Peter Katgert and Alain Mazure
The rest of the members are:
Roland den Hartog (ESTEC, SA division, Noordwijk, The Netherlands)
Pierre Dubath (Observatoire de Geneve, Switzerland)
Giuliano Giuricin and Eric Escalera (SISSA, Trieste, Italy)
Paola Focardi (Bologna University, Italy)
Daniel Gerbal (Institut d'Astrophysique, Paris, France)
Bernard Jones (Theoretical Astrophysics Centre, Copenhagen, Denmark)
Olivier Le Fevre (Lab. Astronomie Spatiale, Marseille, France)
Jaime Perea (Astrophysics Institute of Andalucia, Granada, Spain)
Mariano Moles (Instituto de Matematica y Fisica Fundamental, Madrid, Spain)
George Rhee (University of Nevada, Las Vegas, U.S.A.).
Christophe Adami (Laboratoire d Astronomie Spatiale, Marseille, France)
The extensive observations were carried out with the OPTOPUS multi-fibre spectrograph attached to the ESO 3.6-metre telescope at the La Silla Observatory, during 35 nights in the period from September 1989 to October 1993. With this very efficient spectrograph, the spectra of about 50 galaxies could be recorded simultaneously, dramatically reducing the necessary observing time. In total, the programme has yielded reliable radial velocities for more than 5600 galaxies in the direction of about 100 rich clusters up to a redshift z=0.1.
The new observations approximately double the amount of data available for rich clusters of galaxies. In combination with earlier data, the ENACS has produced a `complete' sample of 128 rich Abell clusters in a region centered near the south galactic pole and comprising about one-fifth of the entire sky.
The data of the ENACS prove
conclusively that 90 percent of the rich, nearby Abell clusters are
real: i.e. many of the galaxies observed in each of these clusters are
indeed at the same distance and they form a physical entity.
About one-quarter of the galaxies in the ENACS do not belong to the
clusters and reside in much smaller galaxy groups or are located in the
space in between. This can be clearly seen in the distribution of the
velocities in the direction of each of the clusters, shown in the
This diagramme gives an overview of the velocities of 5634 individual galaxies which were measured during the ESO Nearby Abell Cluster Survey (ENACS) in the directions of 107 rich and nearby Abell clusters. Each bar plot refers to an individual cluster, the designation of which is shown to the left, together with the total number of galaxy redshifts measured for the cluster.
Each vertical line represents one galaxy, and the position in the bar indicates the measured value of its velocity according to the scale at the bottom. Galaxies that show emission lines in their spectrum are indicated by dashed vertical lines; the others are fully drawn.
A comparison between the ENACS catalogue and the COSMOS Galaxy Catalogue has been made. When cross-correlating the two catalogues it is found that, at least in the areas of the ENACS clusters, the completeness of the COSMOS catalogue is somewhat lower than was estimated previously for the carefully analyzed and well-calibrated part of the COSMOS catalogue known as the Edinburgh-Durham Southern Galaxy Survey (EDSGC). The galaxy positions in the COSMOS and ENACS catalogues are found to be on the same system to within about one arcsecond.
For the clusters for which the photometry in the ENACS and COSMOS catalogues is based on the same survey plates, the two magnitude scales agree very well. It is confirmed that the photometric calibration in the EDSGC subset of the COSMOS catalogue is of higher quality than in the EDSGC complement. The ENACS galaxy samples are unbiased subsets of the COSMOS catalogue as far as the projected galaxy distribution is concerned, except in only a few cases.
For a subset of 80 of these clusters we can calculate a reliable velocity dispersion, based on at least 10 (but very often between 30 and 150) redshifts. The main observational problem that hampers an unambiguous interpretation of the distribution of cluster velocity dispersions, namely the contamination by fore- and background galaxies has been discussed as well as the completeness of the cluster samples for which the distribution of cluster velocity dispersions has been derived. It is found that a cluster sample which is complete in terms of the field-corrected richness count given in the ACO catalogue gives a result that is essentially identical to that based on a smaller and more conservative sample which is complete in terms of an intrinsic richness count that has been corrected for superposition effects.
