Galaxy triplets in Sloan Digital Sky Survey Data Release 7: II. A
  connection with compact groups?
  We analyse a sample of 71 triplets of luminous galaxies derived from the work
of O'Mill et al. (2012). We compare the properties of triplets and their
members with those of control samples of compact groups, the ten brightest
members of rich clusters, and galaxies in pairs. The triplets are restricted to
have members with spectroscopic redshifts in the range 0.01<z<0.14 and
Mr<-20.5. We analyse the stellar mass (SM) content, the star formation rates,
the Dn(4000) parameter and colour index and also analyse different global
properties of these systems as total star formation activity and global
colours. We calculate the probability that the properties of galaxies in
triplets are strongly correlated. We define the triplet compactness as a
measure of the percentage of the system total area that is filled by the light
of member galaxies. Our analysis suggest that triplet galaxy members behave
similarly to compact group members and galaxies in rich clusters. We also find
that systems comprising 3 blue, star-forming, young stellar population galaxies
(blue triplets) are most probably real systems and not a chance configuration
of interloping galaxies. The same holds for triplets composed by 3 red, non
star-forming galaxies, showing the correlation of galaxy properties in these
systems. From the analysis of the triplet as a whole, we conclude that, at a
given total SM content, triplets show a total star formation activity and
global colours similar to compact groups. However, blue triplets show a high
total star formation activity with a lower SM content. From an analysis of the
compactness parameter of the systems we find that light is even more
concentrated in triplets than in compact groups. We propose that triplets
composed by 3 luminous galaxies, should not be considered as an analogous of
galaxy pairs with an extra member, but rather they are a natural extension of
compact groups.
