Optical studies

People involved at OAB: de Ruiter, Stirpe, Zitelli.

Emission lines and variability of AGN

V. Zitelli and G. Stirpe, in collaboration with D. Trevese (Univ. Rome, La Sapienza), have started a collaboration on photometric and spectroscopic monitoring of high-$z$ quasars using both Loiano and Asiago telescopes. Aim of this research is to study the variability of AGN for both continuum and lines. The different physical processes of emission in both continuum and lines, originating in different regions of the AGN, produce a variability on time scales between 1 week and 1 month for Sy 1 and reaching several months for low redshift QSOs. By means of an intense monitoring of these sources it is possible to estimate the BLR size using the cross correlation among lines and continuum light curves, as found by previous observations. At present correlations are found making use of samples with M$_B>-26$ and the analysis is extended to bright objects by extrapolation. With these observations we aim to obtain for the first time primary estimates of the virial mass and Eddington ratio of high luminosity AGN. Preliminary light curves have been obtained from the first year of data using the observations of both sites. The accuracy of the measurements is $\sim 5$%, while the variation in luminosity is of the order of 0.1 mag in the CIV and MgII lines.

Within a collaboration led by A. Marconi (Arcetri Obs.), G. Stirpe is taking part in the monitoring of 2 high-luminosity, high-$z$ QSOs with the ESO VLT, with the purpose of measuring the emission line vs. continuum light curve lag and thus obtain an estimate of the size of the Broad Line Region. These are the highest luminosity AGN monitored in this fashion, which means extending this technique to what are probably the most massive known black holes. Photometric and spectroscopic observations in the $K$ band have been made on a monthly timescale starting in 2001. Light curves have been obtained from the first three years of data. They reveal variations of $\sim$10% in both the continuum and H$\alpha$ line flux. A preliminary cross-correlation analysis shows that the line vs. continuum lags are between 50 and 200 days. Further monitoring is required to lengthen the time baseline of the light curves and decrease the uncertainties of these estimates.

G. Stirpe, in collaboration with A. Robinson and D. Axon (Rochester Institute of Technology), is studying the distribution of properties of the optical broad emission lines in AGN, covering a wide interval of intrinsic luminosity. In particular, estimates of the central black hole masses have been obtained on the basis of the integrated luminosities and widths of H$\alpha$ - a newly-developed technique - and indicate that black hole masses in excess of $10^{10}$ M$_\odot$ may be common in high-luminosity AGN.

In collaboration with J. Sulentic (Univ. of Alabama), P. Marziani, R. Zamanov and M. Calvani (Padova Obs.), and V. Braito (Brera Obs.), G. Stirpe has analyzed the H$\beta$ spectra of a sample of high-$z$ QSOs, obtained at the ESO VLT in the near-IR bands. These are the first H$\beta$ spectra taken in the near-IR with quality sufficient to extend to high luminosities and high redshifts the study of the Boroson-Green Eigenvector 1, which correlates various measured properties of the optical emission lines of AGN. The analysis shows that the main spectral characteristics of broad-line AGN are probably not luminosity-dependent.

H. de Ruiter, in collaboration with J. Lub (Leiden Obs.) constructed a database containing many hundreds of photometric and spectroscopic measurements for a sample of about 15 type 1 and type 2 Seyferts in the southern hemisphere, based on fourteen years of observations. These data have now been made available for the general astronomical community: the calibrated spectra can be inspected directly on the WEB , or downloaded (in FITS format). Animations that show the lightcurves of a spectral region around 5000 Å are now available at the same WEBpage.

HST images of B2 radio galaxies

H. de Ruiter, in collaboration with A. Capetti (Torino Obs.), P. Parma and R. Fanti (IRA-CNR, Bologna), and R. Morganti (ASTRON, Dwingeloo), is studying HST images (in two colours, $V$ and $I$) of about 60 radio galaxies selected from the B2 sample of low luminosity radio galaxies. Brightness profiles were derived for almost all galaxies, and these were used to obtain a detailed mapping of the (circum-)nuclear dust. Fits (with a Nuker law) of the brightness profiles of the B2 radio galaxies have been compared with the profiles of other samples of galaxies (nearby radio quiet ellipticals and brightest cluster galaxies) and have led to the conclusion that radio loud ellipticals have core-type profiles (and not power-laws). This is in line with theoretical studies, which show that merging of massive black holes necessarily produces core-type profiles, and presumably triggers the radio emission.

The environment of AGN

V. Zitelli, in collaboration with P. Focardi and B. Kelm (Astronomy Dept., Univ. of Bologna), is continuing the study of compact groups of galaxies; in particular, the role of active galaxies (AGN) in dense environments is under investigation. Ample evidence has been reported of a complex environment in the local universe around AGN up to $z\sim3$. However, while it is well established that radio-loud quasars, radio galaxies and BL Lacs reside in denser than average regions, the role of the environment and of interactions on Seyfert galaxies is to some extent still controversial. The complexity of the discussion increases because the samples used are limited in number. To limit the statistical uncertainty V. Zitelli and collaborators adopt a strategy based on the analysis of a statistically significant sample of nearby AGN and of appropriate control samples selected on the basis of criteria independent of morphology and environmental properties. A sample of about 300 physical compact groups has been extracted applying an automatic code to 3-D galaxy catalogues. During the current year the occurrence of Seyfert galaxies is being analyzed, in a subsample of these nearby Compact Groups (UZC-CGs) (Kelm et al. 2003). The behaviour of Sy-CGs and non Sy-CGs, when comparing velocity dispersion, number of large scale neighbours and morphological content, shows non-significant differences, though some Seyferts associated to ``extreme'' CGs present larger velocity dispersion, many neighbours and an unusually high number of elliptical members. Another interesting result from this analysis is that Seyfert 2s are preferentially more associated with compact groups than Seyfert 1s. Previous analyses showed a link between local density and larger scale environment; on the basis of this result it is expected that truly isolated galaxy pairs may display the largest evidence of galaxy interaction. To focus this point a new volume-complete sample of bright isolated galaxy pairs has been extracted using the same algorithm. A preliminary analysis of this isolated galaxy pair sample shows a deficit of luminous dominants among E+E and E+S pairs relative to S+S pairs (Zitelli et al. 2004). Optical observations are in progress using the Loiano telescope to obtain a complete and homogeneous spectral classification, and broad-band two-colour photometry for the whole sample. A post-graduate contract has been assigned for this purpose.