2 Galaxy data

  The PPS database was compiled by Giovanelli & Haynes in the last decade (Haynes & Giovanelli 1988; Giovanelli & Haynes 1991; and references therein). It consists mainly of highly accurate 21-cm HI line redshifts, partly unpublished, obtained with the NAIC 305-m telescope in Arecibo and with the NRAO 300-foot telescope formerly in Green Bank (Giovanelli & Haynes 1993, and earlier references therein). The radio data are complemented with optical observations of early-type galaxies carried out at the 2.4-m telescope of the McGraw-Hill Observatory (Wegner et al. 1993). The full redshift survey is magnitude-limited down to $m_{\rm Z} \leq 15.7$,and now it covers the whole region $-2\hbox{$.\!\!^{\rm h}$}00 \le \alpha \le +4\hbox{$.\!\!^{\rm h}$}00$ and $0^{\rm o} \le \delta \le 50^{\rm o}$.

From PPS, we extract a subsample named PPS2 (in analogy with CfA2), complete and magnitude-limited to $m_{\rm Z} \leq 15.5$.We restrict PPS2 to the region $-1\hbox{$.\!\!^{\rm h}$}50 \le \alpha \le +3\hbox{$.\!\!^{\rm h}$}00$ and $0^{\rm o} \le \delta \le 40^{\rm o}$,in order to exclude those parts near the northern edge of PPS heavily affected by Milky Way obscuration. Before the final selection, we also correct Zwicky magnitudes (Zwicky et al. 1961-68) for interstellar extinction as in Burstein & Heiles (1978), yielding $\delta m \leq 0.4$ over most of the selected area (see Fig. 2 in Giovanelli et al. 1986, or Fig. 1 in Park et al. 1994). The solid angle is $\omega= 0.76 \ {\rm sr}$, and the degree of completeness is higher than 95% to this magnitude limit (see Fig. 1 in Iovino et al. 1993, who used a similar but earlier version of PPS).

Regarding redshifts, we consider three different schemes: (a) a correction of observed heliocentric radial velocities as in Yahil et al. (1977), i.e. taking into account the motion of the Sun relative to the rest frame of the Local Group Centroid, $v_{\odot {\rm LGC}}=308 \ {\rm km\ s}^{-1}$ towards $l=105^{\rm o}$, $b=-7^{\rm o}$;(b) a correction of cz for the motion of the Sun relative to the rest frame of the Microwave Background Radiation, $v_{\odot {\rm MBR}}=270 \ {\rm km\ s}^{-1}$ towards $l=265^{\rm o}$, $b=+48^{\rm o}$ (e.g. Peebles 1993); (c) no correction at all. The total number of galaxies is very little affected by purely radial corrections to cz. In complete apparent-magnitude-limited samples, galaxies "flow'' along the lines of constant m in the cz-M plane, but only very few "move across'' the redshift border. (This effect would not be negligible in complete volume-limited samples, i.e. all galaxies within a certain range of cz and absolute magnitude M, where corrections to cz cause "flows'' in the cz-M plane that cross both the redshift and the luminosity edges of the selected sample.) We expect group properties to be very weakly affected too, as all member galaxies within a given group receive similar corrections (they are "moved all together'' in redshift space, and group centroids "follow'' them).

Figure 1 shows PPS2 in redshift+luminosity space. This "4+1-diagram'' (i.e., 3 maps in redshift space and 1 luminosity-distance diagram +1 redshift distribution histogram), eases comparison of group catalogs among them and with their parent galaxy sample. The final sample PPS2 contains $N_{\rm g}=3014$ galaxies, with extinction-corrected magnitude $m_{\rm Z} \leq 15.5$ and MBR-corrected redshift $cz \le 27000 \ {\rm km\ s}^{-1}$ (in practice, almost all galaxies are contained within $cz \sim 15000 \ {\rm km\ s}^{-1}$).

 

Figure 1:  Galaxies in the sample PPS2: 4+1 diagram, i.e. 4 sections of $(\alpha, \delta, cz; M_{\rm Z})$ space + 1 redshift distribution histogram. Each dot is a galaxy. The redshift scale is the same everywhere. Top left: redshift space, $\delta$-cz wedge diagram. Top right: sky-view, $\alpha$-$\delta$ diagram. Bottom right: redshift space, $\alpha$-cz wedge diagram. Bottom left: distance-luminosity diagram in the cz-$M_{\rm Z}$ plane (the lower envelope is the faint cut-off ${m_{\rm lim}}=15.5$), and distance (redshift) distribution histogram (the normalization is arbitrary)

For the sake of comparison, the samples CfA1 (North + South) and the (first two, northern) CfA2 Slices are characterized by $\omega=1.83+0.83$ and $0.42 \ {\rm sr}$, $m_{\rm Z} \leq 14.5$ and 15.5, $N_{\rm g}=1845+556$ and 1766 respectively. The SSRS1 survey is apparent-diameter-limited, with $N_{\rm g}=2028$, $\omega=1.75 \ {\rm sr}$, and $m_{\rm Z} \lower.5ex\hbox{$\;\buildrel < \over \sim \;$}14.8$, slightly deeper than CfA1. The SSRS2 and CfA2 North surveys have $N_{\rm g} \sim 3600$ and 6000, $\omega=1.13$ and $1.2 \ {\rm sr}$, respectively, and $m_{\rm Z} \leq 15.5$.We note that magnitudes in the CfA samples are not corrected for galactic extinctions, and redshifts are usually corrected for the solar motion with respect to the Local Group (sometimes also for infall on the Virgo cluster).