3. The ELG fraction in clusters and the field
3.1. Bias against galaxies without emission lines
In Fig. 2 we show the fraction of ELG as a function of apparent magnitude. The open symbols represent the apparent ELG fraction, calculated as the total number of galaxies in the ENACS with emission lines, divided by the total number of galaxies in the ENACS in the same magnitude range, viz. as:
where n is the number of systems, each containing galaxies with redshifts, of which are ELG. The strong increase of the apparent ELG fraction towards fainter magnitudes is evident. As discussed in Sect. 2.5, this increase must be due to the bias that operates against the successful determination of redshifts for faint galaxies without emission lines.
This bias can be overcome if we calculate the fraction of ELG as the ratio of the number of the ELG for which also an absorption-line redshift is available, by the total number of galaxies with an absorption-line redshift. By definition, the magnitude bias does not operate in this comparison. The filled symbols in Fig. 2 give the resulting ELG fraction as a function of magnitude. As we anticipated, there is essentially no dependence of this corrected, true ELG fraction on magnitude, and it is considerably lower than the apparent fraction, especially at fainter magnitudes. Only for the brighter galaxies, for which there is no bias against the detection of an absorption-line based redshift, are the apparent and true ELG fraction essentially identical.
The apparent ELG fraction in the ENACS is 0.21 (= 1169 / 5634), but the corrected value is 0.12 (= 586 / 5051). In this paper we will always distinguish between the apparent and true ELG fractions, where the latter is calculated from the sample of all galaxies with absorption-line redshifts.
As far as we are aware, the correction for magnitude bias has not been applied in earlier work on the ELG fraction. In comparing our results with other determinations this should always be kept in mind. It is quite possible that some of the earlier results are not affected by magnitude bias, but it is often difficult to find out if that is a reasonable assumption. In a comparison with the results of the ESO Slice Project (ESP, see e.g. Zucca et al. 1995), for which the same instrumentation was used as for the ENACS, there may be differences in bias which influence the result. The reason for this is that the fraction of galaxies with emission lines is larger in the field (the object of study in the ESP) than it is in our clusters.
All ELG fractions based on the ENACS include a small contribution from AGN. Among interlopers and in systems with (which in the ENACS provide the best approximation to the 'field'), the AGN fraction is . For the systems with (real, massive clusters) it is only . These values are lower than the values previously obtained by Dressler et al. (1985), Hill & Oegerle (1993), and Salzer et al. (1989, 1995), but this may be due (at least partly) to the fact that we have been conservative in classifying galaxies as AGN, and have probably accepted only those with the strongest and broadest lines (see Sect. 2.4). The ratio of the AGN fraction in the field and in clusters is , consistent with the value we find for all ELG (see Sect. 3.2). Dressler et al. (1985) found a similar value for the ratio between the AGN fraction in the field and in clusters.
3.2. The fraction of ELG in clusters and in the field
The ELG fractions in clusters and field have been studied by several authors, in order to find out if there is evidence for a difference in the occurence of ELG which can be traced to the influence of the environment in which galaxies live. Even though the ENACS, by its very nature, does not contain many field galaxies, it contains a sufficient number that we can investigate possible differences between the ELG fractions in the field and in clusters.
It is not trivial to identify the field galaxies in the ENACS. The main reason is that galaxies that are in small groups with only a few measured redshifts could, on the one hand, be in the field but, on the other hand, they could equally well be 'tips of the iceberg'. In other words: the number of measured redshifts in a group is not a good criterion for assigning galaxies to the field or to a cluster. One thing that is fairly certain is that the interlopers that were removed from the systems on the basis of their position and velocity (see Sect. 2.2) belong to the field and we consider them to be the best approximation to the field in the ENACS. Second best are the isolated galaxies. Finally, galaxies in groups with at most 3 measured redshifts are acceptable candidates for field galaxies, since the reality of such groups with less than 4 members is doubtful, as the definition of systems with such a small number of members is not at all robust (see Paper I). To a lesser extent the systems with 4 to about 10 redshifts also do not have a very robust definition (ibid.) but those we have included neither as cluster nor as field in the comparison between field and clusters. Finally, systems with at least 10 measured redshifts are very likely to be real clusters or groups.
