Specific star formation rate (sSFR) as a function of the stellar mass. The symbols for the galaxies are the same as Fig. 2. The red line shows Eq. (11) to separate star-forming and passive galaxies (above and below the line, respectively). Right panel: normalized density distribution of sSFR for isolated elliptical galaxies (gray solid histogram) and elliptical galaxies located in the Coma supercluster (red open histogram). The integral of each histogram sums to unity.