Compact groups of galaxies in the TNG100 simulation

Using the TNG100 cosmological simulation, we study the formation and evolution of compact groups of galaxies. Over a redshift range of 0 ≲ z ≲ 0.2, we identify these compact groups as Friends-of-Friends galaxy groups with a high mean surface brightness ($ \overline{\mu}_r < 26.33 \, \mathrm{mag\,arcsec^{-2}} $) and a minimum of four galaxy members. Typically, our compact groups have a median characteristic size of ∼150 kpc, 1D velocity dispersions of 150 km s−1, and stellar masses around 2 × 1011 M⊙. Roughly 1% of galaxies that have a stellar mass above 109 M⊙ lie in physically dense compact groups. We found that these systems do not constitute a separate category within the broader population of galaxy groups; instead, they represent the lower end of the size distribution in the sequence of galaxy group sizes. We traced their evolution backward in time, revealing that they initially form as galaxy systems with a mean low surface brightness that systematically increases to a peak value before stabilizing over time, exhibiting oscillatory behaviour over the following several gigayears during which mergers may occur. Mergers often transform compact groups with typically four members into galaxy pairs or triplets, which eventually can again increase their number of members by accreting a new galaxy. Nevertheless, the full merging of all constituent galaxies into a single massive galaxy is a rare phenomenon.