Probing the population of narrow-line Seyfert 1 galaxies with a southern hemisphere extended sampling

During my Ph.D., I carried out a multi-wavelength study of narrow-line Seyfert 1 galaxies (NLS1s), with the aim of better understanding this peculiar class of AGN. I present a new accurately selected catalog of NLS1s in the southern hemisphere from the Six-degree Field Galaxy Survey (6dFGS) final data release. The classification was based on their optical spectral properties. I further derived for the first time their flux-calibrated spectra that are not provided by the 6dFGS. By analyzing these spectra, I obtained strong luminosity correlations between continuum and optical emission lines. The central black hole masses were estimated with methods based on the reverberation mapping, confirming that NLS1s indeed host a relatively low-mass black hole compared to broad-line Seyfert 1 galaxies (BLS1s). In addition, I present a detailed study of a limited number of NLS1s from the 6dFGS sample that both have optical and X-ray spectroscopic observations. There are five complex NLS1s (C-NLS1s) and six simple NLS1s (S-NLS1s). I propose a possible correlation between [O III] line asymmetry and X-ray complexity. The outflow or wind from the inner accretion disk is commonly present in NLS1s and mostly directed along the system axis. S-NLS1s are sources viewed at a small inclination, where the speed of the wind is high thus the blueshift of the blue wing is large. Such a strong wind could blow away the ionized material, therefore result in a simple X-ray spectrum. At a large inclination, instead, the speed of the wind is low, thus the blueshift is less prominent in C-NLS1s. The presence of ionized material in the weak wind could lead to the X-ray spectral complexity. A fraction of NLS1s are detected at radio frequency. I studied their radio properties at 1.4 GHz and 5 GHz using the NRAO VLA Sky Survey (NVSS) and the Karl G. Jansky Very Large Array (JVLA) respectively. Radio-loud (RL) sources tend to reside in the more distant universe, host a large massive black hole, and have a higher radio and optical luminosity compared to radio-quiet (RQ) sources. To extend the spectral coverage, I generated the test statistic (TS) maps for each radio-emitting source using the Fermi-LAT ten years data set, finding no gamma-ray NLS1 in the sample