tokens: initial selection of compact group candidates. They emphasize that the large angular size of the isolation region for low-redshift systems artificially removes them from a catalog. Figure 8 of Barton et al. ( 1996 ) shows that groups with larger isolation regions have smaller sizes for groups selected from the CfA redshift survey. Here, again the limiting apparent magnitude for the definition of the isolation radius is the limiting apparent magnitude of the catalog rather than a limit three magnitudes fainter than the brightest group member. Eliminating the isolation criterion includes these nearby groups at the expense of including compact group candidates that are substructures in massive systems. However, the compact group candidates within massive systems or in dense regions can be removed after the initial selection. In contrast, the low-redshift systems cannot be recovered. The probability of rejection as a result of the isolation criterion can increase with decreasing redshift depending on the details of the application. The lowest redshift compact group candidates have larger angular size and a correspondingly more extensive isolation region than more distant compact group candidates. Figure 7 (a) shows the difference between the redshift distribution of the MLCG systems and the SDSS DR6 compact groups. Because McConnachie et al. ( 2009 ) remove r 14.5 galaxies, they might miss some nearby groups, thus accounting for the difference. To test this conjecture, Figure 11 plots the redshift distribution of the MLCG systems containing only galaxies in analogy with the SDSS DR6 sample. Although we remove 407 MLCG systems containing bright members, we still identify plenty of compact groups in the local universe ( z 0.05) where SDSS DR6 compact systems are absent or rare. Figure 11. Redshift distribution of the MLCG systems containing only galaxies with (open histograms) compared with the SDSS DR6 compact groups (filled histograms, Sohn et al. 2015 ). Because we do not apply an isolation criterion in the selection algorithm, we also include compact group candidates in dense surroundings (see Figure 10 ). Indeed, the fraction of the MLCG systems in dense environments ( N C > 7) is larger than for the SDSS DR6 compact groups at all redshifts. Interlopers may occur more often in high-density regions, thus producing some of the MLCG systems with inflated line-of-sight velocity dispersions. Barton et al. ( 2007 ) emphasize that a study of some aspects of galaxy evolution in tight pairs and compact groups requires specialization to relatively low-density environments. As in pair studies, candidate systems in dense surroundings can be removed after the general group selection. 5.2. Environmental Effects The diverse local environments of compact groups have been discussed previously (e.g., Ramella et al. 1994 ; Ribeiro et al. 1998 ; Andernach Coziol 2005 ; Mendel et al. 2011 ; Díaz-Giménez et al. 2012 ; Pompei Iovino 2012 ; Díaz-Giménez Zandivarez
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