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تسجيل مستخدم جديدThe ESO Slice Project (ESP) galaxy redshift survey: VI Groups of Galaxies
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In this paper we identify objectively and analyze groups of galaxies in the recently completed ESP survey. We find 231 groups above the number overdensity threshold delta(rho)/rho=80 in the redshift range 5000<cz <60000 km/s. These groups contain 1250 members, 40.5% of the 3085 ESP galaxies within the same redshift range. The median velocity dispersion (corrected for measurement errors and computed at the redshift of the group) is sigma_{ESP,median} = 194 km/s. We show that our result is reliable in spite of the particular geometry of the ESP survey (two rows of tangent circular fields of radius 15 arcmin), which causes most systems to be only partially surveyed. In general, we find that the properties of ESP groups are consistent with those of groups in shallower (and wider) catalogs (e.g. CfA2N and SSRS2). As in shallower catalogs, ESP groups trace very well the geometry of the large scale structure. Our results are of particular interest because the depth of the ESP survey allows us to sample group properties over a large number of structures. We also compare luminosity function and spectral properties of galaxies that are members of groups with those of isolated galaxies. We find that galaxies in groups have a brighter M* with respect to non--member galaxies; the slope alpha is the same, within the errors, in the two cases. We find that 34% of ESP galaxies with detectable emission lines are members of groups. The fraction of galaxies without detectable emission lines in groups is significantly higher: 45%. More generally, we find a gradual decrease of the fraction of emission line galaxies among members of systems of increasing richness. This result confirms that the morphology-density relation found for clusters also extends toward systems of lower density.
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The ESO Slice Project (ESP) is a galaxy redshift survey extending over about 23 square degrees, in a region near the South Galactic Pole. The survey is ~85% complete to the limiting magnitude b_J=19.4 and consists of 3342 galaxies with redshift deter
mination. The ESP survey is intermediate between shallow, wide angle samples and very deep, one-dimensional pencil beams; the spanned volume is ~ 5 x 10^4 Mpc^3 at the sensitivity peak (z ~ 0.1). In this paper we present the description of the observations and of the data reduction, the ESP redshift catalogue and the analysis of the quality of the velocity determinations.
The ESO Slice Project (ESP) is a galaxy redshift survey we have recently completed as an ESO Key-Project. The ESP covers 23.3 square degrees in a region close to the South Galactic Pole. The survey is nearly complete (85%) to the limiting magnitude b
_J=19.4 and consists of 3342 galaxies with reliable redshift determination. In this paper, the first in a series that will present the results of the ESP survey, we describe the main characteristics of the survey and briefly discuss the properties of the galaxy sample. From a preliminary spectral analysis of a large sub-sample of 2550 galaxies we find that the fraction of actively star-forming galaxies increases from a few percent for the brightest galaxies up to about 40% for the galaxies fainter than M= -16.5. The most outstanding feature in the ESP redshift distribution is a very significant peak at z ~ 0.1. The detection of similar peaks, at the same distance, in other surveys in the same region of the sky, suggests the presence of a large bidimensional structure perpendicular to the line of sight. The minimum size of this structure would be of the order of 100 x 50 Mpc, comparable with the size of the Great Wall.
(Abridged) The ESO Slice Project (ESP) is a galaxy redshift survey we have completed as an ESO Key-Project over ~23 square degrees, in a region near the South Galactic Pole. The survey is nearly complete to the limiting magnitude b_J=19.4 and consist
s of 3342 galaxies with reliable redshift determination. The ESP survey is intermediate between shallow, wide angle samples and very deep, one-dimensional pencil beams: spanning a volume of ~ 5 x 10^4 Mpc^3 at the sensitivity peak (z ~ 0.1), it provides an accurate determination of the local luminosity function and the mean galaxy density. We find that, although a Schechter function is an acceptable representation of the luminosity function over the entire range of magnitudes (M < -12.4), our data suggest the presence of a steepening of the luminosity function for M > -17. The amplitude and the alpha and M^* parameters of our luminosity function are in good agreement with those of the AUTOFIB redshift survey (Ellis et al. 1996). Viceversa, our amplitude is significantly higher, by a factor ~ 1.6 at M ~ M^*, than that found for both the Stromlo-APM (Loveday et al. 1992) and the Las Campanas (Lin et al. 1996) redshift surveys. Also the faint end slope of our luminosity function is significantly steeper than that found in these two surveys. Large over- and under- densities are clearly seen in our data. In particular, we find evidence for a local underdensity (for D < 140 Mpc) and a significant overdensity at z ~ 0.1. When these radial density variations are taken into account, our derived luminosity function reproduces very well the observed counts for b_J < 19.4, including the steeper than Euclidean slope for b_J < 17.
We present a detailed discussion of the redshift errors associated to the ESO Slice Project measurements. For a subsample of 742 galaxies with redshifts determined both from the absorption lines (V_{abs}) and from the emission lines (V_{emi}), we fin
d an average difference <V_{abs} - V_{emi}> ~ +100$ km/s. We find that a similar effect is present in another, deeper redshift survey, the Durham/Anglo-Australian Telescope faint galaxy redshift survey, while is absent in surveys at brighter magnitude limits. We have investigated in detail many possible sources of such a discrepancy, and we can exclude possible zero-point shifts or calibration problems. We have detected and measured systematic velocity differences produced by the different templates used in the cross-correlation. We conclude that such differences can in principle explain the effect, but in this case the non-trivial implication would be that the best-fitting template does not necessarily give the best velocity estimate. As we do not have any a priori reason to select a template different from the best-fitting one, we did not apply any correction to the ESO Slice Project velocities. However, as for a small number of galaxies the effect is so large that it is likely to have a physical explanation, we have also taken into account the possibility that the discrepancy can be partly real: in this case, it might help to understand the role of gas outflows in the process of galaxy evolution. In view of the future large spectroscopic surveys, we stress the importance of using different templates and making them publicly available, in order to assess the amplitude of systematic effects, and to allow a direct comparison of different catalogues.
The issue of the approximate isotropy and homogeneity of the observable universe is one of the major topics in modern Cosmology: the common use of the Friedmann-Robertson-Walker [FWR] metric relies on these assumptions. Therefore, results conflicting
with the ``canonical picture would be of the utmost importance. In a number of recent papers it has been suggested that strong evidence of a fractal distribution with dimension D~2 exists in several samples, including Abell clusters [ACO] and galaxies from the ESO Slice Project redshift survey [ESP].Here we report the results of an independent analysis of the radial density run,N(<R)~R^D, of the ESP and ACO data. For the ESP data the situation is such that the explored volume, albeit reasonably deep, is still influenced by the presence of large structures. Moreover, the depth of the ESP survey (z<0.2) is such to cause noticeable effects according to different choices of k-corrections, and this adds some additional uncertainty in the results. However, we find that for a variety of volume limited samples the dimensionality of the ESP sample is D~3, and the value $D = 2$ is always excluded at the level of at least five (bootstrap) standard deviations. The only way in which we reproduce D~2 is by both unphysically ignoring the galaxy k-correction and using Euclidean rather than FRW cosmological distances. In the cluster case the problems related to the choice of metrics and k-correction are much lessened, and we find that ACO clusters have D_{ACO} = 3.07 +- 0.18 and D_{ACO} = 2.93 +- 0.15 for richness class R geq 1 and R geq 0, respectively. Therefore D=2 is excluded with high significance also for the cluster data.
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