Browsing by Author "Lähteenmäki, Anne, Prof., Aalto University, Metsähovi Radio Observatory, Department of Electronics and Nanoengineering, Finland"
Now showing 1 - 1 of 1
- Results Per Page
- Sort Options
- A multifrequency view on the characteristics and evolution of narrow-line Seyfert 1 galaxies
School of Electrical Engineering | Doctoral dissertation (article-based)(2024) Varglund, IreneNarrow-line Seyfert 1 (NLS1) galaxies are a peculiar type of active galactic nuclei (AGN). These sources are identified by their narrow emission lines and are believed to be in an early evolutionary stage, perhaps in their first activity cycle. Originally these sources were thought to have no significant radio emission, but this hypothesis has been proven wrong by the presence of powerful, relativistic jets. The discovery of jets in these sources contradicts both the traditional classification system and the jet paradigm. In this thesis, large samples of NLS1 galaxies have been examined at both radio and optical frequencies by using archival data and data obtained through recent observing time proposals. Due to the difficulty in accurately classifying these sources, the traditional classification system of these sources states that they are incapable of significant radio emission, as well as of hosting powerful relativistic jets, most large samples of NLS1 galaxies are contaminated with other AGN, such as broad-line Seyfert 1 galaxies. Due to this, a study in this thesis focused on obtaining the cleanest large sample of NLS1 galaxies currently available by studying the optical spectrum of 11 001 sources, resulting in roughly 4000 sources deemed as most likely genuine NLS1 galaxies. The host galaxy morphologies of both jetted and non-jetted NLS1 galaxies of both northern and southern NLS1 galaxies were investigated in two separate studies. The aim was to try and decipher whether or not there are any clear differences between these two types of AGN. The results indicate that the predominant host of NLS1 galaxies is disk-like with the jetted and non-jetted sources sharing similar host galaxies. Furthermore, based on these results, major mergers do not seem to correlate with jettedness. In radio, an extensive analysis of the cleanest NLS1 galaxy sample was performed at three different frequencies: 144 MHz, 1.4 GHz, and 3 GHz. Nearly half of the sources were detected in at least one of these frequencies, with the majority of the detections at 144 MHz. Many of these sources present clear AGN activity, with over half of the detections at 3 GHz having a radio luminosity higher than what is typically found in star formation processes. Several compact steep-spectrum sources were also identified. The variability seen in some NLS1 galaxies is unique, with very large flux density changes occurring on shorter-than-expected timescales. Various explanations for the variability have been discussed and deemed as impossible, improbable, and possible. The extraordinary behavior seen in these sources can provide clues on the evolution of them and other AGN.