Galaxy dynamics using simulation models has been an important research field during the last 50 years. In the beginning of the 20th century there were already extensive catalogs of galaxies which showed the large diversity of the galaxy shapes, although there was no theory to explain those. Nowadays, galaxy dynamics can be extensively studied by simulating the dark matter, star, and gas cloud orbits numerically, but before the era of modern computers constructing even a simple model was a considerable challenge. In the beginning of the 1940’s Erik Holmberg (University of Lund) introduced his new integration procedure for galaxy simulations (Holmberg, 1941) which can be considered as a pioneering study in galaxy dynamics. In his work, Holmberg replaced gravitation with light intensity, based on the fact that they both obey the same 1/r² attenuation with distance. He modeled the interaction of two galaxies with light bulbs and came out with the conclusion that some features of the galaxies, for example tails and intergalactic bridges, can be explained by gravitational tidal forces. Holmberg’s work was significant for being the first simulation of galaxy dynamics, which could be upgraded only 20 years later, when the first simulations with electronic computers were started. Despite the significance of the Holmberg’s simulations and the large number of citations they have received, there is no marks that the simulations would have been repeated. During the summer of 2013 I reconstructed the Holmberg’s experimental setting in the University of Oulu. The experiment was made two times: once using exactly the same parameters that Holmberg used, and another time with small changes in the initial parameters. In this thesis I introduce the Holmberg’s experiments and the theory behind that. In chapter 2, I give a brief overview of disk galaxies and methods used in galaxy simulations. In chapter 3, I present the theoretical basis of Holmberg’s analog simulation. Chapter 4 covers the hardware and the practical realization of the experiment. In chapter 4, I represent the results and compare them with the Holmberg’s results.

In this thesis we are interested in the evoluNon of disc galaxies. One of the most common ways to study it is by a deep analysis of the distribuNon of chemical abundances in the stellar populaNons of the disc component. Abundance gradients let us reconstruct the puzzle of the formaNon and evoluNon of this type of galaxies, once we learn about the different elements that are released to the interstellar medium at each stage of the life of stars. In this work we pay special aaenNon to the so-­‐called thick disc component of spiral galaxies, believing that it is a relic of the early galaxy and its understanding opens the door for a complete galaxy formaNon scenario. We analyse thick discs both with observaNonal data and simulaNons because we want to have a wider view of the situaNon and we think that such complementary approaches can help us maximise our knowledge and results. Our simulaNons with an enhanced feedback are the ones that best reproduce the measured data from the Milky Way. The trend for the variaNon of the mean metallicity with galacNc radius at different heights from the plane matches that in the Galaxy. It is negaNve in the mid-­‐plane of galaxies and then becomes posiNve at greatest heights. According to simulaNons, this behaviour is due to a populaNon of relaNvely young and metal-­‐rich stars formed in situ in the outer galaxy, which is missing in the inner thick disc. When looking at the same magnitudes but via observaNons from the CALIFA local universe galaxy sample, we see that the external galaxies exhibit a variety of different behaviours both for the metallicity and age radial gradients with height, in addiNon to the trend found in simulaNons and the Milky Way. We deduce that thick discs probably do not form through a unique mechanism but from a combinaNon of many of them. Finally we want to know the influence of the galacNc mass in the chemical evoluNon of a disc galaxy. By using a fiducial set of simulaNons and comparing the results to observaNonal data we conclude that the smallest systems in our set might have an incorrect feedback efficiency, and suggest that a mass-­‐dependent modulaNon of feedback might improve the result.

High-mass stars play a key role in the energetics and chemical evolution of molecular clouds and galaxies. However, the mechanisms that allow the formation of high-mass stars are far less clear than those of their low-mass counterparts. Most of the research on high-mass star formation has focused on regions currently undergoing star formation. In contrast, objects in the earlier prestellar stage have been more difficult to identify. Recently, it has been suggested that the cold, massive, and dense Infrared Dark Clouds (IRDCs) host the earliest stages of high-mass star formation. The chemistry of IRDCs remains poorly explored. In this dissertation, an observational program to search for chemical variations in IRDC clumps as a function of their age is described. An increase in N2H+ and HCO+ abundances is found from the quiescent, cold phase to the protostellar, warmer phases, reflecting chemical evolution. For HCO+ abundances, the observed trend is consistent with theoretical predictions. However, chemical models fail to explain the observed trend of increasing N2H+ abundances. Pristine high-mass prestellar clumps are ideal for testing and constraining theories of high-mass star formation because their predictions differ the most at the early stages of evolution. From the initial IRDC sample, a high-mass clump that is the best candidate to be in the prestellar phase was selected (IRDC G028.23-00.19 MM1). With a new set of observations, the prestellar nature of the clump is confirmed. High-angular resolution observations of IRDC G028.23-00.19 suggest that in order to form high-mass stars, the detected cores have to accrete a large amount of material, passing through a low- to intermediate-mass phase before having the necessary mass to form a high-mass star. The turbulent core accretion model is inconsistent with this observational result, but on the other hand, the observations support the competitive accretion model. Embedded cores have to grow in mass during the star-formation process itself; the mass is not set at early times as the turbulent core accretion model predicts. The observed gas velocity dispersion in the cores is transonic and mildly supersonic, resulting in low virial parameters (neglecting magnetic fields). The turbulent core accretion model assumes highly supersonic linewidths and virial parameters $sim$1, inconsistent with the observations, unless magnetic fields in the cores have strengths of the order of 1 mG.

