Discovery of four Narrow-line Type-1 Quasars at z = 2.54 -- 6.06 with Subaru 'Onohi'ula PFS-SSP

We report the discovery of four narrow-line type-1 quasars (NLQ1s) from the Subaru 'Onohi’ula Prime Focus Spectrograph Subaru Strategic Program (PFS-SSP). To extend a comprehensive understanding of this population to the distant universe, we searched for broad-line AGNs (quasars) whose rest-UV spectral shapes resemble those of local NLS1 galaxies, utilizing the Ly$\alpha$ line profile. We identified four NLQ1s, including one candidate, at $z=2.54$ – $6.06$. The obtained spectra show a prominent Ly$\alpha$ emission line with a very low continuum level. The measured full width at half maximum (FWHM) of Ly$\alpha$ ranges from $1410\mathrm{k}\mathrm{m}{\mathrm{s}}^{-1}$ to $1510\mathrm{k}\mathrm{m}{\mathrm{s}}^{-1}$ with a mean of $\sim 1430\mathrm{k}\mathrm{m}{\mathrm{s}}^{-1}$. Although we used only line width information for the NLQ1 selection, the resulting equivalent widths (EW) of Ly$\alpha$ are higher (the mean $\sim 190$,\AA \ in the rest frame) than the typical values of quasars. Two quasars have exceptionally large Ly$\alpha$ luminosity, accounting for $\sim 3$ – $4$% of the bolometric luminosity. We measured black hole masses and Eddington ratios for three objects whose C,$\text{textsc}iv$ line is available. The resulting black hole masses are $\sim {10}^{6.9}$ – ${10}^{7.7}{M}_{\odot }$ with high Eddington ratios ($>0.1$), consistent with those of NLS1 galaxies reported in the low-$z$ universe. N,$\text{textsc}v$ and C,$\text{textsc}iv$ broad absorption lines are detected in one object. This quasar can be classified as a mini-BAL quasar, which further supports the idea that this NLQ1 is indeed a luminous version of an NLS1 galaxy.