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Astron. Astrophys. 360, 861-870 (2000)

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2. Observations and data reduction

We select our targets among the candidate EIS clusters in patches C and D, with estimated redshift [FORMULA] (Scodeggio et al. 1999). We do not apply any additional criterion for the selection of our cluster candidates. The size of our sample is one tenth of the total of 36+28 EIS clusters in patches C and D within the above-mentioned redshift range.

Our targets are listed in Table 1. In Col. (1) we list the cluster candidate identification name, in Cols. (2) and (3) the right ascension and declination (J2000), in Col. (4) the cluster richness (see Olsen et al. 1999a), and in Col. (5) the matched-filter redshift estimate, [FORMULA] (see Scodeggio et al. 1999). In Col. (6) we list the number of galaxies targeted for multi-slit spectroscopy in each cluster field, and in Col. (7) the number of successful redshift estimates.


[TABLE]

Table 1. Cluster candidates selected for observation


The observations were carried out at the 3.6 m ESO telescope at La Silla, Chile, during two nights in February 1999. The weather conditions were good, with seeing slightly above 1", in partial moonlight. With a total useful observing time of 16 hours over the two nights, we were able to obtain [FORMULA] min exposures for each cluster.

We observed with EFOSC2 in Multi-Object Spectroscopy (MOS) mode. EFOSC2 was equipped with a Loral CCD of 2048[FORMULA]2048 with 15 µm pixels, allowing for an unvignetted field-of-view of 3.8´[FORMULA]5.5´. We used Grism # 1, giving a spectral range 3185-10940 Å, and a dispersion of 6.3 Å/pixel. On the MOS masks our slits were 1.2" wide.

We obtained spectra for 102 objects in the magnitude range [FORMULA], where [FORMULA] is the apparent magnitude in the [FORMULA] band (Nonino et al. 1999). In Figs. 1-6 we show [FORMULA]-band images of the six EIS candidate clusters. Small circles mark galaxies with redshift, large circles mark galaxies belonging to significant overdensities in redshift space (see Sect. 4).

[FIGURE] Fig. 1. An [FORMULA]-band image of the EIS0533-2353 cluster candidate. The circles indicate those galaxies for which we obtained redshifts. The field is 6.4´[FORMULA]4.3´, centered at [FORMULA]05:33:36.5, [FORMULA]23:53:53 (J2000). North is up, East is to the left.

[FIGURE] Fig. 2. Same as Fig. 1, for the EIS0540-2418 cluster candidate. The field is centered at [FORMULA]05:40:08.5, [FORMULA]24:18:19 (J2000).

[FIGURE] Fig. 3. Same as Fig. 1, for the EIS0950-2154 cluster candidate. Large circles indicate galaxies identified as members of a significant overdensity in redshift space. The field is centered at [FORMULA]09:50:48.6, [FORMULA]21:55:15 (J2000).

[FIGURE] Fig. 4. Same as Fig. 1, for the EIS0951-2047 cluster candidate. Large circles indicate galaxies identified as members of a significant overdensity in redshift space. The field is centered at [FORMULA]09:51:31.8, [FORMULA]20:46:57 (J2000)

[FIGURE] Fig. 5. Same as Fig. 1, for the EIS0955-2113 cluster candidate. Large circles are galaxies members of a significant overdensity in redshift space. The field is centered at [FORMULA]09:55:32.1, [FORMULA]21:13:55 (J2000).

[FIGURE] Fig. 6. Same as Fig. 1, for the EIS0956-2009 cluster candidate. Large circles indicate galaxies identified as members of a significant overdensity in redshift space. The field is centered at [FORMULA]09:56:28.4, [FORMULA]20:09:30 (J2000).

We reduce the data with standard IRAF 1 packages. We determine redshifts using the task XCSAO that implements the cross-correlation technique of Tonry & Davis (1979). We use several real and synthetic templates for the cross-correlation. We use emission lines, where present in the spectrum of the object, to determine the redshift with the task EMSAO. We examine visually all spectra, by overplotting the positions of the major spectral features redshifted at the redshift(s) determined by the automatic techniques described above. We employ particular care in flagging those features that could be contaminated by night-sky lines.

In total, we determine 67 galaxy redshifts, from a minimum of [FORMULA] to a maximum of [FORMULA], with an average [FORMULA]. One of our objects turns out to be a QSO at [FORMULA]. We do not consider this object in our analysis. An internal estimate of the typical redshift uncertainty is [FORMULA]. The success-rate is magnitude-dependent, as can be seen in Fig. 7: it is 85% for [FORMULA] and decreases to 57% for fainter galaxies.

[FIGURE] Fig. 7. The [FORMULA]-band magnitude distribution of the 102 targets for which we attempted spectroscopy (dashed-line histogram), and of the 67 galaxies (plus a QSO) for which we obtained a redshift (solid-line histogram).

We list in Table 2 the galaxies with measured redshift. In Col. (1) we list the name of the EIS candidate cluster field, in Col. (2) a galaxy identification number, in Cols. (3) and (4) the (J2000) right ascension and the declination of the galaxy, in Col. (5) the [FORMULA] magnitude, in Col. (6) the redshift, and in Col. (7) the galaxy set to which the galaxy is assigned. These galaxy sets are defined in Sect. 3 and listed in Table 3.


[TABLE]

Table 2. Galaxies in the six EIS cluster fields



[TABLE]

Table 2. (continued)



[TABLE]

Table 3. Sets of galaxies in the EIS cluster fields


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© European Southern Observatory (ESO) 2000

Online publication: August 23, 2000
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