<p> We develop, test, and apply a statistical photometric parallax method using main sequence turn off (MSTO) stars in the Sloan Digital Sky Survey (SDSS). We show using simulated data that if our density model is similar to the actual density distribution of our data, we can reliably determine the density model parameters of at least three major substructures in the Milky Way halo, and a smooth background component, using the computational resources available on MilkyWay@home (a twenty parameter fit). As a test for the new model, we fit the stellar density in SDSS stripe 19. After confirming that the model is working as intended on both simulated and observed SDSS stripe 19 data, we moved on to fitting stripes 10 through 23 in the SDSS north Galactic cap. We found an oblate halo with an average flattening of $0.58$. Seven streams were found in these fourteen stripes. The Sgr dwarf leading tidal tail detected in 8 stripes, with properties that are consistent with previous fits to the streams. The trailing tidal tail and the ''bifurcated'' stream were found at the previously identified distances. The Parallel Stream was traced across the sky at a distance of 15 kpc, and roughly tracks an orbit that was fit by previous authors to the Virgo Stellar Stream. A new stream, the Perpendicular Stream, was found at a distance of 15 kpc in the region of Virgo, but roughly perpendicular in orientation on the sky to the Parallel Stream. A stream possibly including the globular cluster NGC 5466 was also found at a distance between 5 and 15 kpc from the Sun.</p><p>

Neutron stars (NS) are the most compact objects that can be directly observed. They can be used to study properties of matter at supranuclear densities. This in turn gives us information to separate between numerous theoretical equations of states of dense matter. Thermonuclear (type-I) X-ray bursts from low mass X-ray binaries can be used to address this issue. Some of these bursts can be so energetic that they cause the whole photosphere of the NS to expand. The cooling of these photospheric radius expansion bursts can be compared to theoretical atmosphere models to obtain the mass and radius measurements of the NS. These measurements can then be used to differentiate between the different equations of state. We present a set of differential equations needed to compute these atmospheric models. We introduce an exact treatment of Compton scattering via the relativistic integral equation and an angle-dependent redistribution function. Using these equations, we can construct a set of atmosphere models in plane-parallel approximation in a local thermodynamical equilibrium for hot NSs. The emergent spectra is then fitted by a diluted blackbody to obtain the dilution factor w and the colour-correction factor f_c. On the other hand, the observed spectra from X-ray bursting neutron stars are close to thermal and can be fitted with a blackbody with two free parameters: the observed blackbody temperature T_bb and the normalization K. By equating the dilution factor w and the normalization K, we obtain a relation between the theoretical atmosphere models and the observations. This connection is the main idea of the so called cooling tail method. We then introduce a small correction to this method and discuss the consequences. A common problem encountered using this method is that different bursts from a given system can yield completely different mass and radius measurements. This fact casts a doubt on the robustness of the entire method. We study the burst emission from 4U 1608--52 at various persistent fluxes. We find a strong dependence of the burst properties on the flux before the burst. Bursts that ignite during the hard state at a low accretion rate show strong evolution of the apparent blackbody radius which is consistent with the model predictions of the neutron star atmosphere models. On the other hand, bursts occurring during the soft state at a higher accretion rate show constant apparent radius, which is inconsistent with the models. We then use the hard state bursts only to constrain the neutron star mass and radius from our set of sources. By taking only the physically relevant results into account, we also get information of the chemical composition of the atmosphere. This then gives us a way to conclude if the atmosphere is hydrogen- or helium-rich. After we know the chemical composition, we constrain the NS radius to be between 12 and 16 km. This implies a stiff equation of state of neutron star matter.

