Narrow-line Seyfert 1 galaxies are identified based on their optical spectra; the full-width at half maximum (FWHM) of the broad component of Hβ < 2000 km/s. Also the flux ratio [O III]/Hβ < 3 is required to ensure that we can see the central engine. These sources also often show strong Fe II multiples. The narrow FWHM(Hβ) implies that the black hole masses in narrow-line Seyfert 1s are low, usually less than one hundred million solar masses, as confirmed by reverberation mapping and other proxies for the black hole mass. However, these sources are just as luminous as their older siblings, broad-line Seyfert 1 galaxies, and thus they commonly show very high Eddington ratios.
Their black hole masses, and predominantly disk-like host galaxies suggest that they are unevolved active galactic nuclei, experiencing one of their first activity cycles. Several lines of evidence support this hypothesis, for example, the their large-scale environments: narrow-line Seyfert 1 galaxies reside in regions that in the framework of cosmic downsizing develop later than dense parts, such as superclusters, where most blazars reside.
The discovery of relativistic jets in narrow-line Seyfert 1s broke the conventional jet paradigm in which only the most massive BHs, residing in ellipticals, are able to launch jets. This makes narrow-line Seyfert 1s exceedingly interesting to study as they are the lowest-mass supermassive black holes that are capable of hosting jets, and can thus offer us some clues to the formation of jets in active galactic nuclei. Moreover, a sizable fraction of the active galactic nuclei in the early Universe discovered by the JWST could be classified as narrow-line Seyfert 1 galaxies. Understanding the local population of these sources also opens new avenues in the high-redshift studies.
The unification model states that orientation is the main cause of observational differences in active galactic nuclei. For example, blazars appear as radio galaxies when seen edge-on. Most narrow-line Seyfert 1s are non-jetted, and it is likely that their edge-on counterparts can be found among the Seyfert 2 population, and these sources can be identified by means of spectropolarimetry. On the other hand, the parent population of beamed jetted narrow-line Seyfert 1 sources is elusive. For every beamed source there should be 2Γ2 unbeamed counterparts, where Γ is the Lorentz factor of the jet. For narrow-line Seyfert 1s Γ is ~10, thus the parent population of the known gamma-ray emitting narrow-line Seyfert 1 galaxies should be 8000-10000.
The parent population consists of misaligned Type 1 active galactic nuclei as well as Type 2 sources, with the latter expected to form a majority of the population. The evident Type 1 parent population for the beamed jetted narrow-line Seyfert 1s is the unbeamed jetted narrow-line Seyfert 1 population, i.e. jetted sources with steep radio spectra. However, we are currently aware of less than a hundred of these sources, so it is unlikely that they could make up the whole Type 1 parent population. However, the scarcity of these sources might be partly due to the difficulty of determining the origin of radio emission and identifying jets in narrow-line Seyfert 1 galaixes when high-resolution radio observations are not available.
Identifying the Type 2 counterparts of jetted NLS1s is more challenging since the broad optical emission lines are not detectable. One hypothesis relates jetted NLS1s to peaked soruces that tend to be seen from the side and thus are mostly Type 2 active galactic nuclei. Peaked sources are kinematically young radio sources hosting pc- to kpc-scale jets that are still confined within the host galaxy and often intensely interacting with it. Whereas peaked sources are mostly hosted by ellipticals, subsets of disk-hosted peaked sources have been found. These sources could form some of the Type 2 parent population of jetted narrow-line Seyfert 1 galaxies. We are currently exploring this hypothesis with new observations.
To perform robust population-wise studies, it is imperative that our samples consist of the sources we intend to study - narrow-line Seyfert 1 galaxies in this case. Optical spectra, used to classify NLS1s, are often modelled using automated algorithms and data from large surveys that have poor signal-to-noise ratio. Over the past 20 years there have been several studies reporting identifying large samples of narrow-line Seyfert 1 sources, but upon closer inspection these samples suffer from serious contamination by other types of active galactic nuclei, leading to unreliable, or even wrong, results. We performed a detailed study of one such sample, and found that out of ~11100 sources only ~4000 could be reliably classified as narrow-line Seyfert 1 galaxies. Investigating the genuine narrow-line Seyfert 1 population thus requires very high signal-to-noise ratio data and careful treatment of the spectra. We are currently in the process of obtaining high-quality optical spectra for large samples of candidate narrow-line Seyfert 1 galaxies to compile a pure sample of statistically significant size.
