2021ApJS..253...28R

The study of narrow-line Seyfert 1 galaxies (NLS1s) is now mostly limited to low redshift (z < 0.8) because their definition requires the presence of the Hβ emission line, which is redshifted out of the spectral coverage of major ground-based spectroscopic surveys at z > 0.8. We studied the correlation between the properties of Hβ and Mg II lines of a large sample of SDSS DR14 quasars to find high-z NLS1 candidates. Based on the strong correlation of FWHM(Mg II) = (0.880 ± 0.005) x FWHM(Hβ) + (0.438 ± 0.018), we present a sample of high-z NLS1 candidates having FWHM of Mg II < 2000 km s^–1. The high-z sample contains 2684 NLS1s with redshift z = 0.8-2.5 with a median logarithmic bolometric luminosity of 46.16 ± 0.42 erg s^–1, logarithmic black hole mass of 8.01 ± 0.35 M_☉, and logarithmic Eddington ratio of 0.02 ± 0.27. The fraction of radio-detected high-z NLS1s is similar to that of the low-z NLS1s and SDSS DR14 quasars at a similar redshift range, and their radio luminosity is found to be strongly correlated with their black hole mass.