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Astron. Astrophys. 325, 954-960 (1997)

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1. Introduction

In the course of the last decade redshift surveys have provided a major advance in our knowledge of the large scale distribution of galaxies and its statistical properties (see for example the review of Giovanelli and Haynes 1991).

Bright, wide angle surveys (e.g. CfA2, Geller and Huchra 1989; SSRS2, da Costa et al. 1994; Perseus-Pisces, Giovanelli and Haynes 1988) cover a large fraction of the sky and provide a clear picture of the nearby universe, up to about 10,000 km/s or 100   [FORMULA] Mpc  (where [FORMULA]). These surveys reveal large structures in the distribution of galaxies: voids of sizes up to 50   [FORMULA] Mpc  (de Lapparent et al. 1986), and large ([FORMULA]   [FORMULA] Mpc ) bidimensional sheets as for instance the Great Wall within CfA2 (Geller and Huchra 1989), the Southern Wall within SSRS2 (da Costa et al 1994), or the Perseus-Pisces filament (Giovanelli and Haynes 1988).

Strategies alternative to those adopted for the above-mentioned surveys have been used to study the large-scale structure in depth without paying the price of an excessively large increase of the observing time. These strategies include: a) sparse sampling (see Kaiser 1986), as in the Stromlo-APM redshift survey (Loveday et al. 1992); b) chessboard surveys consisting of separated fields covering a large solid angle; c) pencil beam surveys, as in the Broadhurst et al. (1990; BEKS) survey. These deeper surveys have confirmed the texture detected in shallower surveys up to a depth of 40000 - 50000 km/s.

These strategies, while allowing a faster completion of the surveys, sometimes do not allow an unambiguous interpretation of the data. For example, BEKS report evidence of periodic structures in their first pencil beam redshift survey. However, the real nature of this periodicity is not clear. Moreover, the periodicity is not confirmed in other directions of the sky by the same authors (Koo 1993) and is not detected in other similarly deep redshift surveys even in directions close to the original BEKS pencil beam (Bellanger and de Lapparent 1995).

The present survey (hereafter ESP: ESO Slice Project) was designed to provide an unbiased spectroscopic sample of galaxies brighter than [FORMULA], with a high level of completeness, over a region of the sky significantly extended in the right ascension direction. The geometry of the survey, a slice, is the most efficient for mapping three-dimensional structures like those observed in shallower surveys (de Lapparent et al. 1988), provided its thickness is not smaller than the correlation length of the galaxy distribution. Our sample can be used to derive the basic statistical properties of the galaxy distribution, averaged over a volume large enough to smooth out "local" inhomogeneities. With a magnitude limit [FORMULA] we include [FORMULA] galaxies up to [FORMULA] and obtain a redshift distribution peaking at [FORMULA]. Because of the target selection criteria, the ESP is unbiased with respect to a number of statistical descriptors, such as, for example, the luminosity function (see Zucca et al. 1997, Paper II).

The redshift distribution of the ESP galaxies is similar to that of the galaxies in the Las Campanas Redshift Survey (LCRS, Shectman et al. 1996), which covers over 700 square degrees in six strips, each [FORMULA], and consists of 26418 redshifts of galaxies. The comparison between the two surveys, however, is not straightforward. In fact, ESP galaxies are selected in the photographic blue band, while LCRS galaxies are selected on red CCD frames. Furthermore, contrary to what was done for the LCRS, we did not apply any "a priori" selection on the surface brightness of the target galaxies.

In Sect. 2 we briefly describe the survey region, the galaxy catalogue, the observations and the data reduction; in Sect. 3 we discuss the main spectral properties of the galaxies in our sample; in Sect. 4 we describe qualitatively the large-scale structures detected in our survey. We summarize the main properties of the ESP redshift survey in Sect. 5.

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© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998