Galaxy Morphology In Cluster Environments

Abstract

It is a well known fact that cluster environments favour early-type galaxies over late- type. However, the physical origin of the correlation remains uncertain. We focus specifically on the external environmental mechanisms responsible for morphologi- cal transformation of late-type into early-type galaxies using ∼ 30,000 satellites in 625 clusters from SDSS DR7. All the external processes have one factor in common which is the orbital path taken by a satellite through the cluster that determines the extent of the environmental effects. We use results from orbital libraries extracted from N-body simulations. These give a probabilistic mapping between normalized projected phase-space (R, V ) coordinates to lookback time to cluster infall. The look back time is the key ingredient that traces how far the satellite is in its orbit compared to the cluster centre. Using these results, we explore the effects of a de- crease in disc luminosity caused by disc quenching and disc scale length reduction on satellite morphology quantified by the luminous bulge-to-total (B/T) ratio. Our findings suggest that satellites are quenched and their disc scale lengths are reduced after spending ∼ 5 Gyrs in the cluster which corresponds to ∼ 1 Gyr after pass- ing the pericenter. A drop in disc luminosity due to these processes has a small impact on morphology suggesting that other active processes such as harassment are responsible for further morphological transformations. Fitting to account for such processes over and above the ones responsible for a drop in disc luminosity, we find that satellites with B/T < 0.1 (pure disc systems) experience most drastic changes: their structure changes by developing a bulge component ∼ 5 Gyrs after infall. Similarly, objects with 0.1 < B/T < 0.3, are affected by harassment over a longer time scale ∼ 7 Gyrs suggesting that satellites with a bulge component are more resistant to harassment compared to pure disc systems. We conclude that the density-morphology relationship is a manifestation of both hydrodynamical as well as gravitational processes driving late-type to early-type morphology.