Spectroscopy Data Quality
Previous spectroscopic surveys have yielded redshift measurements for a substantial number of galaxies in common with the WINGS-SPE sample. We emploied these data to test the reliability of our data. We used data from the NASA/IPAC Extragalactic Database (NED) to perform our comparison with the aim of extending our catalogs and exploiting the entire data-set (WINGS+literature) in the kinematical and dynamical analysis. In particular, we intend to use these data to study in detail the properties of clusters and substructures in the WINGS sample. We also separately campare our data with the NOAO Fundamental Plane Survey, which has a good overlap with our sample, and with the SDSS with which we only have 12 clusters in common. In Figure we show the global comparison for the 1325 galaxies in common with NED, the 217 in common with SDSS, and 676 in common with NOAO-FPS. The following plot presents the residuals for redshift measurements obtained for the 3 comparison samples used to perform the quality check: NOAO-FPS, SDSS, NED (from top to bottom). The black line indicates the offset while the red dashed lines indicate the scatter.
As summarized in this table, the mean differences between WINGS and the literature data are very low and much lower than the dispersion, assuring the absence of systematic offsets. Columns are: (1) the comparison sample name, (2) mean difference (in km/s), (3) rms scatter in the differences, and (4) number of galaxies used in the comparison.
Moreover, the dispersion in the measurements is low enough to not greatly influence the measurement of the internal velocity dispersion of galaxy clusters, even in the cases where this quantity is considered low (~300−400 km/s) such as for substructures and groups. This fact is fundamental in view of the subsequent dynamical analyses.
Completeness and Success Rate
It is very important to know the completeness level of the spectroscopic observations as this is a factor that must be accounted for in the derivation of luminosity functions, M/L ratios, as well as any time we want to use the spectroscopic sample to study magnitude-dependent properties (e.g. the different galaxy population fractions inside clusters).
The completeness as a function of magnitude is defined here as:
C(m)= Nz (m)/Nph (m)
where Nz is the number of galaxies with measured redshifts and Nph is the number of galaxies in the parent photometric catalog, taking into account the cuts in color and magnitude, for each given magnitude bin m. Completeness is usually a decreasing function of the magnitude because in observations priority is given to brighter objects. The success rate, i.e. the fraction of galaxies with redshift determination with respect to the total number of observed galaxies, is similarly defined as:
S R(m) = Nz (m)/Ntg (m)
where Nz is defined as in the preceding equation and Ntg is the number of target galaxies we actually observed. The global success rate and completeness as a function of V magnitude are shown for the WINGS-SPE sample in the following Figure.
