Browsing by master's degree program "Alkeishiukkasfysiikan ja astrofysikaalisten tieteiden maisteriohjelma (Particle Physics and Astrophysical Sciences)"
Now showing items 21-29 of 29
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(2022)Strongly coupled matter called quark–gluon plasma (QGP) is formed in heavy-ion collisions at RHIC [1, 2] and the LHC [3, 4]. The expansion of this matter, caused by pressure gradients, is known to be hydrodynamic expansion. The computations show that the expanding QGP has a small shear viscosity to entropy density ratio (η/s), close to the known lower bound 1/4π [5]. In such a medium one expects that jets passing through the medium would create Mach cones. Many experimental trials have been done but no proper evidence of the Mach cone has been found [6, 7]. Mach cones were thought to cause double-bumps in azimuthal correlations. However these were later shown to be the third flow harmonic. In this thesis a new method is proposed for finding the Mach cone with so called event-engineering. The higher order flow harmonics and their linear response are known to be sensitive to the medium properties [8]. Hence a Mach cone produced by high momentum jet would change the system properties and, thus, the observable yields. Different flow observables are then studied by selecting high energy jet events with different momentum ranges. Considered observables for different momenta are then compared to the all events. Found differences in the flow harmonics and their correlations for different jet momenta are reported showing evidence of Mach cone formation in the heavy-ion collisions. The observations for different jet momenta are then quantified with χ 2 -test to see the sensitivities of different observables to momentum selections.
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(2022)I use hydrodynamic cosmological N-body simulations to study the effect that a secondary period of inflation, driven by a spectator field, would have on the Local Group substructures. Simulations of the Local Group have been widely adopted for studying the nonlinear structure formation on small scales. This is essentially because detailed observations of faint dwarf galaxies are mostly limited to within the Local Group and its immediate surroundings. In particular, the ∼ 100 dwarf galaxies, discovered out to a radius of 3 Mpc from the Sun, constitute a sample that has the potential to discriminate between different cosmological models on small scales, when compared to simulations. The two-period inflaton-curvaton inflation model is one such example, since it gives rise to a small-scale cut-off in the ΛCDM primordial power spectrum, compared to the power spectrum of the ΛCDM model with single field power-law inflation. I investigate the substructures that form in a simulated analogue of the Local Group, with initial conditions that incorporate such a modified power spectrum. The most striking deviation, from the standard power-law inflation, is the reduction of the total number of subhalos, with v_max > 10 km/s, by a factor of ∼ 10 for isolated subhalos and by a factor of ∼ 6 for satellites. However, the reduction is mostly in the number of non-star-forming subhalos, and the studied model thus remains a viable candidate, taking into account the uncertainty in the Local Group total mass estimate. The formation of the first galaxies is also delayed, and the central densities of galaxies with v_max < 50 km/s are lowered: their circular velocities at 1 kpc from the centre are decreased and the radii of maximum circular velocity are increased. As for the stellar mass-metallicity and the stellar mass-halo mass relations, or the selection effects from tidal disruption, I find no significant differences between the models.
