Are Narrow Line Seyfert 1s a special class of Active Galactic Nuclei?

TL;DR

Narrow Line Seyfert 1s are not a distinct class of AGN but share properties with all Type 1 AGN, and their perceived uniqueness is likely due to arbitrary classification and observational biases.

Contribution

The paper challenges the notion of NLSy1s as a separate class, showing their properties overlap continuously with BLSy1s and are influenced by selection biases.

Findings

No unique physical properties distinguish NLSy1s from BLSy1s.

The FWHM(Hbeta) boundary at 2000 km/s is arbitrary and not physically meaningful.

Selection biases favor low-mass, high Eddington ratio sources in NLSy1 samples.

Abstract

No. Due to their apparently extreme optical to X-ray properties, Narrow Line Seyfert 1s (NLSy1s) have been considered a special class of active galactic nuclei (AGN). Here, we summarize observational results from different groups to conclude that none of the characteristics that are typically used to define the NLSy1s as a distinct group - from the, nowadays called, Broad Line Seyfert 1s (BLSy1s) - is unique, nor ubiquitous of these particular sources, but shared by the whole Type 1 AGN. Historically, the NLSy1s have been distinguished from the BLSy1s by the narrow width of the broad Hbeta emission line. The upper limit on the full width at half maximum of this line is 2000 km/s for NLSy1s, while in BLSy1s it can be of several thousands of km/s. However, this border has been arbitrarily set and does not correspond to the change of any physical property. All observed parameters in Type 1 AGN cover continues ranges of values, which does not allow to infer the existence of two different kind of populations with FWHM(Hbeta,broad) = 2000 km/s as dividing point. We argue that the usage of this velocity limit to define samples of NLSy1s galaxies - as it is usually done in comparative studies -, together with the well known observational biases, naturally favors the selection of sources with low black hole masses and high Eddington ratios that are hosted by blue spiral galaxies. Therefore selection biases might be responsible for the reported differences between NLSy1 and BLSy1 sources.