In this thesis, the goal is to disentangle the real morphological evolution of galaxies from the observational effects, caused by redshift. This was done by taking a sample of ~1300 high-redshift (0.1 ≤ z ≤ 1.3) galaxies, observed with HST (Hubble Space telescope), from the GOODS-south field and then dividing them into six redshift bins of ∆z ≈ 0.2 and further into absolute magnitude bins of ∆M = 0.5. The sample was then matched to low-redshift (z ≤ 0.025) galaxies from the SDSS with the same comoving number density per ∆M-bin to create a sample of galaxies of the possible descendants of the high-redshift galaxies. Spectroscopic redshifts of the high-redshift sample were fetched from data provided by the 3DHST project. The low-redshift sample was then spatially scaled, dimmed and k-corrected to simulate redshift effects caused by the accelerating expansion of space. To appear as observed with the HST, the images were convolved with a new point-spread function (PSF), created by deconvolving the HST PSF with the SDSS PSF. This results in a change of the PSF 2D-distribution and a resolution change from 0.396”/pix (SDSS) to 0.03”/pix (HST). To achieve this the FERENGI and the KCORRECT codes were used. All three (low-redshift, high-redshift, redshifted) samples were then classified both visually and with CAS (concentration, asymmetry, smoothness) parameters. The results were compared between these samples. The redshifting process did not produce clumpy structures in the low-redshift galaxies as seen in real high-redshift galaxies. Visual classification becomes increasingly difficult for z ≥ 0.55. The concentration index hardly changes with redshift. The asymmetry of the high-redshift galaxies seem to be genuinely higher than that of the low-redshift galaxies. The smoothness parameter values increase with redshift, implying an increasing clumpiness, but some of the change could be due to the redshift effects. For the high-redshift sample, all the CAS parameters between redshift bins show very little evolution for a given morphological type and could be used to differentiate between them. We also note that we found a significant amount of “clumpy” galaxy types, already in the nearest redshift bin of 0.1 ≤ z ≤ 0.3, as opposed to previous studies.

<p> This thesis investigates connections between low albedo asteroids and carbonaceous chondrite meteorites using spectroscopy. Meteorites and asteroids preserve information about the early solar system including accretion processes and parent body processes active on asteroids at these early times. One process of interest is aqueous alteration. This is the chemical reaction between coaccreted water and silicates producing hydrated minerals. Some carbonaceous chondrites have experienced extensive interactions with water through this process. Since these meteorites and their parent bodies formed close to the beginning of the Solar System, these asteroids and meteorites may provide clues to the distribution, abundance and timing of water in the Solar nebula at these times. Chapter 2 of this thesis investigates the relationships between extensively aqueously altered meteorites and their visible, near and mid-infrared spectral features in a coordinated spectral-mineralogical study. Aqueous alteration is a parent body process where initially accreted anhydrous minerals are converted into hydrated minerals in the presence of coaccreted water. Using samples of meteorites with known bulk properties, it is possible to directly connect changes in mineralogy caused by aqueous alteration with spectral features. Spectral features in the mid-infrared are found to change continuously with increasing amount of hydrated minerals or degree of alteration. Building on this result, the degrees of alteration of asteroids are estimated in a survey of new asteroid data obtained from SOFIA and IRTF as well as archived the Spitzer Space Telescope data. 75 observations of 73 asteroids are analyzed and presented in Chapter 4. Asteroids with hydrated minerals are found throughout the main belt indicating that significant ice must have been present in the disk at the time of carbonaceous asteroid accretion. Finally, some carbonaceous chondrite meteorites preserve amorphous iron-bearing materials that formed through disequilibrium condensation in the disk. These materials are readily destroyed in parent body processes so their presence indicates the meteorite/asteroid has undergone minimal parent body processes since the time of accretion. Presented in Chapter 3 is the spectral signature of meteorites that preserve significant amorphous iron-bearing materials and the identification of an asteroid, (93) Minerva, that also appears to preserve these materials.</p><p>