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(2022)Small-scale flux ropes (SFRs) are structures with helical magnetic field and they are frequently observed in the solar wind. In addition to the solar wind, SFRs can also be found within larger structures, like interplanetary coronal mass ejections (ICMEs) and their sheath regions that form between the shock and the leading edge of the ICME if the ICME propagates fast enough. ICME-driven sheaths are composed of shocked and compressed solar wind plasma. SFRs can be swept from the upstream solar wind into ICME sheaths when the upstream wind is deflected and compressed into ICME sheaths. Alternatively, SFRs can be formed within ICME sheaths through number of processes. This thesis includes the first comprehensive study of the occurrence of SFRs specifically in ICME-driven sheath regions. SFRs are identified from spacecraft data in both ICME sheaths and the upstream solar wind using the wavelet analysis method. This method calculates normalized reduced magnetic helicity, normalized cross-helicity, and normalized residual energy and uses them to identify SFRs and Alfvén waves. The method is applied to 55 ICME-driven sheath regions observed by Wind spacecraft. The occurrence of SFRs is studied in three different frequency ranges between 10−2 − 10−4 Hz. SFRs are found to be common structures in ICME-driven sheaths and they are more common in ICME sheaths than in the upstream solar wind. This suggest that SFRs are at least to some extent generated within ICME sheaths. The occurrence of SFRs behaves differently in different frequency ranges. The occurrence of SFRs is relatively constant at high frequencies (smallest scale) while the occurrence of low-frequency (largest scale) SFRs increases towards the leading edge of the ICME. This suggests that high- and low-frequency SFRs are generated by different processes. The occurrence of Alfvén waves was found to be somewhat similar in the upstream solar wind and ICME sheaths. However, there was an increase in the occurrence of Alfvén waves near the shock. This indicates that SFRs and Alfvén waves are generated by different processes and shock related processes might be important in the generation of Alfvén waves while the processes near the leading edge of the ICME are important in the generation of larger scale SFRs.
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(2022)Over the last thirty years more than five thousand exoplanets have been discovered around a wide variety of stellar objects. Most exoplanets have been discovered using the transit method, which relies on observing the periodic brightness changes of stars as their planet transits in front of them. The discovery efficiency of these planets has been strongly enhanced with the advent of space telescopes dedicated to the discovery of planets using the transit method. Planetary signals in the photometric data of active stars can be challenging to find, as the surface features of the stars combined with their rotation might produce signals which are orders of magnitude stronger than those caused by the planetary transit. The question of what statistical methods should be applied to account for the innate variability of stars in order to identify the transits of exoplanets in the lightcurves of active stars is being investigated in this thesis. I test a number of statistical methods in order to combat stellar activity and to identify planetary transit signals. The rotation period of the star is investigated using the Lomb-Scargle and likelihood ratio periodograms. Starspot induced variability is approximated with a number of sinusoids, with periods based on the star's rotation period. Additional stellar activity is filtered out using autoregressive and moving average models. Model fittings are performed with least squares fitting, and using samples generated by the Adaptive Metropolis algorithm. After the lightcurve has been detrended for stellar activity, the likelihoods of planetary transit signals are assessed with a box-fitting algorithm. Models are compared with the Bayesian and Akaike information criteria. Planetary characteristics are then estimated by modeling the shape of the transit lightcurve. These methods are tested and performed on the lightcurve of HD~110082, a highly active young star with one confirmed planetary companion, based on the observations of the TESS space telescope. I find that stellar activity is sufficiently filtered out with a model containing four sinusoid signals. The signal corresponding to the planet is confirmed by the box fitting algorithm, agreeing with results available in scientific literature.
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(2022)Results from cosmological zoom-in simulations focusing on the formation and evolution of massive early-type galaxies starting from redshift z=50 until the present time are presented in this thesis. In addition, the dynamics of supermassive black hole (SMBH) binaries found in a subset of the galaxies as a result of galaxy mergers are studied. The first 12 zoom-in simulations were run with GADGET-3, an N-body hydrodynamical tree code on the Puhti supercomputer at the Finnish IT Centre for Science (CSC). The zoom-in regions were chosen from a low resolution large-volume simulation, run with the University of Helsinki computing cluster Kale. The SMBH binary dynamics were simulated with the regularized integrator code KETJU. In total three simulations using KETJU were run on the CSC supercomputer Mahti. The GADGET-3 simulations included both dark matter only runs and runs containing also baryons. The simulations were run at two different resolutions. No significant differences between the two resolutions were found for the dark matter only runs, whereas the runs including a baryonic component showed large differences. The medium resolution runs had too low particle numbers and too large particle masses to correctly resolve star formation and feedback, leading to rotation curves that missed their central peaks. The results from the high resolution simulations agreed with earlier published results. The rotation curves peak in the central regions of the galaxies with the curves becoming almost constant at large radii. The star formation rates peak in the redshift range z~2-3 and at smaller redshifts star formation can momentarily increase due to the occuring of galaxy mergers. At the present day, all of the studied galaxies include mainly old stellar populations, resulting in red colours for all the galaxies. Still, the galaxy formation efficiency parameter of each galaxy is somewhat higher than what is seen in the observations. Finally, all three SMBH binaries for which the dynamics were studied using KETJU led to a coalescence of the two black holes. The orbital decay of SMBH binaries occur in three phases: dynamical friction, three-body interactions and gravitational wave emission. The merger times strongly depend on the eccentricity of the binaries with the semimajor axis of the binary with the highest eccentricity decreasing the fastest. This is expected from theory, with the evolution from subparsec scales to coalescence agreeing very well with the theoretical evolution taking into account the post-Newtonian correction term 2.5PN, which is the lowest order post-Newtonian term responsible for gravitational wave emission.
