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A quarter of the X-ray sources from recent and XMM- deep surveys has optical counterparts too faint (R≥25) for spectroscopy, even with the 8-10 m class telescopes. In this work we focus on this subsample of X-ray sources with R≥25 and will refer to them as . This criterium reflects an observational limit beyond which we need to rely on accurate photometric measurements over a large baseline and X-ray spectral information to estimate their redshift and to establish their nature. Since the OFS represent a significant fraction of the entire X-ray sample, they have an important impact on statistical studies, such as the X-ray luminosity function, the evolution of the TypeI/TypeII ratio and the overall N distribution. A first study on the nature of the OFS (I≥24) in the Chandra Deep Field North was done by [1].

The most massive galaxies are powerful radio emitters with maximum masses of ≃10 M [6] and massive black holes of ≃ 10 M, even at =4. These results impose short time-scales of mass accumulation and star formation. Moreover from continuous X to radio observations, we show that the powerful 3CR radio galaxies are highly dissipative at ≃ 8microns (Rocca-Volmerange & Remazeilles, in press). By following the evolution of stellar dissipation sources with our evolutionary code, we check that galaxies hosting active nuclei have cooling time-scales of the same order than their self-gravitation time-scales, satisfying to the critic value of the Rees & Ostriker [5] model.

We report here the first results of a program aimed at obtaining high signal to noise (up to ~ 500) optical spectroscopy of a large sample of BL Lac objects using ESO VLT+FORS1. New spectroscopic observations have been obtained for 31 objects, which allows us to obtain new redshift for 13 targets and provide better classification for others. In a number of cases no lines are detected and lower limits to the redshift can be estimated.

We have selected a sample of 12 dust-reddened quasars with extremely red colors, reddenigs and redshifts in the range of 0.4 < z < 2.21 using a combination of various surveys in the radio, infrared and optical. We obtained Chandra observations of these quasars, estimated the column densities towards them, and hence obtained the gas:dust ratios in the material obscuring the quasar. The reddenings do not correlate with the column densities or the hardness ratios in the X-rays. All of the sources show absorbed X-ray spectra, although none of them present Seyfert 2 typical column densities. When we correct the luminosity for absorption, they can be placed among high luminosity quasars, therefore our objects seem to be the high luminosity analogues of the sources contributing to the X-ray background seen in deep X-ray observations.

Almost exactly 40 years ago, quasars were discovered by M. Schmidt [43] and others. Soon thereafter, Zel’dovich [50] and Salpeter [42] suspected that accretion onto compact objects, like black holes, provided the only efficient and plausible way to generate the required luminosities. Little later, Lynden-Bell [28] refined the picture which became to be known as the black hole paradigm for active galactic nuclei.

Little is currently known about the demographics of black holes in the intermediate-mass range 10–10 M, but a small fraction of dwarf galaxies and late-type spirals contain active nuclei that can be used to study black holes in this mass range. We review the properties of the best-known cases and describe a new project that uses AGNs from the Sloan Digital Sky Survey to extend the – relation to masses below 10 M.

Three major feeding mechanisms have been studied in the context of black hole growth: accretion, black hole - black hole mergers, and tidal capture/disruption of stars. While there is ample evidence that accretion is ongoing in active galaxies, observational evidence for the two other processes remained elusive for many years. Given the intense theoretical work on these latter two processes, it is of great interest to see whether such events do occur in nature, how frequent they are, and which properties they show. In this contribution, I will give a review of observational evidence for stellar tidal disruption events concentrating on X-ray flares of giant amplitude detected from the centers of several galaxies. These represent the most dramatic variability events, in terms of amplitude, ever recorded among galaxies.

Using a complete sample of soft X-ray selected AGNs, we show that highly accreting AGNs do not follow the – relation of normal galaxies. This statistical result is robust, with important consequences towards our understanding of black hole formation and growth at present epoch: black holes grow by accretion in well formed bulges, possibly after a major merger. As they grow, they get closer to the – relation for normal galaxies. The relationship between a BH and its bulge appears to be a function of redshift. Our finding of BHs which are still growing at present epoch is consistent with the scenario of downsizing of AGN activity with time or the “anti-hierarchical” BH growth.

We find that Narrow-Line Seyfert 1 Galaxies accrete at moderately super-Eddington rates. This was already suggested with the discovery of extreme soft X-ray excesses with ROSAT, and now can be quantitatively measured with XMM-Newton. NLS1s are, in this context, reminiscent of Galactic Black Hole Binary systems in the soft state. Here we report on the XMM-Newton observation of IRAS 13224–3809 and 1H 0707–495, the latter was observed once in 2000, and again in 2002. Both objects show a common characteristic spectral shape: a strong soft X-ray excess; and a steep hard power-law component, which gradually flattens with increasing energies and abruptly drops at energies above 7 keV. The slim-disc model can explain the strong soft X-ray excesses and high disc temperatures, which exceed the standard Shakura-Sunyaev optically thick and geometrically thin accretion disc expectation. We find that both objects accrete close to, or even above the Eddington rate, and that the accretion rate is on the order of 10 to 20 Eddington units. As a results, NLS1s become normal broad-line Seyfert objects on time scales of only a few 10 Million years, assuming that the high accretion rate ceases. In addition the ionizing continuum is also a function of time as the black hole mass increases. The increasing black hole mass in NLS1s can also account for the evolutionary sequences of: steep to flat ROSAT spectra, the flattening of the 2-10 keV power-law slope, and most probably the decreasing optical Fe II multiplet emission.

The connection between the UV and X-ray emission in AGN depends mainly on two processes: Comptonisation of UV photons into the X-ray range and thermal reprocessing of X-rays into the UV band. Both processes introduce correlations between the light curves of these energy bands but predict opposite time lags. The study of the correlated variability in these bands can in principle determine which of the emission processes dominates in a given object. In this paper we study the variability of the narrow line Seyfert 1 galaxy MCG–6-30-15 using simultaneous X-ray and UV data obtained with . A complete account on this work is given in [1].