In Table 2 we show the resulting ELG fractions for the three classes of environment. Note that for all three categories the fractions have been calculated as in Sect. 3.1. For each class we have calculated the apparent as well as the true ELG fractions. The ELG fractions for the interlopers and the systems are quite similar, and they are both quite different from the average ELG fraction in clusters. Because the galaxies for which the redshift is based solely on emission lines have, on average, fainter magnitudes, the difference appears most striking in the apparent fractions, but it is equally significant in the bias-corrected, true values.
Table 2. The fraction of ELG in different environments
From the numbers in Table 2 we conclude that it is not unreasonable to assume that the interlopers and the galaxies in the systems give a fair estimate of the ELG fraction in the field: combining the two classes we obtain apparent and true ELG fractions of and respectively. It is interesting to note that the corresponding numbers for the systems with are and . This clearly suggests that these systems are indeed intermediate between real clusters and field galaxies. Additional support for the assumption that the systems with are indeed almost all clusters is provided by the ELG fractions for the systems with . For those, there is no doubt at all that they are clusters and their average apparent and corrected ELG fractions are and respectively.
Our apparent ELG fraction for the 'field' is quite similar to that derived by Zucca et al. (1995), who found a value of in the ESO Slice Project. This is quite gratifying, as these authors obtained their spectra using an observational set-up that was essentially identical to ours. It is true that the average redshift in their survey is about a factor of 2 larger than in the ENACS, and their result therefore applies to a larger region in the centre of the galaxies than does ours. Apparently, this has little or no effect on the apparent ELG fraction. The ELG fraction found by Salzer et al. (1995) is 0.31, i.e. intermediate between our apparent and true fractions.
Our apparent ELG fraction for the field is significantly lower than the value of that was found by Gisler (1978). On the contrary, it is higher than the value of found by Dressler et al. (1985), as well as the value of found by Hill & Oegerle (1993). However, as it is not clear whether we should compare the literature values with our apparent or bias-corrected values, the latter two determinations could actually be consistent with our result.
A similar uncertainty is present in the comparison of our cluster ELG fraction with earlier estimates in the literature. Our apparent value of is consistent with the value found by Gisler (1978) in compact clusters (), but quite a bit higher than the values of and found by Dressler et al. (1985), and Hill & Oegerle (1993), respectively. If the latter two literature values should in fact be compared with our bias-corrected value of the agreement becomes somewhat better, although not perfect. As we shall see in Sect. 3.3, part of the remaining difference in the cluster ELG fraction may be due to the composition of the cluster samples with respect to mass (or global velocity dispersion).
There are several other factors of this kind which can, at least in principle, influence the observed ELG fraction. Among these are: the average luminosity of the galaxy sample, the criterion by which cluster members and field galaxies are identified, and (as mentioned earlier) the linear sizes of the average aperture used in the spectroscopy. The latter factor may well explain the differences with the values obtained by Gisler (1978), who used spectra with a larger effective aperture; this may be the reason for the systematically high values that he obtained for the ELG fraction. On the other hand, the sample studied by Dressler et al. (1985) could be biased against late-type spirals and irregulars (see Dressler & Shectman 1988). As these have a relatively high ELG fraction, this might well explain why their ELG fractions (for cluster as well as for the field) are low.
Although the absolute values of the ELG fractions obtained by different authors may thus be difficult to compare (e.g. due to differences in observational set-up etc.), the relative fractions of ELG located in different environments might well be less dependent on such details. In the ENACS the ratio between the ELG fraction in the field and in clusters is (apparent) and (bias-corrected). The average ratios found previously are: (Gisler 1978), (Dressler et al. 1985) and (Hill & Oegerle 1993). The uncertainties are rather large, but there may be some evidence that details of the various techniques and the galaxy and/or cluster selection, have influenced even the relative frequency of occurence of ELG in cluster and field. On the other hand, the mix of the various types of galaxy may not be the same in the different samples so that, with different ELG fractions for the various galaxy types, the ratio between the ELG fractions are expected to be different.
In Table 3 we show the values of the ELG fraction for galaxies in clusters as a function of morphological type. These fractions are based on the ENACS data in combination with the morphologies determined by Dressler (1980b) for the 545 galaxies in the 10 clusters that are common between the ENACS and the Dressler catalogue. Almost all of these (namely 537) have an absorption-line ENACS redshift; 68 of the 537 galaxies (i.e. 13%) also have emission lines. Of the 68 ELG (none of which is an AGN), 60 are spirals or irregulars, 7 are S0s and 1 is an elliptical. We thus find that the fraction of ELG depends strongly on morphological type. Note that the ELG fractions in Table 3 are unbiased, as all galaxies used in the statistics have absorption-line redshifts.
