Extensions around z=2 QSOs

Based on an R-band imaging survey of 6 high redshift (z~2) and high luminosity (M<-28mag) QSOs, we report the detection of extensions in two radio-quiet and one radio-loud QSO. The extensions are most likely due to the host galaxies of these QSOs, with luminosities of at least 3-7 per cent of the QSO luminosity. The most likely values for the luminosity of the host galaxies lie in the range $6-18 per cent of the QSO luminosity.Comment: uuencoded compressed postscript. The preprint is also available at http://www.ast.cam.ac.uk/preprint/PrePrint.htm

Multicolour imaging of z= 2 QSO hosts

We present multicolour images of the hosts of three z=2 QSOs previously detected in R-band by our group. The luminosities, colours and sizes of the hosts overlap with those of actively star-forming galaxies in the nearby Universe. Radial profiles over the outer resolved areas roughly follow de Vaucouleur or exponential disk laws. These properties give support to the host galaxy interpretation of the extended light around QSOs at high-redshift. The rest-frame UV colours and upper limits derived for the rest-frame UV-optical colours are inconsistent with the hypothesis of a scattered halo of light from the active nucleus by a simple optically-thin scattering process produced by dust or hot electrons. If the UV light is indeed stellar, star formation rates of hundreds of solar masses per year are implied, an order of magnitude larger than field galaxies at similar redshifts and above. This might indicate that the QSO phenomenon (at least the high-luminosity one) is preferentially acompanied by enhanced galactic activity at high-redshifts.Comment: Accepted to be published in MNRAS. 11 pages, Latex, uses mn macros, also available at http://www.mpa-garching.mpg.de/~itzia

Photometric Variability and Astrometric Stability of the Radio Continuum Nucleus in the Seyfert Galaxy NGC 5548

The NRAO VLA and VLBA were used from 1988 November to 1998 June to monitor the radio continuum counterpart to the optical broad line region (BLR) in the Seyfert galaxy NGC 5548. Photometric and astrometric observations were obtained at 21 epochs. The radio nucleus appeared resolved, so comparisons were limited to observations spanning 10-60 days and 3-4 yr, and acquired at matched resolutions of 1210 mas = 640 pc (9 VLA observations), 500 mas = 260 pc (9 VLA observations), or 2.3 mas = 1.2 pc (3 VLBA observations). The nucleus is photometrically variable at 8.4 GHz by $33\pm5$% and $52\pm5$% between VLA observations separated by 41 days and 4.1 yr, respectively. The 41-day changes are milder ($19\pm5$%) at 4.9 GHz and exhibit an inverted spectrum ($\alpha \sim +0.3\pm0.1$, $S\propto \nu ^{+\alpha}$) from 4.9 to 8.4 GHz. The nucleus is astrometrically stable at 8.4 GHz, to an accuracy of 28 mas = 15 pc between VLA observations separated by 4.1 yr and to an accuracy of 1.8 mas = 0.95 pc between VLBA observations separated by 3.1 yr. Such photometric variability and astrometric stability is consistent with a black hole being the ultimate energy source for the BLR, but is problematic for star cluster models that treat the BLR as a compact supernova remnant and, for NGC 5548, require a new supernova event every 1.7 yr within an effective radius $r_e =$ 42 mas = 22 pc. A deep image at 8.4 GHz with resolution 660 mas = 350 pc was obtained by adding data from quiescent VLA observations. This image shows faint bipolar lobes straddling the radio nucleus and spanning 12 arcsec = 6.4 kpc. These synchrotron-emitting lobes could be driven by twin jets or a bipolar wind from the Seyfert 1 nucleus.Comment: with 9 figures, to appear in the Astrophysical Journal, 2000 March 10, volume 53

Adaptive Optics observations of LBQS 0108+0028: K-band detection of the host galaxy of a radio-quiet QSO at z=2

We report the first unambiguous detection of the host galaxy of a normal radio-quiet QSO at high-redshift in K-band. The luminosity of the host comprises about 35% of the total K-band luminosity. Assuming the average colour of QSOs at z=2, the host would be about 5 to 6 mag brighter than an unevolved L* galaxy placed at z=2, and 3 to 4 mag brighter than a passively evolved L* galaxy at the same redshift. The luminosity of the host galaxy of the QSO would thus overlap with the highest found in radio-loud QSOs and radio-galaxies at the same redshift.Comment: Accepted to be published in MNRAS. 4 pages, 2 postscript figures. Also available at http://www.mpa-garching.mpg.de/~itzia

Optical Variability in Active Galactic Nuclei: Starbursts or Disk Instabilities?

Aperiodic optical variability is a common property of Active Galactic Nuclei (AGNs), though its physical origin is still open to question. To study the origin of the optical -- ultraviolet variability in AGN, we compare light curves of two models to observations of quasar 0957+561 in terms of a structure function analysis. In the starburst (SB) model, random superposition of supernovae in the nuclear starburst region produce aperiodic luminosity variations, while in the disk-instability (DI) model, variability is caused by instabilities in the accretion disk around a supermassive black hole. We calculate fluctuating light curves and structure functions, $V(\tau)$, by simple Monte-Carlo simulations on the basis of the two models. Each resultant $V(\tau)$ possesses a power-law portion, $[V(\tau)]^{1/2} \propto \tau^{\beta}$, at short time lags ($\tau$). The two models can be distinguished by the logarithmic slope, $\beta$; $\beta \sim$ 0.74--0.90 in the SB model and $\beta \sim$ 0.41--0.49 in the DI model, while the observed light curves exhibit $\beta \sim$ 0.35. Therefore, we conclude that the DI model is favored over the SB model to explain the slopes of the observational structure function, in the case of 0957+561, though this object is a radio-loud object and thus not really a fair test for the SB model. In addition, we examine the time-asymmetry of the light curves by calculating $V(\tau)$ separately for brightening and decaying phases. The two models exhibit opposite trends of time-asymmetry to some extent, although the present observation is not long enough to test this prediction.Comment: 28 pages LaTeX; 9 postscript figures; accepted for publication in ApJ; e-mail to [email protected]