Ohad Shemmer

The X-Ray Properties of the Most Luminous Quasars from the Sloan Digital Sky Survey

This article studies the X-ray properties of a representative sample of 59 of the most optically luminous quasars in the universe spanning a redshift range of z ≈ 1.5-4.5.

Dust Covering Factor, Silicate Emission, and Star Formation in Luminous QSOs

This article presents Spitzer IRS low resolution, mid-IR spectra of a sample of 25 high luminosity QSOs at 2 < z < 3.5.

Observational evidence of quasar feedback quenching star formation at high redshift

Most galaxy evolutionary models require quasar feedback to regulate star formation in their host galaxies. In particular, at high redshift, models expect that feedback associated with quasar-driven outflows is so efficient that the gas in the host galaxy is largely swept away or heated up, hence suppressing star formation in massive galaxies. We observationally investigate this phenomenon by using VLT-SINFONI integral field spectroscopy of the luminous quasar 2QZJ002830.4-281706 at z = 2.4. The spectra sample the optical emission lines redshifted into the near-IR. The [OIII]λ5007 emission-line kinematics map reveals a massive outflow on scales

THE HARD X-RAY SPECTRUM AS A PROBE FOR BLACK HOLE GROWTH IN RADIO-QUIET ACTIVE GALACTIC NUCLEI

This article studies the hard X-ray spectral properties of 10 highly luminous radio-quiet active galactic nuclei at z = 1.3-3.2, including new XMM-Newton observations of four of these sources.

THE 4 Ms CHANDRA DEEP FIELD-SOUTH NUMBER COUNTS APPORTIONED BY SOURCE CLASS: PERVASIVE ACTIVE GALACTIC NUCLEI AND THE ASCENT OF NORMAL GALAXIES

This article presents cumulative and differential number-count measurements for the recently completed 4 Ms Chandra Deep Field-South survey.

Star Formation in the Hosts of High-z QSOs: Evidence from Spitzer PAH Detections

We present Spitzer rest frame mid-infrared spectroscopy of 12 z ~ 2 millimeter-bright type 1 QSOs, selected from unlensed and lensed QSO samples and covering a range of AGN optical luminosities L5100 = 1045–1047 erg s−1. On top of the AGN continuum, we detect PAH emission from luminous star formation in nine objects individually, as well as in the composite spectrum for the full sample. PAH luminosity and rest frame far-infrared luminosity correlate and extend a similar correlation for lower luminosity local QSOs. This provides strong evidence for intense star formation in the hosts of these millimeter-bright QSOs, sometimes exceeding 1000 M☉ yr−1 and dominating their rest frame far-infrared emission. The PAH-based limit on star formation rates is lower for luminous z ~ 2 QSOs that are not preselected for their millimeter emission. Partly dependent on systematic changes of the AGN dust covering factor and the dust spectral energy distribution of the AGN proper, the spectral energy distributions of millimeter-faint high-z QSOs may be AGN dominated out to rest frame far-infrared wavelengths. Toward the most luminous high-z QSOs, there is a flattening of the relation between star formation and AGN luminosity that is observed for lower redshift QSOs. No QSO in our sample has a PAH-measured star formation rate in excess of 3000 M☉ yr−1.

Near-Infrared Spectroscopy of High-Redshift Active Galactic Nuclei. I. A Metallicity-Accretion Rate Relationship

This article presents near-infrared spectroscopic measurements of the Hβ region for a sample of 29 luminous high-redshift quasars.

BLACK HOLE MASS AND GROWTH RATE AT HIGH REDSHIFT

We present new H- and K-band spectroscopy of 15 high-luminosity active galactic nuclei (AGNs) at redshifts 2.3-3.4 obtained with Gemini South. We combined the data with spectra of an additional 29 high-luminosity sources to obtain a sample with 1045.2 < λ Lλ(5100 A) < 1047.3 ergs s−1 and a black hole (BH) mass range, using reverberation mapping relationships based on the Hβ method, of 108.8-1010.7 M☉. We do not find a correlation of L/LEdd with MBH, but we do find a correlation with λ Lλ(5100 A) , which might be due to selection effects. The L/LEdd distribution is broad and covers the range of ~0.07-1.6, similar to what is observed in lower redshift, lower luminosity AGNs. We suggest that this consistently measured and calibrated sample gives the best representation of L/LEdd at those redshifts, and note potential discrepancies with recent theoretical and observational studies. The lower accretion rates are not in accord with growth scenarios for BHs at such redshifts, and the growth times of many of the sources are longer than the age of the universe at the corresponding epochs. This suggests earlier episodes of faster growth at z 3 for those sources. The use of the C IV λ1549 method gives considerably different results and a larger scatter; this method seems to be a poor MBH and L/LEdd estimator at very high luminosity.

The Hard X-Ray Spectral Slope as an Accretion Rate Indicator in Radio-quiet Active Galactic Nuclei

We present new XMM-Newton observations of two luminous and high accretion rate radio-quiet active galactic nuclei (AGNs) at z ~ 2. Together with archival X-ray and rest-frame optical spectra of three sources with similar properties, as well as 25 moderate-luminosity radio-quiet AGNs at z < 0.5, we investigate, for the first time, the dependence of the hard (2 keV) X-ray power-law photon index on the broad Hβ emission-line width and on the accretion rate across ~3 orders of magnitude in AGN luminosity. Provided that the accretion rates of the five luminous sources can be estimated by extrapolating the well-known relation between the broad-line region size and luminosity to high luminosities, we find that the photon indices of these sources, while consistent with those expected from their accretion rates, are significantly higher than expected from the widths of their Hβ lines. We argue that within the limits of our sample, the hard X-ray photon index depends primarily on the accretion rate.

Near-Infrared Spectroscopy of High-Redshift Active Galactic Nuclei. II. Disappearing Narrow-Line Regions and the Role of Accretion

This article presents near-infrared spectroscopic measurements of the Hβ region for a sample of 29 luminous high-redshift quasars, and uses the data to discuss the size and other properties of the narrow-line regions in those sources.