Using six years of spectroscopic monitoring of the luminous quasar HE 0435-4312 (z = 1.2231) with the Southern African Large Telescope, in combination with photometric data (CATALINA, OGLE, SALTICAM, and BMT), we determined a rest-frame time delay of days between the Mg ii broad-line emission and the ionizing continuum using seven different time-delay inference methods. Time-delay artifact peaks and aliases were mitigated using the bootstrap method and prior weighting probability function, as well as by analyzing unevenly sampled mock light curves. The Mg ii emission is considerably variable with a fractional variability of ∼5.4%, which is comparable to the continuum variability (∼4.8%). Because of its high luminosity (L ₃₀₀₀ = 10^46.4 erg s⁻¹), the source is beneficial for a further reduction of the scatter along the Mg ii-based radius–luminosity relation and its extended versions, especially when the highly accreting subsample that has an rms scatter of ∼0.2 dex is considered. This opens up the possibility of using the high-accretor Mg ii-based radius–luminosity relation for constraining cosmological parameters. With the current sample of 27 reverberation-mapped sources, the best-fit cosmological parameters (Ω_m, Ω_Λ) = (0.19; 0.62) are consistent with the standard cosmological model within the 1σ confidence level.

We report 6 yr monitoring of distant bright quasar CTS C30.10 (z = 0.90052) with the Southern African Large Telescope. We measured the rest-frame time lag of days between the continuum variations and the response of the Mg ii emission line, using six different methods. This time delay, combined with other available measurements of Mg ii line delay, mostly for lower-redshift sources, shows that the Mg ii line reverberation implies a radius–luminosity relation very similar to the one based on a more frequently studied Hβ line.

We present the monitoring of the active galactic nuclei continuum and Mg ii broad-line emission for the quasar HE 0413-4031 (z = 1.38) based on the six-year monitoring by the South African Large Telescope (SALT). We manage to estimate a time-delay of days in the rest frame of the source using seven different methods: interpolated cross-correlation function, discrete correlation function (DCF), z-transformed DCF, JAVELIN, two estimators of data regularity (Von Neumann, Bartels), and χ ² method. This time-delay is below the value expected from the standard radius–luminosity relation. However, based on the monochromatic luminosity of the source and the spectral energy distribution modeling, we interpret this departure as the shortening of the time-delay due to the higher accretion rate of the source, with the inferred Eddington ratio of ∼0.4. The Mg ii line luminosity of HE 0413-4031 responds to the continuum variability as , which is consistent with the light-travel distance of the location of Mg ii emission at R _out ∼ 10¹⁸ cm. Using the data of 10 other quasars, we confirm the radius–luminosity relation for the broad Mg ii line, which was previously determined for the broad Hβ line for lower-redshift sources. In addition, we detect a general departure of higher-accreting quasars from this relation in analogy to the Hβ sample. After the accretion-rate correction of the light-travel distance, the Mg ii–based radius–luminosity relation has a small scatter of only 0.10 dex.

The dynamics of the broad line region (BLR) in active galaxies is an open question; direct observational constraints suggest a predominantly Keplerian motion, with possible traces of inflow or outflow. In this paper we study in detail the physically motivated BLR model of Czerny & Hryniewicz based on the radiation pressure acting on dust at the surface layers of the accretion disk (AD). We consider here a nonhydrodynamical approach to the dynamics of the dusty cloud under the influence of radiation coming from the entire AD. We use here a realistic description of the dust opacity, and we introduce two simple geometrical models of the local shielding of the dusty cloud. We show that the radiation pressure acting on dusty clouds is strong enough to lead to dynamical outflow from the AD surface, so the BLR has a dynamical character of a (mostly failed) outflow. The dynamics strongly depends on the Eddington ratio of the source. Large Eddington ratio sources show a complex velocity field and large vertical velocities with respect to the AD surface, while for lower Eddington ratio sources vertical velocities are small and most of the emission originates close to the AD surface. Cloud dynamics thus determines the 3D geometry of the BLR.

As Setti and Woltjer noted back in 1973, one can use quasars to construct the Hubble diagram; however, the actual application of the idea was not that straightforward. It took years to implement the proposition successfully. Most ways to employ quasars for cosmology now require an advanced understanding of their structure, step by step. We briefly review this progress, with unavoidable personal biases, and concentrate on bright unobscured sources. We will mention the problem of the gas flow character close to the innermost stable circular orbit near the black hole, as discussed five decades ago. This problem later led to the development of the slim disk scenario and is recently revived in the context of Magnetically Arrested Disks (MAD) and Standard and Normal Evolution (SANE) models. We also discuss the hot or warm corona issue, which is still under debate and complicates the analysis of X-ray reflection. We present the scenario of the formation of the low ionization part of the Broad Line Region as a failed wind powered by radiation pressure acting on dust (Failed Radiatively Driven Dusty Outflow – FRADO). Next, we examine the cosmological constraints currently achievable with quasars, primarily concentrating on light echo methods (continuum time delays and spectral-line time delays to the continuum) that are (or should be) incorporating the progress mentioned above. Finally, we briefly discuss prospects in this lively subject area.

Context.Broad emission lines are the most characteristic features in the spectra of galaxies with an active galactic nucleus (AGN). They mostly show either single-peaked or double-peaked profiles and originate from a complex dynamics of the likely discrete clouds moving in a spatially extended region known as the broad line region (BLR).Aims.In this paper, we present a large grid of results, which is used to test the model based on calculations of the spectral line generic profiles.Methods.We followed a non-hydrodynamical single-cloud approach to BLR dynamics based on a radiatively dust-driven model. We previously showed in detail that the 2.5D version of the model could provide us with the 3D geometry of the BLR.Results.We show that the shape of profiles not only depends on the accretion rate of the source, the black hole mass, and the viewing angle, but it is most significantly affected by the adopted dust-to-gas mass ratio regulating the strength of the radiation pressure. We also show that the model can aptly explain the low ionized broad emission lines of the mean spectrum of quasars, such as MgII and Hβ.Conclusions.The radiatively dust-driving mechanism can appropriately account for the low-ionized part of BLR of AGNs.