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1 Introduction

The present paper is the fourth of a series presenting the data accumulated by the public ESO Imaging Survey (EIS), being carried out in preparation for the first year of regular operation of the VLT. As described in previous papers (Renzini & da Costa 1997; Nonino et al. 1998, hereafter Paper I) the main goals of the EIS project are to conduct an imaging survey suitable for finding "rare'' objects for follow-up observations with the VLT and to lay down the groundwork for more ambitious wide-field digital, multicolor imaging surveys. The adopted strategy was designed to search for distant clusters of galaxies, quasars, high-redshift galaxies and to identify rare stellar types.

As part of EIS-wide, observations were obtained in three passbands (B, V and I) over an area of about 1.7 square degrees in a region near the South Galactic Pole (EIS-wide patch B). The region was selected because of the high density of known intermediate red-shift quasars. The multicolor data for patch B are now being released and in a separate paper the observations, calibration and the overall quality of the data are described (Prandoni et al. 1998, Paper III). In that paper, preliminary single passband catalogs were presented and evaluated by comparing the star- and galaxy-counts with models and results from other surveys and the EIS patch A, presented in Paper I. The good agreement found from these comparisons indicates that the available data, reduction procedures and object catalogs extracted in each of the passbands are reliable. Furthermore, comparison of the observed color distribution for point-like sources, derived from a preliminary version of a color catalog, with predictions of galactic models also shows reasonable agreement.

Although encouraging, the above results do not fully characterize the color catalog, in particular if it is to be used to identify different types of objects based on color selection criteria. Target selection based on the location of objects in color-color space, require a careful investigation of the performance of the pipeline since any problem may lead to objects with spurious colors. Colors are sensitive to a number of effects such as the observing conditions, the photometric and astrometric solutions in the different passbands and the de-blending algorithm. In order to have a better understanding of the impact of these effects in the color catalog, in this paper the (B-V) versus (V-I) diagram for point-sources is used to select objects lying in potentially interesting regions of the color-color space; such regions have been defined following by an appropriate modelling of the expected colors of such interesting objects. The objects, examined individually, are used to generate lists of candidates for different type of objects.

The goals of the present paper are: 1) to test the performance of the EIS pipeline in the production of object catalogs; 2) to assess the reliability of the color catalogs being extracted from the EIS multicolor data; 3) to provide ESO users with lists of potential VLT targets.

In Sect. 2, the basic characteristics of the color catalog relevant to the present work are described. In Sect. 3, the color-color diagram for point-like sources is inspected and empirical color criteria are used to generate a preliminary list of candidate white-dwarfs, low mass stars and quasars, with a range of red-shifts. Results and the outlook for the future is presented in Sect. 4.


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