High-redshift Narrow-line Seyfert 1 Galaxies: A Candidate Sample

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 H$\beta$ 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 H$\beta$ and Mg II lines of a large sample of SDSS DR14 quasars to find high-$z$ NLS1 candidates. Based on the strong correlation of $\mathrm{FWHM(MgII)=(0.880\pm 0.005) \times FWHM(H\beta)+ (0.438\pm0.018)}$, 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 $46.16\pm0.42$ erg s$^{-1}$, logarithmic black hole mass of $8.01\pm0.35 M_{\odot}$, and logarithmic Eddington ratio of $0.02\pm0.27$. 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.