Modeling of the Quasar Main Sequence in the Optical Plane

Abstract

The concept of the quasar main sequence is very attractive since it stresses correlations between various parameters and implies the underlying simplicity. In the optical plane defined by the width of the Hβ line and the ratio of the equivalent width of the Fe II to Hβ observed objects form a characteristic pattern. In this paper, we use a physically motivated model to explain the distribution of quasars in the optical plane. Continuum is modelled as an accretion disk with a hard X-ray power law uniquely tight to the disk at the basis of observational scaling, and the Broad Line Region distance is determined also from observational scaling. We perform the computations of the FeII and Hβ line production with the code CLOUDY. We have only six free parameters for an individual source: maximum temperature of the accretion disk, Eddington ratio, cloud density, cloud column density, microturbulence, and iron abundance, and only the last four remain as global parameters in our modelling of the whole sequence. Our theoretically computed points cover well the optical plane part populated with the observed quasars, particularly if we allow for super-Solar abundance of heavy elements. Explanation of the exceptionally strong Fe II emitter requires a stronger contribution from the dark sides of the clouds. Analyzing the way how our model covers the optical plane we conclude that there is no single simple driver behind the sequence, as neither the Eddington ratio nor broad band spectrum shape plays the dominant role. Also, the role of the viewing angle in providing the dispersion of the quasar main sequence is apparently not as strong as expected.

Main trends of the quasar main sequence - effect of viewing angle

“…3. The inclusion of the turbulent velocity inside the cloud (microturbulence) recovers the trends that were obtained in Panda et al (2018): the maximum FeII emission and the maximum R FeII values correspond to the case with modest values of microturbulence (10-20 km s −1 ). 4.…”

“…We perform the theoretical modelling of the quasar properties assuming, as in Panda et al (2017Panda et al ( , 2018, that the central black hole is surrounded by an accretion disk which provides the optical/UV continuum, and a hot corona, which is the source of the X-ray radiation. This continuum illuminates the BLR clouds located at a distance given by the BLR size.…”

“…The use of angle-dependent form factor (see Eq. 2 and 5) is a crucial extension of the quasar MS modelling done by Panda et al (2017Panda et al ( , 2018Panda et al ( , 2019a. Viewing angle directly affects the line width, and the object luminosity, and indirectly the estimated distance to the BLR and the location of an object on the quasar MS diagram.…”

“…In our previous works (Panda et al, 2017(Panda et al, , 2018(Panda et al, , 2019a we were successful in modelling almost the entire MS diagram constructed for over 20,000 SDSS quasars. In the past studies, we had incorporated only two of the aforementioned physical parameters of the super massive black hole i.e., black hole mass and accretion rate.…”

Extreme Quasars as Distance Indicators in Cosmology

“…The SED parameters entering in the relation for the virial luminosity areν i , κ i , and more indirectly, n H U . The ionizing photon flux n H U shows remarkably small changes along the MS (i.e., even without restricting the attention to xA sources), as shown by the various estimates reported in Table 4 of Negrete et al (2012): it cannot change by a large factor, otherwise the emission line spectrum would change dramatically (Panda et al, 2018(Panda et al, , 2019. The other SED-related parameters appear as κ i /ν i , and are qualitatively affected in the same way by a change of the ionizing continuum.…”