Reverberation Mapping of High-z, High-luminosity Quasars

We present Reverberation Mapping results after monitoring a sample of 17 high-z, high-luminosity quasars for more than 10 years using photometric and spectroscopic capabilities. Continuum and line emission flux variability is observed in all quasars. Using cross-correlation analysis we successfully determine lags between the variations in the continuum and broad emission lines for several sources. Here we present a highlight of our results and the determined radius--luminosity relations for Ly_alpha and CIV.Comment: Contributed talk at conference "Quasars at all cosmic epochs", held in Padova 2-7 April 2017, published in Frontiers in Astronomy and Space Science

Dynamical and Spectral Modeling of the Ionized Gas and Nuclear Environment in NGC 3783

We present a new approach for calculating the physical properties of highly ionized X-ray flows in active galactic nuclei (AGN). Our method relies on a detailed treatment of the structure, dynamics and spectrum of the gas. A quantitative comparison of our model predictions with the 900 ksec Chandra/HETG X-ray spectrum of NGC 3783 shows that: (1) The highly ionized outflow is driven by thermal pressure gradients and radiation pressure force is less important. (2) A full featured dynamical model that provides a very good fit to the high resolution X-ray spectrum requires a multi-phased flow with a density power spectrum reminiscent of the interstellar medium. (3) Adiabatic cooling is an important factor and so is an additional heating source that may be related to the apparent multi-phase and turbulent nature of the flow. (4) The base of the flow is ~1pc from the central object, in agreement with some, but not all previous estimates. (5) The mass loss rate is in the range 0.01-0.1 solar masses per year which is smaller than previous estimates and of the same order of the mass accretion rate in this object.Comment: 12 pages, 11 figures; accepted for publication in Ap

The largest black holes and the most luminous galaxies

The empirical relationship between the broad line region size and the source luminosity in active galactic nuclei (AGNs) is used to obtain black holes (BH) masses for a large number of quasars in three samples. The largests BH masses found exceed 10^{10} Msun and are correlated, almost linearly, with the source luminosity. Such BH masses, when converted to galactic bulge mass and luminosity, indicate masses in excess of 10^{13} Msun and sigma(*) in excess of 700 km/sec. Such massive galaxies have never been observed. The largest BHs reside, almost exclusively, in high redshift quasars. This, and the deduced BH masses, suggest that several scenarios of BH and galaxy formation are inconsistent with the observations. Either the observed size-L relationship in low luminosity AGNs does not extend to very high luminosity or else the M(BH)-M_B(bulge)-sigma(*) correlations observed in the local universe do not reflect the relations of those quantities at the epoch of galaxy formation.Comment: 12 pages, 2 figures, one table, submitted to ApJ

Black Hole Growth to z = 2 - I: Improved Virial Methods for Measuring M_BH and L/L_Edd

We analyze several large samples of AGN in order to establish the best tools required to study the evolution of black hole mass (M_BH) and normalized accretion rate (L/L_Edd). The data include spectra from the SDSS, 2QZ and 2SLAQ surveys at z<2, and a compilation of smaller samples with 0<z<5. We critically evaluate the usage of the MgII(2798A) and CIV(1549A) lines, and adjacent continuum bands, as estimators of M_BH and L/L_Edd, by focusing on sources where one of these lines is observed together with Hbeta. We present a new, luminosity-dependent bolometric correction for the monochromatic luminosity at 3000A, L_3000, which is lower by a factor of 1.75 than those used in previous studies. We also re-calibrate the use of L_3000 as an indicator for the size of the broad emission line region (R_BLR) and find that R_BLR is proportional to L_3000^0.62. We find that FWHM(MgII)\simeqFWHM(Hb) for all sources with FWHM(MgII)<6000 km/s. Beyond this FWHM, the MgII line width seems to saturate. The spectral region of the MgII line can thus be used to reproduce Hb-based estimates of M_BH and L/L_Edd, with negligible systematic differences and a scatter of 0.3 dex. The width of the CIV line, on the other hand, shows no correlation with either that of the Hb or the MgII lines and we could not identify the reason for this discrepancy. The scatter of M_BH(CIV), relative to M_BH(Hb) is of almost 0.5 dex. Moreover, 46% of the sources have FWHM(CIV)<FWHM(Hb), in contrast with the basic premise of the virial method, which predicts FWHM(CIV)/FWHM(Hbeta)~1.9. Thus, the CIV line cannot be used to obtain precise estimates of M_BH. We conclude by presenting the observed evolution of M_BH and L/L_Edd with cosmic epoch. The steep rise of L/L_Edd with redshift up to z~1 flattens towards the expected maximal value of L/L_Edd~1, with lower-M_BH sources showing higher values of L/L_Edd at all redshifts. [Abridged]Comment: 23 pages, including 2 appendices and 18 figures. Accepted for publication in MNRAS (minor corrections

What triggers black-hole growth? Insights from star formation rates

We present a new semi-analytic model for the common growth of black holes (BHs) and galaxies within a hierarchical Universe. The model is tuned to match the mass function of BHs at z=0 and the luminosity functions of active galactic nuclei (AGNs) at z<4. We use a new observational constraint, which relates the luminosity of AGNs to the star-formation rate (SFR) of their host galaxies. We show that this new constraint is important in various aspects: a) it indicates that BH accretion events are episodic; b) it favours a scenario in which BH accretion is triggered by merger events of all mass ratios; c) it constrains the duration of both merger-induced star-bursts and BH accretion events. The model reproduces the observations once we assume that only 4 per cent of the merger events trigger BH accretion; BHs accretion is not related to secular evolution; and only a few per cent of the mass made in bursts goes into the BH. We find that AGNs with low or intermediate luminosity are mostly being triggered by minor merger events, in broad agreement with observations. Our model matches various observed properties of galaxies, such as the stellar mass function at z<4 and the clustering of galaxies at redshift zero. This allows us to use galaxies as a reliable backbone for BH growth, with reasonable estimates for the frequency of merger events. Other modes of BH accretion, such as disk-instability events, were not considered here, and should be further examined in the future.Comment: accepted to MNRAS, minor changes from version

X-ray Continuum Slope and X-ray Spectral Features in NLS1 Galaxies

The idea that some of the unusual features in the X-ray spectra of Narrow-Line Seyfert 1 galaxies (NLS1s) are due to the steep X-ray continuum is tested by comparing photoionization model calculations with various observed properties of Seyfert 1 galaxies. A meaningful comparison must involve the careful use of the right X-ray ionization parameter, designated here U(oxygen). When this is done, it is found that the strength of the continuum absorption features is insensitive to the exact slope of the 0.1-50 keV continuum. It is also shown that the complex of iron L-shell lines near 1 keV can produce strong absorption and emission features, depending on the gas distribution and line widths. While this may explain some unusual X-ray features in AGN, the predicted intensity of the features do not distinguish NLS1 from broader line sources. Finally, acceleration of highly ionized gas, by X-ray radiation pressure, is also not sensitive to the exact slope of the X-ray continuum.Comment: Invited talk presented at the Joint MPE,AIP,ESO workshop on NLS1s, Bad Honnef, Dec. 1999, to appear in New Astronomy Reviews; also available at http://wave.xray.mpe.mpg.de/conferences/nls1-worksho