Astronomy and Geoscience
Recent Submissions
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Kostić, Petar (b , 2023)[more][less]
Abstract: The models of radio synchrotron emission of supernova remnants (SNRs) imply uniform density ahead of shock wave, so the evolution of luminosity is usu- ally studied in such an environment, most often through the surface-brightness-to- diameter dependence, the Σ–D relation. This field aims to better understand the SNR evolution, the emission models, but also the methods for determining their distance. It is not an easy task because of a very large scatter in the Σ–D Milky Way sample. The dissertation puts a different perspective at the Σ–D relation (usually treated as power-law function), assuming that non-uniform environment around the stars considerably affects its shape and slope, that may vary during the SNR expansion. It makes the ambient density structure an important factor whose impact must be investigated. The numerical code for hydrodynamic (HD) simulations and the emission model were developed. The 3D HD simulations were performed in different non-uniform environments, including low-density bubbles and a variety of clumpy models. Based on the simulation results, a semi-analytical 3D spherically-symmetric model of HD and Σ–D evolution of SNRs in clumpy medium was developed, which is used to generate large Σ–D samples. The results show that after entering the clumpy medium the SNR brightness enhances, but afterward the Σ–D slope steepens, shortening the brightness evolu- tion lifetime. Despite the evident increase in slope in clumpy medium, the Galactic sample average slope flattens at ≈ 13–50 pc. After analyzing the generated SNR samples in clumpy medium it is concluded that the significant flattening and scatter in Galactic sample originates in sporadic emission jumps of individual SNRs in a limited diameter interval. The additional analyses of selection effects are needed to investigate these issues. URI: http://hdl.handle.net/123456789/5606 Files in this item: 1
Kostic_Petar_disertacija.pdf ( 1.947Mb ) -
Milošević, Stanislav (Beograd , 2023)[more][less]
Abstract: In this dissertation, we presented galaxy mergers and the forming of stellar morphological substructures. We assume a large spiral galaxy and dwarf galaxy which is a satellite of the first one. We used N-body simulations to present different scenarios for the merger of two galaxies, and after that, we analyzed the properties of formed structures. The investigation of the formation of these structures and their properties is important for understanding the dynamics and evolution of galaxies. First, we did simulations where we investigated the influence of the properties of dwarf galaxies on the forming structures. We tested: 1. morphology of the dwarf galaxy where we used two models – dwarf with a disk and spheroidal dwarf galaxy; 2. inclination of the orbit in the case of a very radial merger, because in that case, we have the formation of the stellar shells and streams; 3. direction of rotation of the dwarf in the case of a dwarf with a disk. In each case, after the merger, we have stellar shells and streams formed. Morphology, inclination of the orbit, and direction of rotation have their influence on the properties of formed substructures, and on the timescale of disruption of the remnant of the dwarf. In the case of the merger of Andromeda galaxy (M31) and dwarf galaxy, the satellite of M31, we investigate the properties of the Giant Stellar Stream (GSS), as well as the Northeast shell (NE) and West shell (W). The orientation of the GSS, distances, and velocities of the GSS, NE, and W shells from our simulation are in agreement with the observed one. For the first time, we explained the observed metallicity distribution in these substructures. With a linearly decreasing gradient of the initial metallicity in the dwarf galaxy before the merger, using Monte Carlo (MC) simulations, we successfully explained the observed metallicity distribution in these substructures. These results are a contribution to the investigation of metallicity gradients in dwarf galaxies which is important for galaxy evolution in general. URI: http://hdl.handle.net/123456789/5583 Files in this item: 1
SM_Doktorat_18_07_2023.pdf ( 18.08Mb ) -
Racković Babić, Kristina (Beograd , 2022)[more][less]
Abstract: Interplanetary dust grains contain important information about the Solar System. Analyzing these particles is an important aspect of the heliosphere study. Dust impacts have been observed using radio and wave instruments onboard the spacecraft since the 1980s. The interac- tion between the impact-generated plasma cloud and antenna – space- craft system elements generates the characteristic signal waveform. The present work focuses on the detection and interpretation of the dust generated signals from radio instruments onboard various space- craft orbiting at 1 AU. In the first part of the thesis, we aim to develop a model which links the observed electric signals to the dust impact properties. We propose a new model which takes into account the effect of impact - ionization - charge collection and electrostatic-influence. Our model provides an analytical expression for the pulse. It allows us to measure the amount of total ion charge, the fraction of escaping charge, the rise timescale, and the relaxation timescale. The proposed model is simple and conve- nient for large data fitting. To validate the model, we use the Time Do- main Sampler (TDS) subsystem of the STEREO/WAVES instrument, which generates high-cadence time series of voltage pulses for each monopole. Since the beginning of the STEREO mission in 2007, we have collected all the dust events detected by S/WAVES/TDS simul- taneously on all three monopoles at 1 AU. Our study confirms that the rise time vastly exceeds the spacecraft’s short timescale of elec- tron collection by the spacecraft. Aside from electron dynamics, we also obtained interesting results regarding the cloud’s electron tem- perature. The presented model provides an effective