In this thesis, we study slowly rotating relativistic stars by modeling them as perfect fluids. The Hartle formalism is used to perturb a spherically symmetric static solution into slow rotation and the corresponding field equations to second order are presented. Five of the seven equations form a subsystem that decouples and can thus be solved independently. It is solely this system we focus on in this work. Two new variables are introduced to rewrite the field equations such that they become algebraic in one of the variables, the other one is used as a generating function and can be chosen to generate desired perfect fluid configurations. To adapt for numerical integration, the field equations are further reformulated as a coupled system of ordinary differential equations, which is solved numerically using a fourth order Runge-Kutta method. Two choices of the generating function are examined, namely a simple polynomial case as well as the Kuch2 III solution, and the related field equations are solved to find the perturbative functions. By studying physical quantities such as the pressure, energy density and speed of sound to zeroth order, we show that both of the studied configurations correspond to physically realistic solutions within a range of parameter values. Additionally, the shape of the zero pressure surface to second order is analyzed and found to be oblate for both cases, which coincides with the expected shape of a rotating fluid. We also investigate whether the Petrov type D condition is fulfilled, and the results indicate that both solutions fail to satisfy the condition and are thus not of Petrov type D. Moreover, it is shown that both the polynomial case and the perturbed Kuch2 III solution can be matched to a suitable asymptotically flat vacuum exterior, using the Darmois-Israel procedure. It should be noted that even though we are solely examining the slowly rotating limit, our results are in fact valid even up to fairly high rotational velocities. This indicates that the perturbative generating method of this thesis may be used to produce models describing physically reasonable configurations, e.g. rotating neutron stars.

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This thesis introduces briefly physical processes operating in the ISM around massive starsand focuses on the impact strong stellar feedback has in creating large-scale structures in a galaxy.Stellar feedback is ubiquitous in star forming galaxies and its effect on the ISM depends strongly on the energy output from the most massive stars (M$\geq$7 \Mo ) and the properties of the surrounding gas. Starburst galaxies are among the most active in producing %galaxies are among the most active galaxies and produce a large amount of massive star clusters %or even super star clusters (SSC; M$_{cl} \geq 10^5$ \Mo ), with stellar populations up to thousands of massive stars. %The most massive stars (M$\geq$30 \Mo ) are lives short, but they In the first 4 Myr of the star clusters evolution, radiative feedback of the most massive stars (M$\geq$30 \Mo ) are at work. Large amount of ionizing photons are released to the ambient medium while radiative pressure compress the surrounding gas. At the same time their stellar winds inject continuously mechanical energy and momentum in their surrounding. This mechanical feedback is then at later ages, until $\sim$ 40 Myrs, maintained by supernova explosions from the less massive stars. Strong stellar feedback tends to develop large-scale structures such as bubbles, loops, filaments and outflows. These are transient structures and can be seen as imprints of how the released energy is clearing or has cleared paths in the ISM. Strong stellar feedback can have devastating consequences in dwarf galaxies due to their shallow gravitational potential. It can accelerate outflows with velocities larger than their escape velocities. In this way, dwarf galaxies can lose a large fraction of their gas mass, which will be crucial in their subsequent evolution.On the other hand, galactic winds might be responsible to create holes in the ISM, allowing the easily-absorbed ionizing photons (Lyman continuum photons, LyC) to escape the galaxy. Studies on a few LyC leaking galaxies have shown that this mechanisms might have preference from a density-bound scenario, which takes place in galaxies with a highly ionized halo. In my paper I used deep MUSE observations to analyse the impact strong stellar feedback has in the starburst and Lyman continuum emitting galaxy: Haro 11. The paper presents three emission line diagnostics aiming to analyse the condition of the warm ionized gas in this galaxy, which are the \Ha\ emission, the level of ionization in gas and the presence of fast shocks. These diagnostics are presented in maps of 50 \kms\ bins in a velocity range from -400 to 350 \kms . Haro 11 shows a violent ISM whose warm ionized gas is almost completely shaped by effect of stellar feedback from the most massive star forming regions in the centre. Arcs, shells, outflows paths and galactic scale ionizing cones are imprinted in ISM of Haro 11. Our analysis suggests the presence of a kpc-scale superbubble which might have created galactic holes in the ISM. Beside of that, Haro 11 shows a highly ionized halo. Both mechanisms appear to facilitate the escape of LyC in this galaxy. %Finally, we observe emission at velocities up to thousand \kms\ which could hints to gas escaping the galaxy. Finally the paper presents estimates of the gas mass fraction that could escape the gravitational potential of the galaxy.

