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We briefly review some recent results from and studies of the highest redshift () active galactic nuclei (AGNs). Specific topics covered include radio-quiet quasars, radio-loud quasars, moderate-luminosity AGNs in surveys, and future prospects. No significant changes in AGN emission properties have yet been found at high redshift, indicating that the small-scale emission regions of AGNs are insensitive to the dramatic changes on larger scales that occur from . observations are also constraining the environments of high-redshift AGNs, relevant emission processes, and high-redshift AGN demography.

We have obtained deep, multi-band imaging observations around three of the most distant known quasars at redshifts >6. Standard accretion theory predicts that the supermassive black holes present in these quasars were formed at a very early epoch. If a correlation between black hole mass and dark matter halo mass is present at these early times, then these rare supermassive black holes will be located inside the most massive dark matter halos. These are therefore ideal locations to search for the first clusters of galaxies. We use the Lyman-break technique to identify star-forming galaxies at high redshifts. Our observations show no overdensity of star-forming galaxies in the fields of these quasars. The lack of (dust-free) luminous starburst companions indicates that the quasars may be the only massive galaxies in their vicinity undergoing a period of intense activity.

I present here the first estimates of the spatial correlation function of X-ray selected AGN in the 2Msec Chandra Deep Field North (CDFN) and 1Msec Chandra Deep Field South (CDFS). The AGN clustering amplitude, measured for sources with median luminosity ~ 10 erg s and at a median redshift of ~ 0.9, is found to be a factor of ~ 2 higher in the CDFS than in the CDFN, revealing large cosmic variance in these 0.1 deg fields. The extra correlation signal in the CDFS is produced by two prominent redshift structures at =0.67 and =0.73. Within each field, no significant differences are found between the clustering properties of soft and hard X-ray selected sources, or, similarly, between those of type 1 and type 2 AGN. The high ( Mpc) correlation length measured for AGN at ~ 0.9 in the two Msec fields is comparable to that of early type galaxies at the same redshift.

We investigate the origin of the huge luminosities produced by high-redshift SCUBA galaxies using the combination of ultra-deep X-ray observations (the 2 Ms Deep Field-North) and deep optical spectroscopic data. Even though a large fraction of high-redshift SCUBA galaxies host AGN activity (upward of 38%), we argue that in almost all cases the AGNs are not bolometrically important (i.e., contribute <20% of the energetics). Thus, most high-redshift SCUBA galaxies appear to be star-formation dominated. A substantial fraction of high-redshift SCUBA galaxies show evidence for binary AGN activity. Since these systems appear to be interacting and merging at optical/near-IR wavelengths, their super-massive black holes are likely to eventually coalesce.

The first Spitzer images from the GOODS survey have revealed detections of all the Extreme X-ray / Optical sources (‘EXO’s) in the CDF-South. These X-ray sources are completely undetected in our deep optical GOODS HST/ACS imaging, to limits that place them at the extreme end of the x/ parameter space, with values about 100 to 1000 times higher than generally found for Active Galactic Nuclei (AGN). We use their Spitzer detections to investigate two possible scenarios for their nature: (1) their hosts could be extremely faint, obscured high-redshift Extremely Red Objects (EROs), at redshifts about 2 - 5, thus higher redshift than previously studied EROs; (2) some of them could lie at redshifts above about 6 - 7, such that their Lyman-alpha emission is completely redshifted into the IR. In either case, these objects serve as a valuable probe of black hole growth and accretion activity in the early universe.

Hard X–ray surveys efficiently sample the obscured AGN population and in particular those with redder colors which would not be selected by UVX surveys. The discovery that a sizable fraction (~20%) of hard X–ray sources are also associated to extremely red objects (EROs) with optical to near–infrared colour R-K>5 is even more intriguing [1,2,1].

The bulk of the QSO luminosity density was produced at 1<<3, and with 0.5-2 keV luminosities close to the break of the X-ray luminosity function [6,5]. We have used sensitive submillimetre (submm) observations [8,9,12] (using SCUBA at the JCMT) to show that the submm luminosities of unabsorbed and absorbed broad line QSO in those intervals are radically different (>3): only 1 out of 20 unabsorbed QSO has been detected, while 8 out of 19 absorbed QSOs have been detected, all of them are ULIRGs at >1.5 (see Fig. 1). Furthermore, there is a significant correlation between the far infrared luminosity and the redshift of the absorbed QSOs. The FIR emission of these objects is due to dust heated by starbursts, rather than reprocessed QSO emission [8,12]. The implied star formation rates (SFR) are >1000 M/y, sufficient to build a substantial fraction of a galaxy spheroid in only a few 100 Myr.

We report on ESO–VLT near-IR adaptive optics imaging of one radio-loud and two radio quiet quasars at z > 2. In the first case we are able to resolve the QSO and find that it is hosted by an elliptical of M = –27.6. This result, though restricted to a single object, extends up to z=2.5 the finding that cosmic evolution of radio-loud hosts follows the trend expected for luminous and massive spheroids undergoing passive evolution.

High-z quasars are key objects for understanding galaxy formation and evolution, and the physical conditions in the early Universe, since they are the most luminous and distant sources of electromagnetic radiation known. The X-ray emission gives insight into the accretion mechanisms onto super-massive black holes. Current questions are whether quasars exhibit spectral evolution or not and whether absorption in excess of the galactic is a general feature of high-z quasars. The X-ray spectral analysis is essential to address these questions. We have performed a spectral analysis of two radio-loud (PKS 2126-158 and PKS 2149-306) and two radio-quiet (Q 0000-263 and Q 1442+2931) >2 quasars with XMM. For more details see [1].

This is a progress report on a project to measure the black hole masses of the SDSS quasar sample from their broad emission lines and the continuum luminosity. We have developed an automated procedure to fit the continuum, and to deconvolve the broad and narrow line components. We find good agreement of the derived line parameters for quasars in common with other samples. Next, we plan to investigate the connection of the black hole mass and other quasar parameters, taking advantage of the unprecedented size of the SDSS QSO sample.