Astron. Astrophys. 321, 84-104 (1997)

2. The data

2.1. The ESO Nearby Abell Cluster Survey

The ENACS has provided reliable redshifts for 5634 galaxies in the directions of 107 cluster candidates from the catalogue of Abell, Corwin & Olowin (1989), with richness and mean redshift . Redshift estimates are mostly based on absorption lines, but for 1231 galaxies, emission lines were detectable in the spectrum. As described in Katgert et al. (1996, hereafter Paper I), for 62 galaxies the reality of the emission lines is doubtful, as judged from a comparison with the absorption-line redshift (in almost all cases these are galaxies with only one emission line detected in the spectrum). That leaves 1169 galaxies with reliable emission lines. For 586 of these, the redshift is based on both absorption and emission lines, and for the remaining 583 galaxies the redshift estimate is based exclusively on emission lines. The estimated redshift errors range from about 40 to slightly over 100 km/s, with the majority less than 70 km/s. For a detailed description of the characteristics of the ENACS data-base we refer to Paper I.

For almost all of the 5634 galaxies a calibrated R-magnitude estimate is available, from photographic photometry calibrated with CCD-imaging. The R-magnitudes of the galaxies with redshifts range from 13 to about 18, although the majority of the galaxies have R-magnitudes brighter than about 17.

For most of the galaxies in this survey we could not obtain a reliable morphological classification because the galaxies were identified on copies of survey plates made with Schmidt telescopes. However, we can identify star-forming (i.e. presumably late-type) galaxies on the basis of the presence of the relevant emission lines in their spectra. The clear advantage of selecting galaxies on the presence of spectral lines is that the selection is quite effective out to redshifts of , whereas it may already be difficult to reliably determine morphologies of galaxies at a redshift (e.g. Andreon 1993). The disadvantage of a selection on the basis of detectable emission-lines is that the absence of such lines does not decisively prove a galaxy to be an early-type galaxy. In other words: the class of galaxies without detectable emission lines is likely to contain also late-type galaxies with emission lines that are too faint to be detected, or without emission lines. In the following, we will nevertheless refer to the two galaxy populations as ELG and non-ELG. The ELG can be thought of as an almost pure late-type galaxy population (see Sect.  3.2), whereas the non-ELG are a mix of early- and late-type galaxies (which share the property that they do not have detectable emission lines).

2.2. The definition of redshift systems

The 107 pencil beam redshift surveys cover solid angles with angular diameters between 0.5 and about 1.0 . In these 107 redshift surveys, 220 systems were found that are compact in redshift and that contain at least 4 (but often several tens to a few hundred) member galaxies. These systems were identified in redshift space, by using the method of fixed gaps (see Paper I), which separates galaxies within a system (with velocity differences between 'neighbours' less than the chosen gap) from galaxies that do not belong to the system (because the velocity difference with the nearest system member is larger than the chosen gap). For the following discussion we will, in addition, divide the cluster Abell 548 into two components, following the suggestion of Escalera et al. (1994) (which was later confirmed by Davis et al. 1995) because the cluster is clearly bi-modal.

Membership of a given galaxy to a particular system requires that the galaxy has a velocity within the velocity limits of the system as defined with the fixed-gap method. For systems with at least 50 galaxies we applied an additional test for membership which uses both the velocity and position (see den Hartog & Katgert 1996; see also Mazure et al. 1996, hereafter Paper II). This second criterion removes 74 galaxies for which the combination of position in the cluster and relative radial velocity makes it unlikely that they are within the turn-around radius of their host system. These 74 'interlopers' occur in only 25 of the systems.

The 'interloper'-test involves an estimate of the mass-profile of the system, and therefore requires the centre of the system. Following den Hartog & Katgert (1996), we have assumed the centre to be (in order of preference): 1) the X-ray center, 2) the position of the brightest cluster member in the cluster core, provided it is at least one magnitude brighter than the second brightest member, and/or less than 0.25 h^-1 Mpc from the geometric center of the galaxy distribution. If these two methods could not be applied, we determined 3) the position of the peak in the surface density, viz. the position of the galaxy with the smallest distance to its N -th neighbour (with N the number of galaxies in the system). In several cases this position differed by more than 0.1 h^-1 Mpc from that of any of the 3 brightest cluster members. We then used 4) a luminosity weighted average position. If the latter was not nearer than 0.25 h^-1 Mpc to the geometric center, we used 5) the geometric center, as defined by the biweight averages of the galaxy positions (see, e.g., Beers, Flynn, & Gebhardt 1990). For 22 of the 25 systems with at least 50 galaxies, the position of the X-ray peak or that of the brightest cluster member were chosen as cluster centers.