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Synthetic Observations of Simulated Clusters : Bridging the Gap between Observations and Simulations (2023)We investigated the connection between the 3D physical properties of stellar clusters and their measured counterparts from their 2D observed images; primarily focusing on the relationship be- tween the 3D half-mass radius (Rh3D) and the effective radius (Rheff) (also known as the 2D half-light radius) of stellar clusters. We generated an ensemble of 3D models of stellar clusters using the McLuster code. This ensemble is made up of subgroups consisting of different stellar counts, half-mass radius, concentration, maximum mass of the initial mass function, and degree of mass segregation. Each subgroup covered a broad range of their respective property in order to provide a comprehensive overview of the Rh3D to Rheff relationship as a function of these variables. Then, utilizing myosotis, we created synthetic observations of these models and investigated how the Rh3D of the cluster could be inferred from the measured Rheff of the synthetic photometric map. Our analysis reveals that for systems where all stars are of equal mass, independent of their size, the half-mass radius is equal to Rh3D ≈ 1/α Rheff where α ∼ 0.76. We show that the value of α can be inferred by a geometric relationship. We also find that this relationship holds for systems with varying values of concentration. For unsegregated systems of unequal stellar masses, we observe that the value of α oscillates around 0.76, with the amplitude of the oscillations increasing as the maximum mass of the system increases. As Rh3D by construction does not change, the only parameter to cause this variation in α is the Rheff . When we looked at mass segregated systems, we found that the value of Rheff (and similarly α) decreases generally monotonically as a function of the degree of the segregation. The presence of stars of unequal mass is the dominant factor that determines the measurements of Rheff , beyond the geometric effects of projection. The prevalence of this factor is attributed to the non-linear relationship between mass and luminosity that results in a few tens of massive stars greatly influencing the overall luminosity of the cluster, and therefore, its effective radius.
