HST imaging of redshift z>0.5 7C and 3C Quasars

We present preliminary results from HST imaging of radio-loud quasar hosts, covering a ~x100 range in radio luminosity but in a narrow redshift range (0.5 to 0.65). The sample was selected from our new, spectroscopically complete 7C survey and the 3CRR catalogue. Despite the very large radio luminosity range, the host luminosities are only weakly correlated (if at all) with radio power, perhaps reflecting a predominance of purely central engine processes in the formation of radio jets, and hence perhaps also in the radio-loud/-quiet dichotomy at these redshifts. The results also contradict naive expectations from several quasar formation theories, but the host magnitudes support radio-loud Unified Schemes.Comment: To appear in the Proceedings of the ESO/IAC Conference on 'Quasar Hosts' - Tenerife 24-27 September 199

IRAS F10214+4724: the inner 100pc

We use new near-infrared spectroscopy and our published optical spectroscopy of the gravitationally-lensed Seyfert-2 galaxy F10214+4724 to study both the links between the starburst and AGN in this object and the properties of the inner narrow-line clouds. The UV spectrum is consistent with a compact, moderately- reddened starburst providing about half the UV light. Spectroscopy of the Halpha /[NII] line blend has enabled us to distinguish emission from the narrow-line region of the Seyfert-2 and a moderately-reddened emission line region which we argue is associated with the starburst. Estimates of the star formation rate from the UV continuum flux and the Halpha flux are broadly consistent. We can explain the unusual emission line properties of F10214+4724 in terms of conventional models for nearby Seyfert-2 galaxies if lensing is preferentially magnifying the side of the inner narrow-line region between the AGN and the observer, and the other side is both less magnified and partially obscured by the torus. The hydrogen densities of clouds in this region are high enough to make the Balmer lines optically thick and to suppress forbidden emission lines with low critical densities. We have deduced the column density of both ionised and neutral gas in the narrow-line clouds, and the density of the ionised gas. Using these we have been able to estimate the mass of the inner narrow-line clouds to be ~ 1 solar mass, and show that the gas:dust ratio NH/E(B-V) in these clouds must be ~1.3x10^{27}m^{-2}mag^{-1}, significantly higher than in the Milky Way. The cloud properties are consistent with the those of the warm absorbers seen in the X-ray spectra of Seyfert-1 galaxies. Our results favour models in which narrow-line clouds start close to the nucleus and flow out.Comment: 13 pages, 5 figures. Accepted by MNRA

On the 10-micron silicate feature in Active Galactic Nuclei

The 10-micron silicate feature observed with Spitzer in active galactic nuclei (AGN) reveals some puzzling behavior. It (1) has been detected in emission in type 2 sources, (2) shows broad, flat-topped emission peaks shifted toward long wavelengths in several type 1 sources, and (3) is not seen in deep absorption in any source observed so far. We solve all three puzzles with our clumpy dust radiative transfer formalism. (1) We present the spectral energy distribution (SED) of SST1721+6012, the first type 2 quasar observed to show a clear 10-mic silicate feature in emission. We constructed a large database of clumpy torus models and performed extensive fitting of the observed SED, constraining several of the torus parameters. We find that the source bolometric luminosity is ~3*10^12 L_sun. Our modeling suggests that <35% of objects with tori sharing characteristics and geometry similar to the best fit would have their central engines obscured. This relatively low obscuration probability can explain the clear appearance of the 10-mic emission feature in SST1721+6012 together with its rarity among other QSO2. (2) We also fitted the SED of PG1211+143, one of the first type 1 QSOs with a 10-mic silicate feature in emission. Among similar sources, this QSO appears to display an unusually broadened feature whose peak is shifted toward longer wavelengths. Although this led to suggestions of non-standard dust chemistry in these sources, our analysis fits such SEDs with standard galactic dust; the apparent peak shifts arise from radiative transfer effects. (3) We find that the distribution of silicate feature strengths among clumpy torus models closely resembles the observed distribution, and the feature never occurs deeply absorbed. (abridged)Comment: 11 pages, 9 figures, 4 tables, accepted for publication in ApJ; minor revision, added reference in Section

Thermal-infrared imaging of 3C radio galaxies at z~1

We present the results of a programme of thermal-IR imaging of nineteen z~1 radio galaxies from the 3CR and 3CRR samples. We detect emission at L' (3.8um) from four objects; in each case the emission is unresolved at 1" resolution. Fifteen radio galaxies remain undetected to sensitive limits of L'~15.5. Using these data in tandem with archived HST data and near-IR spectroscopy we show that 3 of the detected `radio galaxies' (3C22, 3C41, and 3C65) harbour quasars reddened by Av<5. Correcting for this reddening 3C22 and 3C41 are very similar to coeval 3C quasars, whilst 3C65 seems unusually underluminous. The fourth radio galaxy detection (3C265) is a more highly obscured (Av~15) but otherwise typical quasar which previously has been evident only in scattered light. We determine the fraction of dust-reddened quasars at z~1 to be 28(+25)(-13)% at 90% confidence. On the assumption that the undetected radio galaxies harbour quasars similar to those in 3C22, 3C41 and 3C265 (as seems reasonable given their similar narrow emission line luminosities) we deduce extinctions of Av>15 towards their nuclei. The contributions of reddened quasar nuclei to the total K-band light ranges from ~0 per cent for the non-detections, through ~10 per cent for 3C265 to ~80 per cent for 3C22 and 3C41. Correcting for these effects does not remove the previously reported differences between the K magnitudes of 3C and 6C radio galaxies, so contamination by reddened quasar nuclei is not a serious problem for drawing cosmological conclusions from the K-z relation for radio galaxies. We discuss these results in the context of the `receding torus' model which predicts a small fraction of lightly-reddened quasars in samples of high radio luminosity sources. We also examine the likely future importance of thermal-IR imaging in the study of distant powerful radio sources.Comment: 17 pages incl 14 figures, accepted by MNRA

Searches for galaxies at z > 4 through Lyman-limit imaging

We present preliminary results of a search for galaxies at z>4 through Lyman-limit imaging of the fields of known high-redshift radio-galaxies. Objects were selected by means of their broad-band colours, and spectroscopy of candidate objects in one of the fields has been performed through multi-slit spectroscopy at the 4.2m William Herschel Telescope. These spectra show some of the first z>4 galaxies to be identified using the Lyman break technique.Comment: 4 pages, LaTeX, requires paspconf.sty (included). To appear in Proceedings of "The Young Universe", Rome Observatory, Sep 29 - Oct 3 199