<p>This thesis explores the ways in which observations of the low surface brightness universe can inform our views of galaxy evolution and, specifically, the extent to which galaxies assemble stellar mass through mergers and accretion events. I first present a study of the variation in the stellar halos of galaxies, using data from the Dragonfly Nearby Galaxies Survey (DNGS). The survey consists of wide field, deep (&mu;<i>g</i> > 31 mag aresec^-2) optical imaging of nearby galaxies using the Dragonfly Telephoto Array. The sample in question includes eight spiral galaxies with stellar masses similar to that of the Milky Way, inclinations of 16 &ndash; 90 degrees and distances between 7&ndash;18 Mpc. I construct stellar mass surface density profiles from the observed <i>g</i>-band surface brightness in combination with the <i>g</i> &ndash; <i>r</i> color as a function of radius, and compute the halo fractions from the excess stellar mass (relative to a disk+bulge fit) beyond 5 half-mass radii. I find a mean halo fraction of 0.009 &plusmn; 0.005 and a large RMS scatter of 1.01[special characters omitted] dex. The peak-to-peak scatter of the halo fraction is a factor of > 100 &ndash; while some galaxies feature strongly structured halos resembling that of M31, three of the eight have halos that are completely undetected in our data. I conclude from this sample that spiral galaxies as a class exhibit a rich variety in stellar halo properties, implying that their assembly histories have been highly non-uniform. I find no convincing evidence for an environmental or stellar mass dependence of the halo fraction in the sample.</p><p> In addition to being the future building blocks of the stellar halos of galaxies, dwarf satellite galaxies are a key probe of dark matter and of galaxy formation on small scales and of the dark matter halo masses of their central galaxies. They have very low surface brightness, however, which makes it difficult to identify and study them outside of the Local Group. I used the Dragonfly Telephoto Array to search for dwarf galaxies in the field of the massive spiral galaxy M101, and identified seven large, low surface brightness objects in this field, with effective radii of 10-30 arcseconds and central surface brightnesses of &mu;<i>g</i>,0 ~ 25.5 &ndash; 27.5 mag aresec^-2. Given their large apparent sizes and low surface brightnesses, these objects would likely be missed by standard galaxy searches in deep fields. Their radial surface brightness profiles are well fit by Sersic profiles with a very low Sersic index (<i>n</i> ~ 0.3 &ndash; 0.7). The properties of the sample are similar to those of well-studied dwarf galaxies in the Local Group, such as Sextans I and Phoenix.</p><p> Finally, follow-up observations of these low surface brightness objects with the Hubble Space Telescope subsequently revealed that three of the seven objects were bonafide satellite galaxies of the M101 group. I show that, unexpectedly, the other four galaxies are ultra-diffuse galaxies in a group environment. The galaxies have effective radii of 10 &ndash; 38 and central surface brightnesses of 25.6 &ndash; 27.7 mag aresec^-2 in the <i>g</i>-band. They remain persistently unresolved even with the spatial resolution of HST/ACS, which implies distances of <i>D</i> > 17.5 Mpc. I show that the galaxies are most likely associated with a background group at ~ 27 Mpc containing the massive ellipticaLs NGC 5485 and NGC 5473. At this distance, the galaxies have sizes of 2.6-4.9 kpc, and are classified as UDGs. They are similar to the populations that have been revealed in clusters such as Coma, Virgo, and Fornax, yet have on average even lower surface brightness. The discovery of four UDGs in a galaxy group demonstrates that the UDG phenomenon is not exclusive to cluster environments. Furthermore, their morphologies seem less regular than those of the cluster populations, which may suggest a different formation mechanism or be indicative of a threshold in surface density below which UDGs are unable to maintain stability.</p><p>

<p> Near-Earth Objects (NEOs) are generally small, dark, and fast-moving. Multiple observations over time are necessary to constrain NEO orbits. Orbits based on observational data are inherently uncertain. Here we describe code written in Python and Fortran used to generate synthetic asteroids and compare calculated orbital fit based on noisy ephemeris using the a distance criteria, D-value. Observational sessions separated by more than one month produce very good orbital fits (low D-values) even at the highest noise level. Daily observational sessions show the highest D-values, as expected, since observed points on the orbital ellipse are not well separated. D-value is closely correlated to differences in the eccentricity and inclination of compared orbits.</p><p>

One of the primary drivers in the development of large format millimeter detector arrays is the study of sub-millimeter galaxies (SMGs) - a population of very luminous high-redshift dust-obscured starbursts that are widely believed to be the dominant contributor to the Far-Infrared Background (FIB). The characterization of such a population requires the ability to map large patches of the (sub-)millimeter sky to high sensitivity within a feasible amount of time. I present this dissertation on the design, integration, and characterization of the 144-pixel AzTEC millimeter-wave camera and its application to the study of the sub-millimeter galaxy population. In particular, I present an unprecedented characterization of the “blank-field” (fields with no known mass bias) SMG number counts by mapping over 0.5 deg 2 to 1.1mm depths of ∼1mJy - a previously unattained depth on these scales. This survey provides the tightest SMG number counts available, particularly for the brightest and rarest SMGs that require large survey areas for a significant number of detections. These counts are compared to the predictions of various models of the evolving mm/sub-mm source population, providing important constraints for the ongoing refinement of semi-analytic and hydrodynamical models of galaxy formation. I also present the results of an AzTEC 0.15 deg 2 survey of the COSMOS field, which uncovers a significant over-density of bright SMGs that are spatially correlated to foreground mass structures, presumably as a result of gravitational lensing. Finally, I compare the results of the available SMG surveys completed to date and explore the effects of cosmic variance on the interpretation of individual surveys.