Galaxy triplets in Sloan Digital Sky Survey Data Release 7 - II. A connection with compact groups?

Abstract

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 absolute r-band luminosities brighter than M r = −20.5. For these member galaxies, we analyse the stellar mass content, the star formation rates, the D n (4000) parameter and (M g − M r ) colour index. Since galaxies in triplets may finally merge in a single system, we analyse different global properties of these systems. We calculate the probability that the properties of galaxies in triplets are strongly correlated. We also study total star formation activity and global colours, and define the triplet compactness as a measure of the percentage of the system total area that is filled by the light of member galaxies. We concentrate in the comparison of our results with those of compact groups to assess how the triplets are a natural extension of these compact systems.Our analysis suggest that triplet galaxy members behave similarly to compact group members and galaxies in rich clusters. We also find that systems comprising three blue, starforming, 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 three 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 stellar mass 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 stellar mass 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 three luminous galaxies, should not be considered as an analogous of galaxy pairs with a third extra member, but rather they are a natural extension of compact groups.

Galaxy triplets in Sloan Digital Sky Survey Data Release 7 – III. Analysis of configuration and dynamics

“…This is in agreement with the results given in Duplancic et al (2013) who show that SDSS-triplets present an important fraction of bulge-type, red galaxies, with low star formation activity indicators, similarly to galaxy members of compact groups.…”

Dissecting galaxy triplets in the Sloan Digital Sky Survey Data Release 10 – I. Stellar populations and emission line analysis

“…Only three triplets contain the majority of star-forming galaxies (classified as 111, 112 and 113), mostly in the low mass bin. This result is in agreement with Duplancic et al (2013), who found that blue triplets are located in the less massive tail of the total stellar mass distribution and present an efficient star formation activity when compared to compact groups, in the same stellar mass bin. As the fractions of AGNs and passive galaxies increases, the number of triplets with a dominance of passive and retired galaxies (mainly 233 and 333) increases.…”

“…Further analysis carried out by Duplancic et al (2013), based on photometric colours and star formation activity, indicates that galaxy triplets present similar properties to compact groups and that these systems, comprising luminous galaxies, are actually an extension of compact galaxy groups with a smaller number of members. A dynamical analysis, employing the AA-diagram (Agekyan & Anosova 1968) and mock catalogues, also indicates that their configurations provide better conditions for interaction and mergers between galaxy members (Duplancic et al 2015).…”

An isolated compact galaxy triplet

“…In O-sample, there are about one tenth of the triplets with spectroscopic redshifts for all their members. Duplancic et al (2013Duplancic et al ( , 2015 analyzed the configuration and dynamics of this sub-sample, and compared them with galaxy pairs and clusters. They found that the triplet galaxy members are more similar to the galaxies in compact groups and rich clusters than in galaxy pairs and concluded that the galaxy triplets may not be formed hierarchically.…”