and the exact solution in Mikowski space for such strings is found. Also we generalize the study of the scaling regimes for string networks evolution in the framework of the VOS model.

The properties of cosmic superstrings are discussed in the forth chapter. We consider dynamics and conditions for the production of string junctions both for superstrings in an expanding universe and for superstrings with currents. In addition, we suggest an approach for integrating junction dynamics in the VOS model. For this case we revisit the averaged treatment of strings collisions and averaged correlation function evolution. This leads to the formulation of the generalized VOS model with dynamical junctions.

The possible observational outcomes of this work are studied in chapter five. We review the main observational opportunities to confirm/restrict the existence of cosmic (super)strings. In particular, we revisit the cosmic microwave background anisotropies produced by cosmic strings. We generalize this study for the case of strings with non-trivial internal structure, i.e. with presence of currents. Specifically we study anisotropies caused by wiggly and superconducting (chiral) cosmic strings. In addition, we explore the stochastic gravitational background from cosmic strings and highlight the main adjustments that should be implemented in order to extend the study for non-conventional cosmic strings.

The outlook and conclusions end the thesis in chapter six.

Publications: Paper 1Paper 2

Advisors: Carlos J. A. P. Martins (IA-U.Porto), Anastasios Avgoustidis(University of Nottingham)

Fellowship: PD/00040/2012 – SFRH/BD/52699/2014 – FCT

University: Porto

Finish date: 2018/12/18