NLS1s represent a special and important stage in the evolution of AGN. They are "young'" sources, with low black hole masses and high Eddington ratios, and much alike to recently discovered high-redshift active galactic nuclei. This similarity offers us an opportunity to study this early evolutionary stage using larger, more easily accessible samples in the local Universe. This knowledge can be applied at higher redshift, helping us understand the active galactic nuclei evolution and growth at the cosmic dawn - including the launching of relativistic jets - and quantify the role of feedback from rapid accretors. However, our knowledge of narrow-line Seyfert 1s is not comprehensive enough to use them for this purpose; we still lack the understanding of some of their basic properties, such as the nature of their radio emission and the jets' contribution to it.
About 5% of narrow-line Seyfert 1 galaxies host relativistic jets. These jets have been found to be kinematically young, as expected. In contrast, most narrow-line Seyfert 1s (85%) have never been detected in radio, or are weak radio sources (10%). According to our recent study of a carefully selected sample of narrow-line Seyfert 1s, the detection fraction might be even lower (7%). Narrow-line Seyfert 1 galaxies exhibit diverse radio morphologies, as confirmed by our Karl G. Jansky Very Large Array survey, and the origins of their radio emission is a complicated issue. A detailed investigation of a sample of 44 narrow-line Seyfert 1 galaxies showed that these sources range from jet-dominated to totally host-dominated. Narrow-line Seyfert 1s are amazingly diverse, and no assumptions should be made based on the mere classification. Instead, both individual and statistical approaches are needed to truly uncover the nature of this unique class.
Our understanding of narrow-line Seyfert 1 sources in radio was further complicated when in the Metsähovi Radio Observatory (MRO) active galactic nuclei monitoring programme we discovered a small sample of radio-weak narrow-line Seyfert 1 galaxies flaring at Jy-levels at 37 GHz. Such rapid, high-amplitude variability strongly implies that these sources host relativistic jets. However, despite our relentless multiwavelength - from radio to X-rays - attempts to detect the jets, we have not been able to see them. In fact, the latest results indicate that the properties of their radio flares might not be explainable by standard jet models, and we might be witnessing a totally new kind of variability in AGN. Our investigation of these puzzling sources continues.
The intra- and interclass evolutionary paths of narrow-line Seyfert 1 galaxies are largely unknown. Our preliminary results employing a somewhat clean sample of ~4000 narrow-line Seyfert 1s indicate that the line profile of the broad lines gradually changes from turbulence-dominated Lorentzian to Keplerian motion-dominated Gaussian as the black hole mass increases, but these results need to be confirmed with more detailed spectral modelling. Furthermore, there is some evidence that their X-ray properties and Eddington ratios are part of the evolutionary sequence, but quantitative proof is still missing. Also the duty cycle, and the frequency and role of the relativistic jets in it, are unknown. The duty cycle is also closely related to the interclass evolution since as the black hole grows, narrow-line Seyfert 1s will eventually turn into other kind of active galactic nuclei. How long this takes and what are the observational consequences, especially when it comes to jetted narrow-line Seyfert 1 galaxies, is unclear, though it has been suggested that jetted narrow-line Seyfert 1s might turn into flat-spectrum radio quasars and radio galaxies. The paucity of relic radio emission and/or evidence for several activity cycles in narrow-line Seyfert 1s might indicate that jets are a feature of the most evolved sources. However, narrow-line Seyfert 1s are the lowest-mass supermassive black holes that host relativistic jets, and thus crucial for establishing what are the necessary circumstances and physical properties that enable an AGN to launch relativistic jets.