Observing the galaxy environment of QSOs

We outline our recently started program to investigate the galaxy environment of QSOs, in particular of radio–quiet objects at intermediate redshifts. 1 Studies of QSO environments Galaxy environment studies of QSOs and of their host galaxies are important since tidal interaction with neighbouring galaxies is thought to be one fundamental mechanism for triggering AGN activity (e.g. Osterbrock(1993)). The search for galaxy density enhancements (clusters) and/or for the presence of close companions near QSOs in support of this scenario is only part of the story. In addition, one wants to look for qualitative differences in the environments associated with intrinsic QSO properties (e.g. host galaxy types, radio–loud/quiet), and for the evolution with redshift. E.g. previous studies (cf. Ellingson and Yee (1994), Yee and Ellingson (1993)) for z < 0.6 show basically that radio–loud QSOs tend to lie in galaxy clusters while radio–quiet QSOs are found in poorer environments. On the contrary, a few recent observations at z > 1 (Hutchings et al. (1995), Hutchings (1995)) show hints for both radio–loud and radio–quiet QSOs to reside in compact groups of galaxies. This seems to imply an evolution in density and composition of the cluster environment of radio–quiet QSOs at the intermediate redshifts. A careful comparison and interpretation of several studies of this kind seems necessary as principle difficulties exist for the observation and analysis of QSO environment data, e.g. the interpretation of different statistical methods for detecting and measuring galaxy clustering. 2 A research at intermediate redshifts There are nearly no data of QSO environments within 0.6 < z < 1. We have started a program on the observation of fields around radio–quiet QSOs at these redshifts. This research is part of an extensive QSO environment study over a large redshift range. Our main goals are the detection of galaxy groups or clusters around the QSOs, an investigation into the density evolution of the environments, into the role of the host galaxy types and of gravitational interactions with companions for the development of QSO activity. The first step of our program is a deep imaging survey (multicolor broad/narrow–band) of fields around a large sample of QSOs to detect host galaxies, close companion– and cluster candidates. Beforehand all fields were checked for contamination by 2 Klaus Jäger, Klaus J. Fricke, and Jochen Heidt known foreground galaxy clusters, radio sources, bright foreground galaxies or stars. As a second step spectroscopy of selected targets is intended to verify the physical association of environment galaxies with the QSOs and to assess their state of activity. A preliminary analysis of our obtained data holds already some promising results. A comparison of R– and I– band observations of fields around the QSOs reveals evidence for the physical association of companion galaxies with some QSOs. While the galaxies are easily detected in the I–band the comparable R–band image shows these objects to be more diffuse and fainter. This is to be expected at redshifts z > 0.75 due to the restframe wavelength ranges covered by the selected broad–band filters (eg. in the R–band below the 4000 Å break). Fig. 1 shows e.g. the compact group of objects near the QSO 2249– 0154 at z = 0.83 (I–band image, 50 min., Calar Alto 2.2m telescope). 11 objects within 20 arcsec or within a projected distance of about 300 kpc around the QSO can be seen (one being a likely foreground star). There are some other cases where we found close–companion galaxies or even cluster candidates revealing the typical shape and size of a cluster to be expected at this redshift. In summary, the first inspection of our data supports the presumption that radio– quiet QSOs reside in a variety of environments at intermediate redshifts. Fig. 1. The close environment of the radio–quiet QSO 2249–0154.(cf. text)