Table 3. The fraction of ELG for cluster galaxies of different morphological types
It is also of interest to determine the fraction of spirals that we have detected as ELG. Of the 180 spirals in the sample of 537 galaxies, only 60 are ELG. So, while most of our ELG are late-type galaxies, the ELG represent only about of the total spiral population in our clusters.
Since the mix of galaxy types is a strong function of the density of the environment, one may ask whether the difference between the ELG fractions in the clusters and in the field can be totally attributed to a lower fraction of late-type galaxies in clusters. Following Dressler et al. (1985), we have used the ELG fractions of cluster galaxies for the different galaxy types, and convolved that with the distribution over galaxy type of field galaxies. This should yield the ELG fraction that clusters would have if their morphological mix were the same as that of the field. In the ENACS there are only very few field galaxies with known type. Therefore, we have assumed the field type mix given by Oemler (1974), with which we calculate an expected field ELG fraction of . This value, which is based on the assumption that the dependence of ELG fraction over morphological type is identical in cluster and field, is of course fully consistent with our observed, bias-corrected value for the field ELG fraction.
This result is at variance with all previous findings on this point (Osterbrock 1960, Gisler 1978, Dressler et al. 1985, Hill & Oegerle 1993). It can be rephrased by saying that environmental effects probably do not affect the fraction of ELG, or the emission-line activity. Note that, had we not accounted for the magnitude bias (the fact that the apparent ELG fraction increases towards faint magnitudes), we would have come to the same conclusion as the above-mentioned authors. However, the magnitude bias is stronger for the field galaxies than for the cluster sample (because our field galaxies are on average fainter than our cluster galaxies). As a result, the need for different emission-line characteristics of field and cluster galaxies disappears if the bias is taken into account.
At this point we must come back to the selection against late-type spirals which is inherent in our calculation of the true ELG fraction, since the latter is based only on the ELG with absorption-line redshift (see Sect. 2.5). We have attempted to take this factor into account, by assuming that most of the ELG without absorption-line redshift in the field are late-type spirals. Our best estimate of the fraction of late-type spirals among our field spirals is about 50%, although we cannot exclude that it is 70%. Using the former fraction together with the ELG fractions for early- and late-type spirals in Table 3, we estimate an expected ELG fraction in the field of instead of . This is still consistent with the observed true ELG fraction in the field of , so that the conclusion in the preceding paragraph is not likely to be the result of the selection against late-type spirals in the calculation of true ELG fractions.
3.3. The ELG fraction as a function of velocity dispersion
In Sect. 3.2 we found that is practically independent of N for systems with . On the other hand, we also noted that some of the differences between our ELG fractions and those of other authors might be due to different composition of cluster samples in terms of mass, or some other physically relevant parameter. An obvious question is therefore if, within the ENACS data, we observe a dependence of the ELG fraction on the global velocity dispersion of the system. In Fig. 3 we show as a function of velocity dispersion, where was calculated as in Sect. 3.1 in three separate intervals of . For this figure, we have used only the 75 systems with of sample 3, as these are very likely to be bona-fide rich clusters. It is clear that there is a significant decrease of the ELG fraction with increasing velocity dispersion, by a factor of 1.5 over the range of dispersions sampled. Within the errors, the same result is obtained if we use the sample of all 120 systems with listed in Table 1.
On average, clusters with smaller velocity dispersions are less rich than clusters with larger velocity dispersions (see, e.g., Paper II). Since essentially all ELG are spirals, the above result is thus consistent with van den Bergh's (1962) finding that the fraction of late-type galaxies is higher in poorer clusters. We must point out that the dependence on is not induced by different sizes of the area over which we obtained spectroscopy for the different clusters. This could have an effect, in principle, as a consequence of the morphology-density relation and because the clusters for which the observations covered a larger area have a (slightly) higher than average. However, if we consider only galaxies within 1 h-1 Mpc of their respective cluster center, in those 51 clusters (with ) observed at least out to 1 h-1 Mpc , the relation between and is unchanged.
We conclude therefore that there is a significant decrease of the fraction of ELG, with increasing , which must reflect a dependence on mass.
© European Southern Observatory (ESO) 1997
Online publication: June 30, 1998