2.3. The various samples of Galaxy systems

Our discussion of the differences between the average velocities of ELG and non-ELG within individual clusters will only be based on the 58 systems that contain at least 5 ELG: we consider this a minimum number for the estimation of a meaningful average velocity. In general, such systems also contain at least 5 non-ELG. However, for A3128 (), the number of non-ELG is less than 5 and we have therefore not considered it in the analysis for individual systems. That leaves a sample of 57 systems (sample 1) with both at least 5 ELG and 5 non-ELG.

In discussing velocity dispersions of individual systems we have limited ourselves to the subset of 18 systems with at least 10 ELG (all of which also have at least 10 non-ELG). I.e. we applied a lower limit to the ELG population that is identical to the one used in Paper II, in the discussion of the distribution of velocity dispersions of a complete volume-limited sample of rich clusters. The same restriction was applied in estimating projected harmonic mean radii: from numerical modeling we find that such estimates are biased if they are based on less than 10 positions. The sample of 18 systems with at least 10 ELG will be referred to as sample 2.

Finally, we will also discuss results for a sample of 75 systems with at least 20 members (sample 3). The requirement that the total number of galaxies in a system be at least 20 ensures that the centre of the system can be determined with sufficient accuracy. This sample also defines a 'synthetic' average cluster, which contains 3729 galaxies of which 559 are ELG.

In Table 1 we list some characteristics of all 87 systems in the 3 samples defined above, as well as of the 33 systems with a total number of members from 10 to 19, of which less than 5 are ELG. The total number of galaxies in these 120 systems is 4333, of which 809 are ELG. In col.(1) the ACO number (Abell et al. 1989) of the (parent) ACO system is given, and in col.(2) the average redshift of the system. Col.(3) gives the position of the centre of the system (B1950.0). The number of member galaxies, and the number of member ELG among these are given in col.(4) (note that these numbers do not include the 74 interlopers), and col.(5) lists the projected distance, , in h^-1 Mpc , of the galaxy farthest from the cluster centre.

Table 1. The data-set of 120 systems

Table 1. (continued)

Table 1. (continued)

If there are several systems along the line-of-sight to a given cluster, these are identified by their average redshift, which was obtained using the biweight estimator. Throughout this paper, averages are determined with the biweight estimator, since this is statistically more robust and efficient than the standard mean in computing the central location of a data-set (see Beers et al. 1990). When at least 15 velocities are available, velocity dispersions were also computed with the biweight estimator; however, for smaller number of redshifts we used the gapper estimator. These estimators yield the best robust estimates of the true values of location and scale of a given data-set, particularly when outliers are present.

2.4. The emission-line galaxies

In the wavelength range covered by the ENACS observations, and for the redshifts of the clusters studied, the principal emission lines that are observable are [OII] (3727 Å), H (4860 Å) and the [OIII] doublet (4959, 5007 Å). Note that because of the small aperture of the Optopus fibers (2.3 arcsec diameter), we have only sampled emission-lines in the very central regions of the galaxies. For the redshifts of our clusters the diameters of these regions are  h^-1 kpc . This should be kept in mind when making comparisons with other datasets for which the information about emission lines may refer to much larger or smaller apertures.

The emission lines were identified independently by two of us, in two different ways; first by examining the 2 - D Optopus CCD frames, and second by inspecting the uncleaned wavelength-calibrated 1-D spectra. Two lists of candidate ELG were thus produced, and for the relatively small number of cases in which there was no agreement, both the 2-D frames and 1-D spectra were examined again. The inspection of the 2-D frames allowed easy discrimination against cosmic-ray events (emission lines are soft and round as they are images of the fiber), and against sky-lines (since these are found at the same wavelength in all spectra). While examining the 1-D spectra we also obtained the wavelengths of the emission lines by fitting Gaussians superposed on a continuum to them.

The combined list of galaxies that show emission lines contains 1231 ELG. As mentioned earlier, for 62 of these we have good evidence that the emission line(s) are not real; in the large majority of these cases there is only one line. For a subset of 586 of the remaining 1169 ELG, the reality of the emission lines is borne out by the very good agreement between the absorption- and emission-line redshifts (see Paper I). For the other 583 ELG, no confirmation of the reality of the lines is available; we expect that in at most 10% of these cases the lines are not real.

Among the 1169 ELG there are 78 active galactic nuclei (AGN). These were identified either through the large velocity-width of the H line, or through the intensity ratios of the [OIII] and H lines, and the relative intensity of the [OII] line (if present). We are convinced that our criteria were sufficiently strict that all 78 galaxies that we classify as AGN are indeed bona fide AGN. However, at the same time, our criteria were probably too strict to identify all AGN in our dataset.

