The morphology-density relation in the Sloan Digital Sky Survey

We have studied the morphology-density relation and morphology-cluster-centric-radius relation using a volume-limited sample (0.05 < z < 0.1, Mr* < -20.5) of the Sloan Digital Sky Survey (SDSS) data. Major improvements compared with previous work are: (i) automated galaxy morphology classification capable of separating galaxies into four types; (ii) three-dimensional local galaxy density estimation; and (iii) the extension of the morphology-density relation into the field region. We found that the morphology-density and morphology-cluster-centric-radius relation in the SDSS data for both of our automated morphological classifiers, Cin and Tauto, as fractions of early-type galaxies increase and late-type galaxies decrease toward increasing local galaxy density. In addition, we found that there are two characteristic changes in both the morphology-density and the morphology-radius relations, suggesting that two different mechanisms are responsible for the relations. In the sparsest regions (below 1 Mpc^-2 or outside of 1 virial radius), both relations become less noticeable, suggesting that the physical mechanisms responsible for galaxy morphological change require a denser environment. In the intermediate-density regions (density between 1 and 6 Mpc^-2 or virial radius between 0.3 and 1), intermediate-type fractions increase toward denser regions, whereas late-disc fractions decrease. Considering that the median size of intermediate-type galaxies is smaller than that of late-disc galaxies, we propose that the mechanism is likely to stop star formation in late-disc galaxies, eventually turning them into intermediate-type galaxies after their outer discs and spiral arms become invisible as stars die. For example, ram-pressure stripping is one of the candidate mechanisms. In the densest regions (above 6 Mpc^-2 or inside 0.3 virial radii), intermediate-type fractions decrease radically and early-type fractions increase in turn. This is a contrasting result to that in intermediate regions and it suggests that yet another mechanism is more responsible for the morphological change in these regions.

We also compared the morphology-density relation from the SDSS (0.01<z<0.054) with that of the MORPHS data (z~0.5). Two relations lie on top of each other, suggesting that the morphology-density relation was already established at z~ 0.5 as in the present Universe. A slight sign of an excess elliptical/S0 fraction in the SDSS data in dense regions might suggest the additional formation of elliptical/S0 galaxies in the cluster core regions between z= 0.5 and 0.05.