The HCG database table is based on the Hickson Catalog, which is a list of 100 compact groups of galaxies that were identified by a systematic search of the Palomar Observatory Sky Survey red prints. Each group contains four or more galaxies, has an estimated mean surface brightness brighter than 26.0 magnitude per arcsec<sup>2</sup> and satisfies an isolation criterion. Dynamical parameters which were derived for 92 of the 100 groups are also included in the database. (Note that the Hubble constant was assumed to be Ho = 100 km/s/Mpc.) This database table essentially contains the information given in Table 1 of Hickson, P. (1982, ApJ, 255, 382) and Table 3 of Hickson, P. et al. (1992, ApJ, 399, 353). Consequently, the information on individual galaxies in the Hickson groups that is also given in these references, e.g., in Table 2 of Hickson, P. et al. (1992, ApJ, 399, 353), is not in the HCG database table; however, the latter data can be found in the related HEASARC database table HCGGALAXY. This online catalog was created by the HEASARC in August, 1999, based on machine-readable tables obtained from the ADC/CDS data centers (<a href="https://cdsarc.cds.unistra.fr/ftp/cats/VII/213:">CDS catalog VII/213:</a> files groups.dat and dynamics.dat). The HEASARC refined the coordinates, corrected the dynamics_flag values, and updated the table's metadata in August, 2005. This is a service provided by NASA HEASARC .
