2025A&A...699A.309Z

Strong broad and narrow emission lines from central broad emission line regions (BLRs) and narrow emission line regions (NLRs) are fundamental spectroscopic characteristics of broad-line active galactic nuclei (BLAGNs). The unique subclass of the true Type 2 AGNs can be identified without central hidden BLRs, which provides clues on the formation and/or the suppression of AGN BLRs. Whether were there BLAGNs that lack the central NLRs is still an open question. In the blue quasar SDSS J1251+0613, blue continuum emission and broad emission lines are both clearly detected in its SDSS spectrum, but there are no apparently detected narrow emission lines in the optical/near-UV bands, which leads to no central normal NLRs in the blue quasar SDSS J1251+0613. In order to explain the lack of NLRs, evolving NLRs are proposed in which the radial outflows carry material from BLRs to NLRs, and the current narrow line emission material lies closer to the outer side of the central BLRs in SDSS J1521+0613. Our results indicate a new unique subclass of BLAGNs, that is, BLAGNs without central normal NLRs. They might provide clues on the physical origin and/or evolution of AGN NLRs.