Do Anomalous Narrow Line Quasars Cast Doubt on Virial Mass Estimation?

Abstract

Anomalous Narrow-Line Quasars (ANLs) are a population of quasars with narrow H\beta, and sometimes [O III] broader than ~1000 km/s, in total comprising \sim 10-30% (most likely ~25%) of Type I quasars at 0.2 < z < 0.8. We find that virial masses using the H\beta and Mg II lines systematically differ for ANLs by an average of as much as 0.5 dex. Because the broad H\beta component width increases in ANLs but Mg II does not, we might suspect H\beta-based virial masses for ANLs are wrong but Mg II masses are correct. If this is due to an outflow reaching the lower-ionization potential H\beta line, C IV masses will be similarly flawed. However, we cannot be certain of this explanation without followup work, and may be unable to identify which quasars are ANLs at z > 0.8. Therefore, it is essential that ANLs be well-understood and well-modeled in order to allow the use of virial mass estimators on large optical spectroscopic catalogs, particularly at z < 0.4 and z > 2.0 where only one broad line is available for use in mass estimation.