Browsing by discipline "Astronomi"

In 2012, a sudden and significant increase in the radiocarbon 14C abundance of tree rings at around AD 775 was found by Miyake et al (2012). Since then, various explanations for the cause of the event have been offered. These include a supernova explosion, a short gamma-ray burst, a comet collision with the Sun causing an energetic burst, a comet disintegration in the Earth's atmosphere and an especially energetic solar flare. Even though there have been a lot of studies considering the event, the definite cause is still unclear. Most of the knowledge regarding the event comes from 14C measurements made from trees that grew in various locations. These include Japan, Germany, USA and Russia. To further increase the understanding of the event and its cause, we have measured the event from a subfossil Lapland tree. This measurement is unique, since the other measurements come from locations significantly further from the magnetic pole. Thus, they might be affected by different atmospheric and geomagnetic dynamics. To date, there has not been a considerable effort in trying to quantify possible differences in the various measurements. Only the maximum increase from the background value has been considered. However, it might not be the most robust indicator of the event intensity, since it is susceptible to statistical fluctuations. For this reason, we have adopted a peak fitting method to better quantify the various properties of different measurements. Special interest was put into calculating the area under the curve of the fit to get a more robust indicator of the event intensity. Here we report that the measurement from a Lapland tree shows a significantly stronger 14C signal than what has been found earlier. Furthermore, our peak analysis demonstrates that there is a clear dependency between the latitude, where the trees have grown, and the intensity of the 14C signal, indicating that higher latitude trees have stronger signals. The connection is even more evident when, instead of the latitude, the distance from the North magnetic pole is used. It is known that the production of 14C by charged particles is significantly higher near the polar regions due to geomagnetic effects. Hence, a solar proton event is consistent with the observed latitude effects, whereas a gamma-ray burst or an atmospheric comet disintegration is not. Therefore, a solar origin is strongly implicated. These findings have a societal significance, since a solar storm poses a considerable threat to various infrastructures. We advice that the AD 775 event should be used as a new worst-case scenario when evaluating different risk mitigation strategies.

To understand the formation of disk galaxies it is also important to understand different feedback mechanisms that affect the formation process. Without a feedback process to delay star formation the disk galaxies should not have ongoing star formation in the present day Universe. However, this is not the case since star formation is still taking place. For example, in the Milky Way the star formation rate is still ~1 solar mass per year. Moreover, during the formation process most of the gas inside galaxies is not bound into stars. Instead when disk galaxies form inside a dark matter halo there is much more baryonic matter initially available in gaseous form than in stars. This contradicts the basic CDM model, according to which most of the gas should cool down and form stars in the absence of feedback. The goal of this thesis is to first introduce the theory behind disk galaxy formation and the feedback mechanisms affecting the galaxy formation process with the main focus being on the supernova feedback. After introducing the theory the aim is to compare how supernova feedback affects the formation of a massive Milky Way-like galaxy and a less massive dwarf galaxy using a simulation code developed by Efstathiou (2000). For both galaxies four cases are simulated. Two of them represent a basic galaxy formation model presented in this thesis. One observes a situation in which the galaxy would have a very high star formation efficiency and the second concentrates on a slightly refined model including some parameters, which are ignored in the basic model. The work conducted in this thesis proves that supernova feedback may work throughout the galaxy's lifetime and causes a significant portion of the gas to escape the galaxy. This also shows that supernova driven feedback might be a reason why disk galaxies in the present day Universe still have ongoing star formation. Also the analytic model is surprisingly realistic and produces results which not only explain why there still is star formation in the present day disk galaxies, but also why the stellar mass in disk galaxies is lower than what is predicted by the basic CDM model. In dwarf galaxies with circular speed 70 km/s the ejected gas mass may be up to 60% of the total initial gas mass and in a high star formation case the ejected gas mass may be equal to the final stellar mass. Dwarf galaxies are also more sensitive to changes in the initial parameters compared to massive galaxies. In more massive galaxies with circular speed 280 km/s the ejected gas mass is smaller, but still may be 20% of the total gas mass. Another result was that massive galaxies are not very sensitive to changes in the initial conditions and the effects of supernova feedback. Finally, in the massive galaxies gas may join a galactic fountain, which was not observed in the dwarf galaxies, in which the gas was lost.