5 Catalogue presentation

In Table 2 we list the fields we observed with OPTOPUS (see Sect. 1 and Paper I) and their properties. Column (1) gives the number of the field, whose centre coordinates ($\alpha$ and $\delta$) are reported in Cols. (2) and (3). Col. (4) gives the total number of objects classified as galaxies in the photometric catalogue ($N_{\rm T}$), while Cols. (5), (6), (7) and (8) give the number of redshifts ($N_{\rm Z}$), of not-observed objects ($N_{\rm NO}$), of failed spectra (i.e. not useful to obtain a redshift determination, $N_{\rm F}$) and of stars ($N_{\rm S}$), respectively. From these numbers the redshift completeness of each field can be derived (see also Paper I and Zucca et al. 1997) as

$${ { N_{\rm Z} } \over { N_{\rm T} - N_{\rm S} - 0.122 * N_{\rm NO} } }\cdot$$ (2)

This equation assumes that the failed spectra correspond to galaxies, because stars, being point-like objects, have on average a better signal-to-noise ratio than galaxies, and that the percentage of stars in not-observed objects spectra is the same as in the spectroscopic sample (i.e. $\sim\! 12.2\%$).

Note that the field centres are separated by 30 arcmin both in right ascension and declination whilst the OPTOPUS fields have a diameter of 32 arcmin. Therefore there is a small overlap between the fields which results in the fact that some galaxies belong to two adjacent fields; the galaxies in the overlap areas have been assigned to the field whose centre is closer to the object position.

 

Table 3: Sample page of the catalogue (the whole catalogue is available only in electronic form)

We observed a total of 4043 objects, corresponding to $\sim\! 90\%$ of the parent photometric sample of 4487 objects. Out of the 4043 observed objects, 493 turned out to be stars and 207 have a too low signal-to-noise ratio to provide a reliable redshift (failed spectra). In the end, our final sample consists of a total of 3343 objects with reliable redshifts (3342 galaxies and 1 QSO).

The survey data are available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html. In Table 3 we provide a sample page of the catalogue, which is sorted in right ascension. The columns contain the following information:
Column (1): ESP galaxy number.
Column (2): OPTOPUS field number.
Column (3): Right Ascension (1950).
Column (4): Declination (1950).
Column (5): $b_{\rm J}$ magnitude.
Column (6): Heliocentric Radial Velocity from absorption lines in km/s.
Column (7): Associated Internal Error in km/s.
Column (8): Value of the R parameter (Tonry & Davis 1979) from cross correlation.
Column (9): Heliocentric Radial Velocity from emission lines in km/s.
Column (10): Associated Error in km/s.

The codes -9999 and -8888 in the velocity columns indicate stellar spectra (hence stars misclassified in the EDSGC) and spectra not useful for radial velocity measurements, respectively. Objects which have not been observed have a zero in the velocity columns. For two galaxies (# 13489 = NGC 7410 and # 17863) the reported velocities are from da Costa et al. (1991) and Metcalfe et al. (1989), respectively. The object # 31954, with the velocity coded as 99999, is a quasar with $z \sim 1.174$.Finally, a few galaxies have a measure for $v_{\rm abs}$ but have R=0: this fact indicates low quality spectra for which the cross-correlation was "forced", choosing the correlation peak by hand.

In the case of multiple observations of the same galaxy, we report in Table 3 the best measurement only.

For what concerns the use of these data for scientific analyses, when a galaxy has both $v_{\rm abs}$ and $v_{\rm emiss}$ the choice of the velocity can be done on the basis of the minimum error: however, the differences between $v_{\rm abs}$ and $v_{\rm emiss}$ are so small that different choices do not produce appreciable effects on the results of most of the scientific analyses. Note that the velocity errors reported in Cols. (7) and (10) are formal errors: for the conversion factors from these internal errors to the true errors, see the discussion in Sect. 6.2.