It should be realized that our ELG sample is not complete with regard to a well-defined limit in equivalent width of the various emission lines. Furthermore, the poorly-defined limit in equivalent width is probably not sufficiently low that essentially all galaxies with emission lines will have been identified as ELG. Therefore, the sample of non-ELG is very likely to contain a mix of real non-ELG (i.e. galaxies without emission-lines) and unrecognized ELG with emission lines that are too weak to be detected in the ENACS observations. Any difference between ELG and non-ELG that we may detect is therefore probably a reduced version of a real difference. For the same reason, the absence of an observable difference between ELG and non-ELG does not prove conclusively that there is no difference between the ELG and the other galaxies.

2.5. Completeness with regard to apparent magnitude

As was discussed in Paper I, spectroscopy was attempted for all galaxies in the fields of the target ACO clusters down to well-defined limits in isophotal magnitude. However, the success rate of the determination of an absorption-line redshift depends strongly on the signal-to-noise ratio in the galaxy spectrum. This in turn depends primarily on the surface brightness of that part of the galaxy that illuminates the fibre entrance. As a result, the success rate is highest for intermediate magnitudes, and decreases somewhat for brighter galaxies (as those are large, so that only a small fraction of the total flux is sampled), and quite noticeably for fainter galaxies for which the total flux is smaller. On the contrary, the succes rate for the detection of emission lines does not appear to depend significantly on the brightness of the galaxy. Therefore, the relative distribution with regard to magnitude of ELG and non-ELG can be different, as it is generally easier to obtain a redshift for a faint galaxy if it has emission-lines in its spectrum, than if it has not.

This is illustrated in Fig. 1 where we show the apparent magnitude distribution of the 4447 galaxies with redshifts determined only from absorption lines, of the 585 galaxies with redshifts determined using both absorption and emission lines, and of the 583 galaxies for which the redshift is based only on emission lines (for 19 of the 5634 galaxies magnitudes are not available). The magnitude distribution of the galaxies with redshift based on emission lines only is significantly different from the other two (with probability, according to a Kolmogorov-Smirnov test, see e.g. Press et al. 1986). This figure clearly illustrates the fact that at faint magnitudes it is generally more difficult to obtain a redshift from absorption lines than from emission lines.

Fig. 1. The normalized distribution with regard to apparent magnitude (R25) for three subsets of the ENACS: the 4447 galaxies with redshift based solely on absorption lines (heavy-line histogram), the 585 galaxies with redshift based both on absorption and emission lines (solid line histogram), and the 583 galaxies with redshift based solely on one or more emission lines (dotted-line histogram).

From Fig. 1 it is clear that the apparent fraction of ELG varies considerably with magnitude. When calculating the intrinsic ELG fraction one must take this magnitude bias into account (see also Sect.  3.1). However, the magnitude bias is unlikely to be relevant in the analysis of the kinematics and the space distribution of ELG and non-ELG. Since it has been established that velocities and projected clustercentric distances are only very mildly correlated with magnitude (see, e.g., Yepes, Dominguez-Tenreiro & del Pozo-Sanz 1991, and Biviano et al. 1992, and references therein), it seems safe to assume that the different magnitude distributions of ELG and non-ELG will not affect our analysis of the observed space distribution and kinematics.

The magnitude bias in Fig. 1 could affect distributions of clustercentric distance if in the ENACS the magnitude limit would vary with distance from the cluster center. However, when we compare the catalogues of cluster galaxies for which we obtained an ENACS redshift with the (larger) catalogues of all galaxies brighter than our magnitude limit (see Paper I), we find that no bias is present. In other words: in all clusters that we observed in the ENACS the completeness of the redshift determinations does not depend on distance from the cluster center. This conclusion is strengthened by a comparison of our spectroscopic catalogue with the nominally complete photometric catalogues of Dressler (1980b), for the 10 clusters that we have in common. Again, we detect no dependence of the completeness on clustercentric distance.

We conclude therefore that the magnitude bias, which causes the apparent fraction of ELG to increase strongly towards the magnitude limit of the ENACS, only affects the estimation of the intrinsic ELG fraction. As is apparent from Fig. 1, that bias can be avoided by restricting the analysis to the 585 ELG for which also an absorption-line redshift could be obtained. However, it must be realized that this remedy against the magnitude bias for ELG has one disadvantage: it is likely to select against late-type spirals as these occur preferentially in the class of ELG without absorption-line redshift. We will come back to this in Sect.  3.2.

For surveys of (cluster) galaxies at higher redshifts (and fainter apparent magnitudes), which therefore have an inevitable observational bias against galaxies without detectable emission lines, this bias can in general not be corrected. Unless one has redshifts for all galaxies, e.g. down to a given magnitude limit, conclusions drawn from such 'incomplete' samples can be seriously biased, as they refer mostly to ELG. One obvious example is the determination of the fraction of ELG as a function of redshift, but e.g. also the determination of the evolution of cluster properties can be seriously affected. This problem may be aggravated if, as we will discuss below (see Sect.  5), the spatial distributions of ELG and non-ELG are not the same.

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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