tool for analyzing vii dust waveforms, and is applicable for different space missions which investigate the distribution of dust particles, e.g., Solar Orbiter and Parker Solar Probe. In the second part of the thesis, we focus on the interstellar dust (ISD). Interplanetary and interstellar dust are the two main dust pop- ulations at 1 AU. Our objective is to search for interstellar dust by analyzing the data sets collected by STEREO and Wind, starting from the beginning of the missions. Between 2007 and 2012, while being at the solar minimum with a solar dipole pointing southward, all three spacecraft recorded ISD flux at 1 AU. However, before and after that period, the disappearance of the interstellar component was noticeable. The observed change of the impact rate suggests that the flux of inter- stellar dust at 1 AU varies with the solar cycle. Each time the magnetic dipole field changes its polarity during the solar cycle, small interstel- lar grains experience focusing or defocusing. Consequently, the dust grains are systematically deflected either towards, or away from the solar magnetic equator plane by the solar wind magnetic field which thus affects the dust dynamics and the total interstellar dust flux in the inner heliosphere. Our study provides the first quantitative de- scription of the time variation of ISD flux at 1 AU. URI: http://hdl.handle.net/123456789/5547 Files in this item: 1
Teza_KRB.pdf ( 10.95Mb ) -
Pavlović, Marina (Beograd , 2022)[more][less]
Abstract: The study of galaxies through high redshifts are key to understanding the evolution of galaxies through cosmic times. As such objects are very difficult to observe directly, they are mainly examined using empirically derived tools such as the numerous correlations between their different parametric characteristics, one of them being the linear relationship between far-infrared and radio emission in star-forming galaxies, named the Far Infrared-Radio (FIR) Correlation.Although the correlation was considered to be stable in terms of linearity, recent works, which include galaxies at high redshifts (0 < z < 6), showed a large deviation from the correlation in these systems. The goal of this doctoral dissertation is an examination of the physical processes that lead to this kind of behavior. As a possible cause of this evolution, we will assume for the first time, and examine interactions between galaxies (collisions and close approaches). Interactions be- tween galaxies lead to the formation of shock waves on large scales that can lead to changes in the relationship between infrared and radio emissions. Our hypoth- esis was tested in several stadiums and the main results are as follows: 1. We developed models of the evolution of the FIR correlation with redshift as functions of the galaxy interaction rate. We tested the models on a sample of galaxies with an already determined morphology separately for disc galaxies and for galaxies that have recently been or are currently interacting - irregular galaxies. 2. In a small sample of 34 galaxies that we took from paper Miettinen et al. (2017), it was shown that there is an indication that the interaction between galaxies can be responsible for the evolution of the correlation with the redshift. 3. The next analysis was performed on a much larger sample of star-forming galaxies taken from COSMOS field, where we did not find any evolution of corre- lation with the redshift. Also, it was shown that the mean value of the correlation parameter is lower in irregular galaxies than in disk galaxies. Although recent observations indicated an evolution of the FIR correlation with redshift, the results of this research failed to reproduce that evolution and showed that the FIR correlation is stable with redshift. However, it was also shown that due to the interaction of galaxies, the evolution of the FIR correlation is possible if the representation of interacting systems in the sample is higher. URI: http://hdl.handle.net/123456789/5460 Files in this item: 1
Disertacija_Marina_Pavlovic.pdf ( 8.624Mb ) -
Rakić, Nemanja (Beograd , 2022)[more][less]
Abstract: Active Galactic Nuclei (AGN) are galaxy cores in which a supermassive black hole is being surrounded with an accretion disk, which emits powerful continuum emission. This continuum ionizes the surrounding gas which than emits the broad lines, detected in the optical band. The aim of this thesis is to model complex optical spectra of large numbers of type 1 AGNs (AGNs with broad emission lines) and study physical properties of the emitting gas, so-called broad line region. Understanding the physics of the broad line region, as well as the physics of AGN as a whole is important for understanding the galaxy formation and evolution. This thesis investigate a large sample of type 1 AGNs taken from the SDSS spectral database. In total, over 1500 spectra were analyzed, for which a completely new software for modeling of optical AGN spectra (named FANTASY) was developed. Automated simultaneous modeling of all emission components in the optical spectrum enables reliable measurement of spectral parameters, which can have significant application in future investigation of large AGN samples, collected within the upcoming large spectroscopic sky surveys. The physical properties in the broad line region were studied through the analysis of a still unexplained (anti)correlation between the equivalent width of the emission lines and the conti- nuum flux (the so-called Baldwin effect). It has been confirmed that the intrinsic Baldwin effect exists in the Balmer hydrogen lines in type 1 AGNs, and that a potential physical explanati- on for this effect might be presence of an additional non-ionizing optical continuum emission, which originates from the broad line region. Finally, the significant outcome of this thesis is is the new publicly available software package FANTASY, which can be widely used in the future spectral analysis of AGN. URI: http://hdl.handle.net/123456789/5451 Files in this item: 1
Doktorska_disertacija_nrakic.pdf ( 5.806Mb )