The less significant role of large-scale environment than optical AGN in nearby, isolated elliptical galaxies

Abstract

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. Here we study the role of large-scale environment in some physical properties of 573 isolated elliptical galaxies out to z=0.08. We use three environmental estimators of the large-scale structure within a projected radius of 5 Mpc around isolated galaxies: the tidal strength parameter, the projected density eta_k, and the distance to the fifth nearest neighbor galaxy. 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 eta_k. Almost all the isolated ellipticals in the densest large-scale environments are red or quenched, where a third of them are low-mass galaxies. The percentage of isolated elliptical galaxies located in the AGN region of the BPT diagram is 64%. We have identified 33 blue, star-forming isolated ellipticals using both color and sSFR. Half of them are star-forming nuclei in the BPT diagram, which is 5% of the galaxies in this diagram. The large-scale environment is not playing the primary role to determine the color or sSFR of isolated elliptical galaxies. The large-scale environment seems to be negligible from a stellar mass scale around 10^10.6 Msun, 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. AGN might also be an essential ingredient to keep most of the low-mass isolated elliptical galaxies quenched.