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In this thesis, we have constructed new analytical solutions for initial data of the Einstein equations. Such solutions are valuable for gaining a better understanding of problems involving strong gravitational and/or electromagnetic interactions in general relativity. In this process, we have examined an inhomogeneous cosmological model consisting a lattice of regularly arranged, charged black holes with initial data corresponding to the maximum expansion of a cosmological solution. We have also refined the method in such a way that the values of the mass and charge of the sources can be prescribed beforehand subject to certain constraints dictated by the field equations. Then we studied a two dimensional 'equatorial' cross-section of the initial data space and presented the behaviour of the local curvature for the slices of the Platonic bodies 8, 16, 24, 120 and compared black hole lattices with a Friedmann universe of a unit radius. We see that the black hole lattice is not close to this, or any other, round sphere as far as its local curvature is concerned. For all Platonic solutions, the black hole regions are located where the curvature assumes its minimum value, and it strikes the eye that they are well isolated from each other and do not distort each other noticeably. On the other hand, we confirm that black holes themselves are remarkably round in the strong curvature regions. The recent surge of interest in these solutions was driven by the cosmological averaging problem. Is it likely to affect our understanding of the dark side of the universe in a significant way? We feel that the right way to go may well be to find other interesting toy models, where agreement can be reached quickly.

Gamma-ray bursts (GRBs) are the brightest events in the Universe, for a short time outshining the rest of the Universe combined, as they explode with isotropic equivalent luminosities up to $10^{54}$ erg s$^{-1}$. These events are believed to be connected to supernovae and to binary compact object mergers, such as binary neutron stars or neutron star -- black hole systems. The origin of the so-called prompt emission in GRBs remains an unsolved problem, although some progress is being made. Spectral analysis of prompt emission has traditionally been performed with the Band function, an empirical model with no physical interpretation, and it is just recently that physical models have started to be fitted to data. This thesis presents spectral analysis of GRB data from the Fermi Gamma-ray Space Telescope using a physical model for subphotospheric dissipation. The model is developed using a numerical code and implemented as a table model in {\scriptsize XSPEC}. Paper \rom{1} presents the model and provides a proof-of-concept of fitting GRB data with such a model. Specifically, two GRBs are fitted and compared with the corresponding Band function fits. In paper \rom{2}, a sample of 37 bursts are fitted with an extended version of the model and improved analysis tools. Overall, about a third of the fitted spectra can be described by the model. From these fits it is concluded that the scenario of subphotospheric dissipation can describe all spectral shapes present in the sample. The key characteristic of the spectra that are not fitted by the model is that they are very luminous. Within the context of the model, this suggests that the assumption of internal shocks as a dissipation mechanism cannot explain the full population of GRBs. Alternatively, additional emission components may required. The thesis concludes that subphotospheric dissipation is viable as a possible origin of GRB prompt emission. Furthermore, it shows the importance of using physically motivated models when analysing GRBs. / <p>QC 20170221</p>

The first stars, known as Population III (Pop III) stars, formed when the Universe was about 100 Myr to 200 Myr old. In contrast to the present-day Universe, star formation took place in a basically metal-free environment. Despite several decades of research, Pop III stars have not yet been directly observed.  This work investigates the prospects of the ArmazoNes high Dispersion Echelle Spectrograph (ANDES), used with the Extremely Large Telescope (ELT) to observe the supernova ejecta of Pop III stars. If those supernova ejecta enrich a pristine gas cloud, the imprints in the absorption spectrum from a background source may be observable.  First, the prospects of the unique identification of Pop III stars in contrast to non-Pop III stars is studied by using theoretical yields and abundance ratios of supernova simulations. The elements C, O and Si, used in a previous proposal for ANDES, can help to differentiate pair-instability supernovae of Pop III stars from any population and mixture of core-collapse supernovae. The inclusion of N, Al, Mg or Fe supports the discrimination of core-collapse supernovae from different populations.  Second, with a new code to simulate synthetic spectra with resolution and noise relevant for ANDES, the potential to identify a Pop III star is studied. Besides the suggested absorption lines of C, O and Si in the previous ANDES science document, the strong lines of Mg and Fe are potential indicators.

The origin of supermassive black holes (SMBHs,∼ 10^9M⊙) in the first billion years of cosmic history remains a mystery. Direct Collapse Black Holes (DCBHs,∼ 10^4M⊙) are considered one of the most promising hypotheses for the progenitor of SMBHs, as they are believed to provide a favorable starting point as massive seeds for SMBH formation. Based on the models of Pacucci et al. (2016), the DCBHs at birth are expected to exhibit an unusual steep (red) spectral energy distribution (SED) in the 1-5 micrometer range. In this study,we investigate degeneracies between DCBHs and dusty galaxies using synthetic data from the Pacucci model. Furthermore, we employ James Webb Space Telescope (JWST) photometric data obtainedfrom the Cosmic Evolution Early Release Science (CEERS) survey,which has been processed and reduced by Gabriel Brammer using the Grizli pipeline. By analyzing this JWST photometric data, we aim to identify potential DCBH candidates for further spectroscopic observations. We find that the dusty galaxies over a wide range ofredshift could mimic the SED shape of DCBHs, and that obscured active galactic nuclei (AGN) could potentially act as interlopers as well. The similarities in certain observational characteristics between DCBHs and dusty galaxies/AGN can obscure their distinct identities. In a localized region of the Extended Groth Strip field, we identify 12, 17, and 77 objects in three catalogues with increasing aperture sizesthat show relatively good fits with the DCBH models. However, upon further examination of their images, we do not find any candidatesthat are particularly compelling or convincing.