Dwarf galaxies with AGN and their environments in observations and simulations

This thesis is a study of dwarf galaxies with active galactic nucleis (AGN) characteristics, their environments, and identification of them in both observations and simulations. More specifically, it attempts to answer the questions of what environmental conditions are favourable for AGN activity, if environmental has any influence at all, and to what degree current AGN identification tools are suitable for dwarf galaxies. Using the observational catalogue NASASloan Atlas and the Baldwin-Philips-Terlevich (BPT) and WHAN diagrams as diagnostics, no connection between AGN activity and environment is found based on 62 258 dwarf galaxies, although a weak connection cannot be refuted in a redshift-limited sample of BPT galaxies, while the IllustrisTNG simulation shows an increase in AGN occupation fraction of its 6 771 dwarf galaxies if they have recent mergers. Additionally, dense environments are found to be detrimental for AGN activity, but this finding may be due to numerical reasons. Machinelearning does not rank environmental features highly for identifying AGN, but predicted AGN galaxies reside closer to a massive galaxy and denser neighbourhoods. Preliminary results indicate that the best model relies internal features. Other studies find multi-wavelength data provide the best venue to obtain a complete set of AGN in dwarf galaxies, and simulations are now utilising higher resolution and improved black hole (BH) modelling, enabling accurate evolutionary paths of dwarf galaxies. The seemingly contradictory results between different approaches can in part be explained selection bias (e.g BPT favours unobscured AGN), numerical effects (e.g overmassive BH seeding), and statistical framework used toquantify differences. Future work involves constructing a more complete and accurate sample of dwarf AGN, achieved through using multi-wavelength data, higher sensitivity observations like integrated field unit spectroscopy, and simulations with improved dwarf galaxy and BHmodelling, tying together the many strings by a fine tuned machine learning approach

The recurrence time of Dansgaard-Oeschger events and limits on the possible periodic component

By comparing the high-resolution isotopic records from the GRIP and NGRIP icecores, we approximately separate the climate signal from local noise to obtain an objective criterion for defining Dansgaard-Oeschger events. Our analysis identifies several additional short lasting events, increasing the total number of DO events to 27 in the period 12-90 kyr BP. The quasi-regular occurrence of the DO events could indicate a stochastic or coherent resonance mechanism governing their origin. From the distribution of waiting times we obtain a statistical upper bound on the strength of a possible periodic forcing. This finding indicates that the climate shifts are purely noise driven with no underlying periodicity.Comment: 9 figure

On the theory of coupled modes in optical cavity-waveguide structures

Light propagation in systems of optical cavities coupled to waveguides can be conveniently described by a general rate equation model known as (temporal) coupled mode theory (CMT). We present an alternative derivation of the CMT for optical cavity-waveguide structures, which explicitly relies on the treatment of the cavity modes as quasinormal modes with properties that are distinctly different from those of the modes in the waveguides. The two families of modes are coupled via the field equivalence principle to provide a physically appealing yet surprisingly accurate description of light propagation in the coupled systems. Practical application of the theory is illustrated using example calculations in one and two dimensions.Comment: 14 pages, 9 figure