6 Discussion

The selection of HQS candidates led to an efficiency of 29% $\pm$ 10% taking into account the probable BL Lac object HS 1309+2605. Similarly, the $U-V\le 0$ OMHR candidates with undetected proper motion gave a success rate for identified QSOs of 33% $\pm$ 14%.

The OMHR selection actually gives a success rate of 54% $\pm$ 14% if the 8 previously known QSOs falling under the same selection criteria are included, which is a good score for this range of (bright) optical magnitudes. Similarly, deep pointed ROSAT surveys towards low galactic neutral hydrogen column density regions ([Shanks et al. 1991]; [Hasinger etal. 1993], 1994) give a selection of AGNs in X-rays that is very successful with typical efficiencies of up to $\sim 60$% [(Zickgraf etal. 1997)]. We also note that the fact that all OMHR targets are either stars or bona fide QSOs confirms the quality of the diffuse-object rejection by Moreau & Reboul (1995).

Out of a total of 51 selected candidates, 16 new active galaxies (AGNs and HII region galaxies) have been identified, and a probable BL Lac object discovered. This leads to a 33% $\pm$ 8% global selection efficiency. Compared to the catalogue of [Véron-Cetty & Véron (1998)], the number of confirmed QSOs/AGNs brighter than $18^{\rm th}$ magnitude has increased by 22% within the inner $2\hbox{$^\circ$}$, and by 36% in the annulus between $2\hbox{$^\circ$}$ and $4\hbox{$^\circ$}$ (see Fig. 2).

  

Figure 2: Projected distribution of the new AGNs brighter than $B\approx 18$behind Coma (filled stars) together with the AGNs listed in the compilation by Véron-Cetty & Véron (1998) brighter than $V\approx 18$ in the same redshift range (empty circles). The dots indicate galaxies with measured redshifts within that solid angle (see text)

No less than 50 QSOs/AGNs with B or are now available behind Coma within a radius of $\approx 4\hbox{$^\circ$}$ of its centre, and they constitute the largest sample of QSOs/AGNs available behind a cluster, with a more uniform coverage from the inner Abell radius to the outskirts related to the large-scale filaments. Figure 2 gives their angular distribution and the positions of galaxies with known redshifts from a variety of surveys ([Karachentsev & Kopylov 1990];$\!\!\!$[Huchra et al. 1995]; [Biviano etal. 1995]; [Doi et al. 1995]; [Donas etal. 1995]; [Colless & Dunn 1996]). Note that there are far many more galaxies known in this field ([Godwin & Peach 1977]; [Slezak etal. 1988]), whose redshifts are likely to be measured in the near future by the Sloan Digital Sky Survey. Moreover, there are $\sim 15$ QSO/AGN pairs with separations smaller than $30\hbox{$^\prime$}$. The sample thus provides ideal targets for observation with HST and for optical monitoring with small/medium-size telescopes to shed some light on the nature of the Lyman-$\alpha$clouds at low redshift, to study the spatial distribution of the warm gas in and around the Coma cluster of galaxies, and its baryonic dark matter content.

Acknowledgements

We are grateful to A. Biviano, C. Lobo, F. Durret and M. Doi for providing galaxy and star positions in advance of publication, to M.-P. Véron-Cetty for clarifying the status of some rejected quasars, and to the CAI/MAMA team for their kind assistance. This research has made use of the SIMBAD database operated at CDS (Observatoire de Strasbourg, France) and of the NASA/IPAC Extragalactic Database (NED) operated by JPL (California Institute of Technology, U.S.A.). The OMHR survey is supported by the Programme National de Cosmologie (INSU/CNRS, France). The Hamburg Quasar Survey was supported by the Deutsche Forschungsgemeinschaft (DFG) under grants Re 353/11 and 22. The Hamburg/RASS identification project has been funded by the DFG under Re 353/22 and by the BMBF under DARA 50 0R 96016.