4 Redshift determination

Redshifts were measured for absorption-featured spectra using the task XCSAO (Kurtz et al. 1992), a cross-correlation program (Tonry & Davis 1979) developed at the Smithsonian Astrophysical Observatory as a part of the RVSAO package, a contributed IRAF package.

The program allows the user to adapt the several working parameters to the characteristics of its own data-set. It is in fact possible to control if and how to rebin the data in log wavelength, to set an initial guess for the radial velocity, to define the dimension of the apodization region to be applied to the spectra, to fit and subtract the continuum from both spectra and templates, to choose the appropriate Fourier filtering and how to fit the peak of the correlation function. Once these parameters are defined, the program can be run in batch mode. An interactive graphical mode is available, during which all the main parameters can be modified and tested.

Redshifts for emission line objects were instead determined using EMSAO (Mink & Wyatt 1995), the XCSAO companion task. EMSAO was developed to find emission lines automatically, to compute redshifts for each identified line and to combine them into a single radial velocity. The results may be graphically displayed or printed. The graphic cursor can be used to interactively change fit and display parameters.

4.1 Templates

A crucial point for minimizing the error of the cross-correlation is the construction of a good template system, i.e. a set of spectra closely reproducing the characteristics of the observed spectra, having for example a range of relative absorption line depths similar to that of the sample galaxies of different morphological types.

After the first observing run we realized that some of the spectroscopic stars, which were misclassified as galaxies in the EDSGC, could represent such a set of templates.

A system of eight stars (6 of F type and 2 of K type) was chosen and used. Note that the use of such cold stars is a valuable criterion for cross-correlation procedure, since the profile of strong lines such as the calcium K and H, the G band, $\rm H\beta$ and $\rm MgI$ is not affected by gravity and/or metal abundance: it remains narrow, with a Doppler core and Stark broadened wings.

We made the assumption that such a group of stars could define a self consistent system with an average radial velocity of zero km/s. This assumption has been verified cross-correlating against this template system three standard radial velocity stars we purposely observed with OPTOPUS. We measured a zero point shift of $-7.4 \pm 3.8$ km/s; this correction has not been applied to the data of the catalogue.

4.2 Redshift measurement

Absorption spectra were measured with XCSAO and we decided to adopt as the "absorption velocity" the one associated with the minimum error in km/s from the cross-correlation against the eight stellar templates. In the great majority of cases, this coincided also with the maximum R parameter of Tonry & Davis (1979). Generally, the best performing templates were the F stars, but in a few cases better measurements were obtained from the two K templates.

Spectra showing both absorption and emission features were generally measured with both tasks (XCSAO and EMSAO). Emission features were manually excised prior to performing the cross-correlation analysis.

As a consistency check, all spectra were examined also visually, in order to verify the assigned velocity. A few examples of spectra of different quality are shown in Fig. 1.

  

Figure 1: Three examples of spectra: a) Standard quality spectrum (R=3.8) of a galaxy with $v_{\rm abs}=54315\pm 46$ km/s. b) High quality spectrum (R=20.2) of a galaxy with $v_{\rm abs}=12764\pm 20$ km/s. c) Spectrum with prominent emission lines of a galaxy with $v_{\rm emiss}=19110\pm 10$ km/s

 

Table 2: OPTOPUS fields (available also in electronic form)