2018A&A...620A.117L

Context. The formation and evolution of elliptical galaxies in low-density environments are less understood than classical elliptical galaxies in high-density environments. Isolated galaxies are defined as galaxies without massive neighbors within scales of galaxy groups. The effect of the environment at several Mpc scales on their properties has been barely explored. We study the role of the large-scale environment in 573 isolated elliptical galaxies out to z=0.08.
Aims. We aim to explore whether the large-scale environment affects some of the physical properties of the isolated galaxies studied in this work.
Methods. We used three environmental estimators of the large-scale structure within a projected radius of 5Mpc around isolated galaxies: the tidal strength parameter, projected density η_k,LSS, and distance to the fifth nearest neighbor galaxy. We studied isolated galaxies regarding stellar mass, integrated optical g-i color, specific star formation rate (sSFR), and emission lines.
Results. We find 80% of galaxies at lower densities correspond to "red and dead" elliptical galaxies. Blue and red galaxies do not tend to be located in different environments according to η_k,LSS. Almost all the isolated ellipticals in the densest large-scale environments are red or quenched, of which a third are low-mass galaxies. The percentage of isolated elliptical galaxies located in the active galactic nucleus (AGN) region of the BPT diagram is 64%. We identified 33 blue, star-forming (SF) isolated ellipticals using both color and sSFR. Half of these are SF nuclei in the BPT diagram, which amounts to 5% of the galaxies in this diagram.
Conclusions. The large-scale environment does not play the primary role in determining the color or sSFR of isolated elliptical galaxies. The large-scale environment seems to be negligible from a stellar mass scale around 10^10.6M_☉, probably because of the dominant presence of AGN at higher masses. For lower masses, the processes of cooling and infall of gas from large scales are very inefficient in ellipticals. Active galactic nuclei might also be an essential ingredient to keep most of the low-mass isolated elliptical galaxies quenched.