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1 Introduction

Bright quasars (QSOs) can be used as background sources to probe the physical properties of foreground objects. When the foreground is a cluster of galaxies, the quasars behind the cluster can be used to map not only the warm gas content of the intra-cluster medium (via absorption lines), but also its dark matter distribution (via micro-lensing).

The discovery of a diffuse extreme UV-excess in nearby clusters by the Extreme Ultraviolet Explorer (EUVE) satellite ([Lieu et al. 1996a],b) was initially interpreted as evidence for a warm gas component at about 500 000 K in the intra-cluster medium. This would constitute a major mass component and raises enormous problems [(Fabian 1996)]. A non-thermal explanation, based on inverse Compton scattering, seems more plausible and consistent with the hard X-ray tail observed by BeppoSAX ([Ensslin & Biermann 1998]; [Sarazin & Lieu 1998]). However the distorted shape of the EUV isophotes may be more consistent with a shock heated medium, so the situation remains very confused. Although attempts have been made to detect the warm gas through UV emission lines [(Dixon et al. 1996)], absorption lines are far more sensitive to the presence of this component. Hence, Hubble Space Telescope (HST) spectroscopy of background QSOs should reveal absorption lines at the redshift of the cluster if the thermal interpretation is correct.

In addition, background QSOs can be used as probes of intervening gaseous clouds and of their evolution along the line of sight. Adjacent lines of sight provided by close pairs of QSOs can also constrain the size of the absorbers, their spatial distribution and their connection with galaxies. In this context, targeting quasars behind low-redshift clusters is a very efficient way to reveal the connection of the intergalactic medium on any scale with these concentrations of galaxies. Coma (Abell 1656, at a redshift of $<z_{\rm Coma}\gt\ =0.0229$) is ideal in this respect since it is one of the best studied areas of the sky as far as redshift coverage and galaxy properties are concerned [(Biviano 1998)].

Quasar pairs with projected separations of no more than a few arcminutes yield interesting constraints on the size, physical structure and kinematics of galactic haloes, clusters and filaments ([Smette etal. 1995]; [Petitjean etal. 1998]; [D'Odorico etal. 1998]). The ideal project however would be to observe a large number of QSOs in a small solid angle on the sky to probe all scales at the same time. Since absorption is extremely sensitive to the total amount of warm gas, this would probe even small column densities inside the cluster and also reveal, in the outskirts of the cluster, the connection with the large-scale filamentary network ([Cen et al. 1994]; [Petitjean etal. 1995]; [Hernquist et al. 1996]; [Miralda-Escudé et al. 1996]).

Besides the mapping of the warm gas in the cluster and in the neighbouring environment, a third goal of a QSO survey behind clusters is the mapping of their baryonic dark matter content. This can be achieved by monitoring the background QSOs searching for the expected micro-lensing signature in their light curves. The micro-lensing optical depth produced by a cluster is very large, a few 10-3, and for lens masses ranging from 10-5 to $10^{-3} M_\odot$ the events will have typical time scales of days ([Walker & Ireland 1995]; [Tadros et al. 1998]; [Wu & Xue 1998]). A program to monitor a few tens of QSOs behind Coma for instance, for 5 months with a daily sampling rate, should reveal dozens of micro-lensing events and produce a 2-D mapping of the baryonic dark matter within the cluster.

There are however very few quasars known to date behind low-redshift clusters and clearly additional background quasars, and more generally galaxies with an active galactic nucleus, are badly needed for all these studies. These sources have to be bright enough to be accessible for medium-resolution spectroscopy with the HST and for optical monitoring with small/medium-size ground-based telescopes.

Until now optical searches for quasars have only been performed systematically behind Virgo ([He et al. 1984]; [Impey & He 1986]), where 29 QSOs were found out of 82 candidates observed at the Palomar 5-meter telescope. There are a few X-ray selected QSOs found in ROSAT images whose optical counterparts were identified in the Digitized Sky Survey around several clusters [(Knezek & Bregman 1998)]. Although interesting, this technique only provides a handful of QSOs in the vicinity of clusters. There are also unpublished surveys around Abell 2029 and Abell 2255 [(Horowitz et al. 1994)]. We report here on a survey [(Ledoux et al. 1998)] to complete the extant catalogues of QSOs/AGNs within a projected radius of $\approx\!4\hbox{$^\circ$}$ from the centre of the Coma cluster, down to a limiting magnitude of about B<18. We describe the selection methods in Sect. 2 and present the observations in Sect. 3. In Sects. 4 and 5, we discuss the characteristics of the extragalactic objects identified in this way, and conclude in Sect. 6 on the global efficiency of the selection.

Throughout this paper, we assume a standard Einstein-De Sitter cosmological model and a Hubble constant $H_0=100 \, h$ km s-1 Mpc-1.


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