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

4. The large-scale structure

One of the main goals of our survey is to study the properties of the large-scale structure in the local Universe. Our survey is significantly deeper than local surveys (e.g. CfA2 & SSRS2) since galaxies are sampled out to , corresponding to a comoving distance   Mpc  (), which is the effective depth of the sample. This depth, together with a wide extension in the right ascension direction ( corresponding to   Mpc  at the effective depth), could be enough to ensure the characteristic of a "fair sample" to ESP. Clearly, the discovery of structures larger than those observed in the existing shallower surveys may prevent even ESP from being a fair sample.

In particular, ESP is deep enough to detect at least the first peaks of the BEKS survey and it is sufficiently wide and thick (   Mpc  at ) to clarify the nature of the peaks that could correspond either to isolated groups/clusters or to the intersection of the beam with a larger connected structure like the Great Wall.

Detailed statistical results, including an analysis of the groups and clusters in the survey, will be presented in forthcoming papers; here we describe the main characteristics of the volume sampled by the ESP.

Fig. 4 shows the histogram of the distribution in comoving distance of the 3342 galaxies with measured redshift (panel a) and the corresponding wedge diagram (panel b). For comparison we show in panel c) the wedge diagram corresponding to the ESP right ascension range from the nearest strip of the LCRS centered at (Shectman et al. 1996).

The most outstanding feature in Fig. 4a is the peak at   Mpc . Although about 50 redshifts in this peak are due to galaxies within 1 Abell radius from the centers of two Abell clusters (ACO 2840 and ACO 2860) both located near the eastern edge of strip A, this peak remains highly significant even if these galaxies are removed from the sample. In fact, it is clear from Fig. 4b that the peak in the redshift distribution corresponds to an ensemble of large-scale structures nearly perpendicular to the line of sight. If this ensemble can be considered as a single, connected structure, it would have a linear dimension of about 120   Mpc , before entering into the gap between regions A and B. At least part of this structure is visible in the LCRS data (panel c) where it seems to extend towards west and possibly connecting to other structures.

Fig. 4a-c. a Galaxy distribution in comoving distance (); error bars represent Poissonian uncertainties. The solid line shows the expectation resulting from a uniform distribution of the galaxies in the sample. Vertical lines correspond to the positions of the BEKS peaks (see text). b Wedge diagram of ESP galaxies projected in right ascension; the step of the grid is . c The same as panel b for galaxies in the LCRS strip centered at

Other foreground/background peaks in the redshift histogram correspond to structures less evident to the eye in the wedge diagram.

Close examination of the wedge diagram shows the existence of many more dense regions and structures, some of which are elongated along the line of sight such as the one in region B stretching from about 130   Mpc  to 200   Mpc .

The LCRS data in panel c) allow to follow some of these structures, even if the different sampling, color selection and depth of the samples do not allow a quantitative comparison.

As well as all the other redshift surveys (see for instance panel c), our data suggest the presence of large underdense regions. Fig. 4a clearly shows one such region at   Mpc  and a second one corresponding to a nearby (   Mpc ) underdensity.

On the basis of our data and the comparison with the LCRS data, we conclude that this nearby underdense region is statistically significative (see Fig. 4a); however, given the small solid angle covered by ESP and LCRS in this region, it is impossible to assess its extension in the transverse direction. This underdensity has interesting consequences for the interpretation of the bright galaxy counts as we discuss in Paper II.

The vertical lines in Fig. 4a show the location of the regularly spaced density enhancements found in the BEKS pencil beam survey, located at , , i.e. north of the eastern part of the ESP. The two main peaks in our redshift distribution (at comoving distances of and   Mpc ) are in reasonably good agreement with the BEKS peaks. The same two peaks are clearly visible in the LCRS data extracted from their slice immediately north () of our strip. Moreover, the peak at   Mpc  appears to be present in the deep pencil beam of Bellanger and de Lapparent (1995), centered at and , i.e. away from the BEKS direction. It is also visible in the shallower OPTOPUS probes obtained in this region of the sky by Ettori et al. (1995, see their Fig. 2).

The coincidence in depth of the two nearest peaks of the redshift distribution within ESP with similar peaks in all other surveys in the same region suggests the presence of large extended structures (walls), approximately orthogonal to the line of sight. Under this hypothesis, the structure at would have minimum linear dimensions of the order of   Mpc , comparable with the size of the Great Wall (Geller and Huchra 1989).

The existence of these two peaks does not imply, however, a periodicity with a preferred scale as claimed by Broadhurst et al. (1990), which can be tested only with much deeper samples.

Fig. 5 shows the density of the planar projection of the data. Isodensity contour levels are spaced as , with : they represent the ratio of the ESP projected number density , Gaussianly smoothed over 5   Mpc , to the projected number density of a similarly smoothed random field, derived from an average of 100 random samples.

Fig. 5. Smoothed galaxy distribution, obtained by taking into account the survey selection function and the fields incompleteness (see text).

Each galaxy was given a weight inversely proportional to the selection function (as derived from the ESP luminosity function, see Paper II), and to the incompleteness of its field. This procedure implicitly assumes that the galaxies we did not observe follow the same distribution in depth as the observed galaxies; this assumption is justified by our random selection of targets in overdense fields. The weighting with the selection function allows to detect structures even at large distances, which would be otherwise "washed out"; however, as any magnitude-limited sample, such structures are inferred from the few high luminosity galaxies that can still be detected, and are therefore affected by a large uncertainty.

The random samples were built using the ESP average luminosity function and K-corrections described in Paper II, and within a three-dimensional space which faithfully takes into account the sample geometry, i.e. not only the unobserved region, but also the presence of a few "holes" in the original catalogue, due to the presence of bright stars.

The final result is a representation of the large-scale galaxy distribution which is more "objective" than the cone-diagram.

In Fig. 5 the dense regions corresponding to the peaks in Fig. 4a are clearly identifiable, in particular the two structures crossing the entire region from one side to the other at comoving distances of and   Mpc . Note also that the most prominent structure at large comoving distance in the small strip B appears to be elongated along the line of sight.

© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998

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