Early phase of the active galactic nuclei lifetime: narrow-line Seyfert 1 galaxies

Among several types of active galactic nuclei (AGN), narrow-line Seyfert 1 (NLS1) galaxies represent a challenging subclass whose physical properties are still not completely understood. In the optical regime, their spectra are characterised by the narrowness of the permitted lines which has been hypothesised to be related to the presence of a low-/intermediate- mass supermassive black hole (SMBH). Together with a high Eddington ratio, these properties may suggest that NLS1s are fast-growing AGN at an early stage of evolution. According to this evolutionary scenario, NLS1s are considered the low-z analogue of the first AGN formed in the early Universe. Some authors, instead, explained their spectral features in a geometrical context, claiming the inclination effect as responsible for the width of the emission lines. If this were the case, the SMBH mass would be underestimated. Disentangling between these two scenarios is possible through morphological studies of the NLS1 host galaxies. Moreover, NLS1s have demonstrated that the presence of relativistic jets in an AGN is not strictly related to its radio-loudness, the SMBH mass, or the host galaxy type. A remarkable example of this statement is the radio-quiet NLS1, J0354-1340. From Jansky Very Large Array observations, I found this source hosting the largest radio jets found to date in an NLS1, with a viewing angle indicating an intrinsically low SMBH mass. By means of the photometric decomposition of near-infrared images, I proved that this NLS1 is hosted by a disc-like galaxy. My results confirm that powerful jets can also be launched and sustained by less massive SMBHs in spiral galaxies. In this scenario, I also determined the host galaxy morphology of 10 low-luminosity compact sources (LLCs), radio AGN at the first stage of evolution, aiming to confirm them as a candidate parent population of NLS1s. Since their small SMBH masses and low-power jets, it was suggested that LLCs and jetted NLS1s may be the same class of objects seen under different orientations. From Panoramic Survey Telescope $\&$ Rapid Response System multi-band data, I found six out of ten late-type galaxies, probably with pseudo-bulges, along with three point-like sources and one object of uncertain classification. These results support the trend observed in NLS1s, indicating late-type host galaxies for LLCs and supporting the validity of the parent population scenario. Last but not least, I also analysed optical and X-ray spectra of a peculiar source, 2MASX J0413-0050, which was classified as NLS1s in 2004 within the Six-degree Field Galaxy Survey. This source has changed its spectral type (from NLS1, going through a hybrid phase to a Seyfert 1.8) and showed huge variation in the X-ray flux several times in the last year 20 years, bringing to a changing-look (CL) classification. The analysis of several spectra of J0413-0050, taken in different epochs, showed de-coupled variability in the continuum and line emission, probably induced by changes in the accretion rate of the SMBH or maybe due to a disc-wind shielding the contribution from the broad-line region . Also, from optical images, its host galaxy resembles a late-type one. NLS1s should be quite inclined to experience CL phase during their life-cycle, being unevolved AGN accreting at high rates and showing fast variability in the the soft X-ray. The principal aim of this entire work was to validate the evolutionary scenario as an explanation of the nature of NLS1s, explaining their observational properties as the consequence of the high accretion rate around small mass SMBHs in late-type galaxies fed by secular processes rather than mergers.