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(2024)This thesis studies gas-rich galaxy mergers at redshifts of z ∼ 1-2 using numerical simulations, with a particular focus on the effect of feedback from active galactic nuclei (AGNs). In total, 16 galaxy mergers at redshifts z = 1 and z = 2 were modeled using the simulation codes KETJU and GADGET-3. The simulations were performed on the supercomputer Mahti located at the Finnish IT Centre for Science (CSC). AGN feedback can be described as the radiative and mechanical energy released through accretion, which act to heat and disperse the remaining gaseous material surrounding the central supermassive black hole (SMBH). The feedback mechanisms include, for example, photoionization heating due to high-energy photons and winds and jets driven by the AGN. Numerically, AGN feedback was implemented using two models in this thesis: thermal and kinetic AGN feedback, in which the gas particles are either heated or ‘kicked’, respectively. In addition to AGN feedback, the simulations included metal-dependent gas cooling, stochastic star formation, and stellar feedback. The simulated progenitor galaxies were gas-rich spirals consistent with observed galaxies at redshifts z = 1 and z = 2. The virial masses of the progenitors were set to correspond to typical massive galaxies at their redshifts using the Press-Schechter mass function, while the initial masses for the central SMBHs were set using observed MBH-M⋆ and MBH-σ⋆ relations. The gas fractions and metal abundances of the progenitors were calibrated using observational data at their respective redshifts. The KETJU and GADGET-3 simulations produced very similar results for the overall evolution of a given merger configuration. Consistent with earlier studies, the kinetic feedback was observed to be significantly more effective at removing gas from the galaxies than the thermal feedback. The combined effect of AGN and stellar feedback was observed to strongly suppress star formation, with the star formation of one merger being almost completely shut down. The thermal and kinetic feedback models caused noticeable differences in the orbital evolution of the SMBH binaries. Merger timescales were significantly longer for the SMBHs in the KETJU simulations with kinetic feedback. In general, the merger timescales increased with decreasing initial eccentricity for the SMBH binary. The merger remnants were compared to observed MBH-σ⋆, R1/2-M⋆, fgas-M⋆, and mass-metallicity relations. Overall, the remnants were reasonably consistent with the observed relations. Hence, we can conclude that AGN feedback plays a crucial role in galaxy evolution and that both the thermal and kinetic feedback models are able to produce realistic high-redshift galaxies.
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(2023)The aim of this study is to inspect fluctuations in the solar wind magnetic field for four different types of solar wind time series. The events considered are fast and slow solar wind, along with magnetic clouds and sheath regions, which are found in coronal mass ejections (CMEs). Time series measurements of these processes are analysed using methods from Information Theory and Complex Network Analysis. The techniques that are used here are the Fisher-Shannon information plane, the Jensen-Shannon complexity-entropy plane, and Horizontal Visibility Graph Analysis. Statistical and information theory measures as well as network analysis have recently been applied to studying time series in an attempt to determine their internal structure. There is promising research into these methods quantifying data as either chaotic, stochastic, or periodic. Knowing whether a process has e.g. a deterministic origin could shed light on the creation of said process. Applying these methods to solar wind, more information could be found about its formation at the Sun. In general, the solar wind data analysed in this thesis was found to be stochastic, which agrees with previous studies. In addition, when analysing magnetic field magnitude B, magnetic clouds appear to have more internal structure in the time series signal than the other types of solar wind data tested. The results obtained here are promising in terms of finding differences in structure within solar wind, and could be investigated further with the use of more solar wind data.
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(2023)The high luminosity upgrade of the Large Hadron Collider (LHC) will result in higher collision rates and current equipment is not up to par with this future era of operations. Identification and reconstruction of hard interactions may be hampered by the spatial overlapping of particle tracks and energy deposits from additional collisions and this often leads to false triggers. In addition, current particle detectors suffer from radiation damage that severely affects the accuracy of our results. The new minimum ionizing particle (MIP) timing detector will be equipped with low gain avalanche detectors which have a comparably small timing resolution that helps with the track reconstruction and their thin design limits the radiation damage over time. In this thesis, we build an experimental set-up in order to study the timing resolution of these detectors closely. In order to find the timing resolution, we take the time difference between the signals from two detectors and put it in a histogram to which we apply a Gaussian fit. The standard deviation of this Gaussian is called the time spread from which we can estimate the timing resolution. We first build, characterize and improve our experimental set-up using reference samples with known timing resolution until our set-up is able to reproduce the reference value. Then we repeat the measurements with irradiated samples in order to study how radiation damage impacts timing. We were able to adjust our setup with reference samples until we measured a timing resolution of 33$\pm$2~ps. We use this result to calculate the timing resolution of an irradiated sample ($8.0 \times 10^{14}$ proton fluence) and we found a timing resolution of 62$\pm$2~ps. This thesis also discusses the analysis of the data and how the data can be re-analyzed to try to improve the final result.
Now showing items 21-29 of 29