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

Abstract

We analyse the spatial configuration and the dynamical properties of a sample of 92 galaxy triplets obtained from the SDSS-DR7 (SDSS-triplets) 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. The configuration analysis was performed through Agekyan & Anosova map (AA-map). We estimated dynamical parameters, namely the radius of the system, the velocity dispersion, a dimensionless crossing-time and the virial mass. We compared our results with those obtained for a sample of triplets from the catalogue "Isolated Triplets of Galaxies" (K-triplets) and a sample of Compact Groups. We have also studied a mock catalogue in order to compare real and projected configurations, and to estimate the three dimensional dynamical parameters of the triple systems. We found that the SDSS-triplets prefer alignment configurations while K-triplets present an uniform distribution in the AA-map. From the dynamical analysis we conclude that the SDSS-triplets, K-triplets and Compact Groups present a similar behaviour comprising compact systems with low crossing-time values, with velocity dispersions and virial masses similar to those of low mass loose groups. Moreover, we found that observed and simulated triplets present similar dynamical parameters. We also performed an analysis of the dark matter content of galaxy triplets finding that member galaxies of mock triplets belong to the same dark matter halo, showing a dynamical co-evolution of the system. These results suggest that the configuration and dynamics of triple systems favour galaxy interactions and mergers.

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

“…Taken as a whole, these results are consistent with the environment, through galaxy interactions, playing a major role in triplet evolution. This is consistent with an analysis of triplets in mock catalogues, which indicates that these systems are dynamically evolved and probably have undergone merger events (Duplancic et al 2015), favouring the for-mation of early-type galaxies, classified spectroscopically as passive or retired galaxies. The spectroscopic incompleteness represents the main source of fake triplets in our sample.…”

“…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).…”

“…For this reason, we impose the condition that there are no bright (Mr < −20.5 + 5 log h70) neighbours within 0.5h −1 M pc from the triplet centre and with the same restriction on |∆V | used to identify triplet members. Duplancic et al (2015) used these criteria to isolate triplet galaxy systems. Moreover, 95% of the systems are at distances greater than 3h −1 M pc from their massive neighbours, implying that our isolation criterion is effective in selecting systems far away from groups and clusters of galaxies.…”

The global environment of small galaxy systems

“…This criteria excludes neighbours within a fixed aperture of 500 kpc (∆V ≤ 700 km s −1 ). In Duplancic et al (2015) we use these re- strictions to isolate galaxy triplets finding that 95 per cent of the systems are at distances greater than 3 Mpc from clusters, therefore the isolation is effective in the identification of systems far away from high density regions. In this context, pairs studied here are not only interacting galaxies but individual systems composed by two member galaxies and inhabiting low density environments.…”

“…For example, to select compact groups "Hickson criteria" (Hickson 1982) states that there can not be significant neighbours within a concentric circle three times greater than the smallest circle encompassing the geometric centres of the galaxies in the group. On the other hand, O'Mill et al (2012) and Duplancic et al (2015) selected isolated triple systems with no significant neighbours within a fixed aperture of 0.5 Mpc from the centre of the system and with the same restriction on the radial velocity difference used to identify triplet members. For galaxy pairs the isolation is usually re-stricted to the absence of a third companion within the projected distance and radial velocity cut used to define the pair, and/or by requiring that galaxies in the pair sample do not belong to larger structures as groups or clusters of galaxies (i.e.…”