Statistical analysis of Al III and C III] emission lines as virial black hole mass estimators in quasars

Abstract

Aims. We test the usefulness of the intermediate ionisation lines Aliiiλ1860 and Ciii]λ1909 as reliable virial mass estimators for quasars. Methods. We identified a sample of 309 quasars from the Sloan Digital Sky Survey Data Release 16 (SDSS DR16) in the redshift range 1.2≤z≤1.4 that have [Oii]λ3728 recorded on the same spectrum as Aliiiλ1860, Siiii]λ1892, and Ciii]λ1909. We set the systemic quasar redshift using careful measurements of [Oii]λ3728. We then classified the sources as Population A (Pop. A), extreme Population A (Pop. xA), and Population B (Pop. B), and analysed the 1900 Å blend using multi-component models to look for systematic line shifts of Aliiiλ1860 and Ciii]λ1909 along the quasar main sequence. Results. We do not find significant shifts of the Aliiiλ1860 line peak in Pop. B or in the vast majority of Pop. A. For Pop. xA, a small median blueshift of -250 km s −1 was observed, motivating a decomposition of the Aliii line profile into a virialised component centred at rest frame and a blueshifted component for an outflow emission. For Pop. B objects, we proved the empirical necessity to fit a redshifted very broad component (VBC), clearly seen in Ciii], and analysed the physical implications on a Pop. B composite spectrum using CLOUDY simulations. We find consistent black hole mass estimations using Aliii and Ciii] as virial estimators for the bulk of Pop. A. Conclusions. Aliii (and even Ciii]) is a reliable virial black hole mass estimator for Pop. A and Pop. B objects. The Pop. xA sources deserve special attention due to the significant blueshifted excess observed in the line profile of Aliii, although it is not as large as the excess observed in Civλ1549.

Accretion/Ejection Phenomena and Emission-Line Profile (A)symmetries in Type-1 Active Galactic Nuclei

“…A second assumption is the relatively constant physical conditions within the BLR. This is likely the case if the restriction is made to objects belonging to Population B (i.e., showing a prominent redward asymmetry in the Balmer lines) in the innermost part of their emitting regions, the so-called very broad line region (e.g., [103,104]): for the VBLR, the ionization parameter has been estimated as high as log U ∼ −0.5-0, with a moderate density logn H ∼ 10 (cm −3 ) [5,105], with solar or somewhat subsolar metallicity [105]. These physical conditions have been derived since the late 1970s [106]-before the realization that a large fraction of type-1 AGN emit substantial FeII-and are appropriate for sources that are weak FeII emitters.…”

“…The emitting region radius was found to be a factor of 10 smaller than previously thought [54,55], and its inner radius might be just of the order of a few hundred gravitational radii. The smaller distance of the emitting gas from the central continuum source has important implications for the physical conditions of the gas; the conventional view of the broad-line region (BLR) as a system of emitting clouds characterized by high ionization, typical densities n H ∼ 10 9 cm −3 , and column density N c = 10 23 (cm −2 ) are unable to account for the strong FeII emission in quasars [56,57] observed in Population A (if FeII emission is strong (R FeII 0.5), a high density is needed to maintain the ionization parameter, within reasonable limits; it is now accepted that the bulk of the low-ionization lines requires a high column density, high particle density, low-ionization degree, and high metallicity [58,59]), but it is expected to remain valid for Population B [5].…”

“…• Low-ionization lines (LILs, ionization potential χ ion 20 eV) that include the lines of the Balmer and Paschen series of Hydrogen, MgIIλ2800, the CaII IR Triplet, and FeII features. (UV intermediate-ionization lines (IILs) such as AlIIIλ1860, SiIII]λ1892, and CIII]λ1909 are due to parent species with 15 χ ion 30 eV that show a behavior similar to the LILs and are frequently associated with them (e.g., [5]). ) The most widely studied line is the HI Balmer line Hβ.…”

High-redshift quasars along the main sequence

“…The VBC FWHM is expected to be ≥ 4000 km s −1 , and the NC FWHM ∼ 1000 km s −1 (there is only one source that shows a significant NC in C iii]). Differently from C iii]λ1909, lines such as the Al iiiλ1860 doublet, Si iii]λ1892, and Si ii λ1816 are assumed not to present a very broad component (Buendia-Rios et al 2022). They are fit by only one BC.…”

High Eddington quasars as discovery tools: current state and challenges

“…Low-ionization lines retain fairly symmetric profiles that indicate virial motions and therefore that their width is suitable for virial broadening estimation without the need of introducing large corrections (Marziani et al, 2013;2019;2022). M BH estimates remain reliable even if the effect of partially resolved outflows (in radial velocity) has to be taken into account (Negrete et al, 2018;Marziani et al, 2022;Buendia-Rios et al, 2023).…”