It is found that the large apparent spread in the relation between velocity dispersion and richness count (based either on visual inspection or on machine counts) must be largely intrinsic; i.e. this spread is not primarily due to measurement uncertainties. One of the consequences of the (very) broad relation between cluster richness and velocity dispersion is that all samples of clusters that are defined complete with respect to richness count are unavoidably biased against low sigma_V clusters. For the richness limit of our sample this bias operates only for velocity dispersions less than 800 km/sec.
A statistically reliable distribution of global velocity dispersions is obtained which, for velocity dispersions sigma_V > 800 km/s, is free from systematic errors and biases. Above this value of sigma_V the distribution agrees very well with the most recent determination of the distribution of cluster X-ray temperatures, from which it is concluded that beta = 1.
The observed distribution n(>sigma_V), and especially its high tail above 800 km/s, provides a reliable and discriminative constraint on cosmological scenarios for the formation of structure.It stresses the need for model predictions that produce exactly the same information as do the observations, namely dispersions of line-of-sight velocity of galaxies within the turn-around radius and inside a cylinder rather than a sphere, for a sample of model clusters with a richness limit that mimics that of the sample of observed clusters.
Well over 90% of the ELG in the ENACS appear to be spirals; however, we estimate that the detected ELG represent only about one-third of the total spiral population. The apparent fraction of ELG increases towards fainter magnitude, as redshifts are more easily obtained from emission lines than from absorption lines. From the ELG that have an absorption-line redshift as well, a true ELG fraction in clusters is derived of the order of 0.10, while the apparent fraction is 0.16. The apparent ELG fraction in the field is 0.42, while the true fraction is 0.21. The true ELG fractions in field and clusters are consistent if the differences in morphological mix are taken into account. Thus, it is not necessary to assume that ELG in and outside clusters have different emission-line properties. The average ELG fraction in clusters is found to depend on global velocity dispersion.
Combining the data for 75 clusters, it is found that velocity dispersion of the ELG is, on average, 20% larger than that of the other galaxies. The spatial distribution of the ELG is significantly less peaked towards the centre than that of the other galaxies. This causes the average projected density around ELG to be 30% lower than it is around the other galaxies. In combination with the inevitable magnitude bias against galaxies without detectable emission lines, this can lead to serious systematic effects in the study of distant clusters.
The virial estimates of the cluster masses based on the ELG only are, on average, about 50% higher than those derived from the other galaxies. This indicates that the ELG are either on orbits that are significantly different from those of the other galaxies, or that the ELG are not in virial equilibrium with the other galaxies, or both. The velocity dispersion profile of the ELG is found to be consistent with the ELG being on more radial orbits than the other galaxies. For the ELG, a ratio between tangential and radial velocity dispersion of 0.3 to 0.8 seems most likely, while for the other galaxies the data are consistent with isotropic orbits. The lower amount of central concentration, the larger value of the velocity dispersion and the possible orbital anisotropy of the ELG, as well as their content of line-emitting gas would be consistent with a picture in which possibly all spirals (but certainly the late-type ones) have not yet traversed the virialized cluster core, and may even be on a first (infall) approach towards the central, high-density region.
The 19 clusters with the most regular projected galaxy distributions appear to define a quite narrow FP, which is qualitatively similar to the FP found by Schaeffer et al., who used other clusters. The ENACS cluster FP appears to be quite different from that of ellipticals. The FP of cluster galaxies also differs substantially from the virial prediction. This may imply that the dynamical structure of rich clusters spans a wide range, or that M/L varies with other cluster properties in different ways for different clusters, or that some of the clusters are not fully virialized, despite their regular appearance. An important part of the observed scatter around the FP is likely to be intrinsic.
If this intrinsic spread is exclusively due to deviations from a Hubble flow it implies cluster peculiar velocities not greater than about 1000\ks.
For 20% of these, the distribution of galaxies in the COSMOS catalogue does not allow a reliable centre position to be determined. For the other 62 clusters, we first determined the centre and elongation of the galaxy distribution. Subsequently, we made Maximum-Likelihood fits to the distribution of COSMOS galaxies for 4 theoretical profiles, two with `cores' (generalized King- and Hubble-profiles) and two with `cusps' (generalized Navarro, Frenk and White, or NFW, and de~Vaucouleurs profiles).
