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Quasars are the most luminous sources in the Universe. They are currently observed out to redshift z ≈ 7 when the Universe was less than one tenth of its present age. Since their discovery 50 years ago astronomers have dreamed of using them as standard candles. Unfortunately quasars cover a very large range (8 dex) of luminosity making them far from standard. We briefly review several methods that can potentially exploit quasars properties and allow us to obtain useful constraints on principal cosmological parameters. Using our 4D Eigenvector 1 formalism we have found a way to effectively isolate quasars radiating near the Eddington limit. If the Eddington ratio is known, under several assumptions it is possible to derive distance independent luminosities. We discuss the main statistical and systematic errors involved, and whether these “standard Eddington candles” can be actually used to constrain cosmological models. [ABSTRACT FROM AUTHOR]

We review the basic techniques for extracting information about quasar structure and kinematics from the broad emission lines in quasars. We consider which lines can most effectively serve as virial estimators of black hole mass. At low redshift the Balmer lines, particularly broad H β , are the lines of choice. For redshifts greater than 0.7 – 0.8 one can follow H β into the IR windows or find an H β surrogate. We explain why UV C iv λ 1549 is not a safe virial estimator and how Mg ii λ 2800 serves as the best virial surrogate for H β up to the highest redshift quasar known at z ≈ 7 . We show how spectral binning in a parameter space context (4DE1) makes possible a more effective comparison of H β and Mg ii . It also helps to derive more accurate mass estimates from appropriately binned spectra and, finally, to map the dispersion in M BH and Eddington ratio across the quasar population. FWHM MgII is about 20% smaller than FWHM H β in the majority of type 1 AGN requiring correction when comparing M BH estimates from these two lines. The 20% of sources showing narrowest FWHM H β ( < 4000 km s −1 ) and strongest FeII ( R Fe ≳ 1.0 ) emission (we call them bin A3-4 sources) do not show this FWHM difference and a blueshift detected in MgII for these sources suggests that FWHM H β is the safer virial estimator for these extreme Eddington emitters. [ABSTRACT FROM AUTHOR]

We present preliminary results from a pencil-beam spectroscopic survey of low-luminosity quasars at z ≈ 2.2 - 2.5 . Our goal is to compare these sources with low redshift analogues of similar luminosity. High s/n and moderate resolution spectra were obtained for 15 sources using the faint object spectrograph Osiris on the 10m Gran Telescopio Canarias. The new data make possible an almost unprecedented comparison between sources with the same (moderate) luminosity at widely different cosmic epochs. Preliminary analysis of our spectra confirms the presence of a relatively evolved population of quasars radiating at modest L / L Edd . A notable difference between the low and high z quasars may involve the presence of lower metallicity quasars at high redshift. [ABSTRACT FROM AUTHOR]

Black hole mass estimation in quasars, especially at high redshift, involves the use of single-epoch spectra with signal-to-noise ratio and resolution that permit accurate measurement of the width of a broad line assumed to be a reliable virial estimator. Coupled with an estimate of the radius of the broad-line region (BLR) this yields the black hole mass MBH. The radius of the BLR may be inferred from an extrapolation of the correlation between source luminosity and reverberation-derived rBLR measures (the so-calledKaspi relation involving about 60 low-z sources). We are exploring a different method for estimating rBLR directly from inferred physical conditions in the BLR of each source. We report here on a comparison of rBLR estimates that come from our method and from reverberation mapping. Our "photoionization" method employs diagnostic line intensity ratios in the rest-frame range 1400-2000 Å (Al iii λ1860/Si iii] λ1892, Civ λ1549/Al iii λ1860) that enable derivation of the product of density and ionization parameter with the BLR distance derived from the definition of the ionization parameter. We find good agreement between our estimates of the density, ionization parameter, and rBLR and those from reverberation mapping. We suggest empirical corrections to improve the agreement between individual photoionization-derived rBLR values and those obtained from reverberationmapping. The results in this paper can be exploited to estimate MBH for large samples of high-z quasars using an appropriate virial broadening estimator. We show that the width of the UV intermediate emission lines are consistent with the width of Hβ, thereby providing a reliable virial broadening estimator that can be measured in large samples of high-z quasars. [ABSTRACT FROM AUTHOR]

In order to investigate where and how low ionization lines are emitted in quasars we are studying a new collection of spectra of the CaII triplet at λ 8498, λ 8542, λ 8662 observed with the Very Large Telescope (VLT) using the Infrared Spectrometer And Array Camera (ISAAC). Our sample involves luminous quasars at intermediate redshift for which CaII observations are almost nonexistent. We fit the CaII triplet and the OI λ 8446 line using the H β profile as a model. We derive constraints on the line emitting region from the relative strength of the CaII triplet, OI λ 8446 and H β . [ABSTRACT FROM AUTHOR]

We describe an alternate way to estimate Broad Line Region (BLR) radii for type-1 AGN based on determination of physical conditions in the BLR under the assumption that the line emitting gas is photoionized by a central continuum source. We derive “diagnostic” intensity ratios involving UV lines Al iii λ 1860, Si iii ] λ 1892 and C iv λ 1549 which enable us to compute the ionizing photon flux, and hence BLR radius from the ionization parameter definition. We compare our estimates of BLR radii with values independently obtained from reverberation monitoring of H β and, in a few cases, of C iv λ 1549. We analyze the interpretation of the photoionization estimates in the 4D eigenvector 1 context, and discuss in some detail the case of 3C 390.3. For this object we are able to provide not only the ionizing photon flux, but also an estimate of density and ionization parameter from the measured diagnostic ratios. We also compare black hole masses obtained from this method with values derived from widely-applied correlations between mass, line broadening and luminosity. Good agreement is found for both radius and black hole mass comparisons. [ABSTRACT FROM AUTHOR]