Average core radii (or characteristic radii for the profiles without core) are obtained of 128, 189, 292 and 1582 kpc for fits with King, Hubble, NFW and de~Vaucouleurs profiles respectively, with dispersions around these average values of 88, 116, 191 and 771 kpc. There is very good agreement on the outer logarithmic slope of the projected galaxy distribution, which is that for the non-generalized King- and Hubble-profile with the corresponding values for the two other model-profiles).
The Likelihood ratio is used to investigate whether the observations are significantly better described by profiles with cusps or by profiles with cores. Taking the King and NFW profiles as `model' of either class, it is found that about 75% of the clusters are better fit by the King profile than by the NFW profile. However, for the individual clusters the preference for the King profile is rarely significant at a confidence level of more than 90%. When limited to the central regions it appears that the signifance increases drastically, with 65% of the clusters showing a strong preference for a King over an NFW profile. At the same time, about 10% of the clusters are clearly better fitted by an NFW profile than by a King profile in their centres.
Composite clusters from the COSMOS and ENACS data are constructed, taking special care to avoid the creation of artificial cusps (due to ellipticity), and the destruction of real cusps (due to non-perfect centering). When adding the galaxy distributions to produce a composite cluster, either no scaling is applied of the projected distances, scaling with the core radii of the individual clusters or scaling with r_200, which is designed to take differences in mass into account. In all three cases it is found that the King profile is clearly preferred (at more than 95% confidence) over the NFW profile (over the entire aperture of 5 core-radii). However, this `preference' is not shared by the brightest (M_b > -18.4) galaxies. It is concluded that the brighter galaxies are represented almost equally well by King and NFW profiles, but that the distribution of the fainter galaxies clearly shows a core rather than a cusp.
Finally, the outer slope of the galaxy distributions in our clusters is compared with results for model calculations for various choices of fluctuation spectrum and cosmological parameters. It is concluded that the observed profile slope indicates a low value for Omega_0. This is consistent with the direct estimate of Omega_0 based on the M/L-ratios of the individual clusters.
It is found evidence of luminosity segregation for galaxies brighter than M_R < -21.5, i.e. typically the four brightest members of each cluster. It is also found evidence of morphological segregation: both the core-radius and the velocity dispersion increase along the Hubble sequence (ellipticals - S0 - early spirals - late spirals).
Galaxies of different types have different velocity dispersion profiles, being steeper for later type galaxies. Simple modelling allows to show that elliptical (and, to a lesser extent, S0) orbits are mostly tangential in the cluster core, and nearly isotropic outside, while spiral (in particular late-spiral) orbits are predominantly radial.
A viable interpretation of these results is that (1) late spirals, at variance with other type galaxies, are a non-virialized cluster population, still on partially radial infalling orbits, (2) the elliptical phase-space distribution is evolving towards energy equipartition through the process of dynamical friction, (3) S0 and early-spirals have intermediate distributions between these two extremes.
A standard candle is defined to determine the distance of clusters of galaxies and to investigate their peculiar velocities by using the nth rank galaxy (m_n). The question of the universality of the luminosity function is addressed for a sample of 28 rich clusters of galaxies (cz < 20000 km/s) in order to model the influence on m_n of cluster richness. This luminosity function is found to be universal and the fit of a Schechter profile gives alpha=-1.50 +- 0.11 and M_b =-19.91 +- 0.21 in the range [-21,-17]. The uncorrected distance indicator m_n is more efficient for the first ranks n. With n=5, it has a dispersion of 0.61 magnitude for the (m_n,5log(cz)) relation. When it is corrected for the richness effect and for the background galaxies the uncertainty is reduced to 0.21 magnitude with n=15.
Simulations show that a large part of this dispersion originates from the intrinsic scatter of the standard candle itself. These provide upper bounds on the amplitude of cluster radial peculiar motions. At a confidence level of 90%, the dispersion is 0.13 magnitude and the random velocity is limited to 1200 km/s.
Distribution of velocity dispersions
Kinematical behaviour and distribution of ELG.
Fundamental Plane for Clusters of Galaxies.
Distribution and Kinematics of Early- and Late-type galaxies.
Segregation in Clusters
Galaxy Associations in Clusters
Cluster LF - Distance indicator.