The morphology-density relation in the Sloan Digital Sky Survey

Abstract

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.

Local Environmental Dependence of Galaxy Properties in a Volume-Limited Sample of Main Galaxies

“…In order to study the morphology-density relation and morphology-cluster-centric-radius relation, Goto et al(2003) measured local galaxy density in the following way. For each galaxy, they calculated the projected distance to the 5th nearest galaxy within ±1000 km s 1 − in redshift space in a volume limited sample (0.05< z <0.1, M r < −20.5) of the Sloan Digital Sky Survey (SDSS) data.…”

“…In this paper, we modify Goto et al(2003)'s method and measure local three-dimensional galaxy density in the following way. For each galaxy, we calculate the three-dimensional distance to the 5th nearest galaxy.…”

Evidence of pre-processing and a dependence on dynamical state for low-mass satellite galaxies

“…In particular, galaxies at large radii show enhanced star formation and are more likely to have spiral morphologies compared to galaxies near the centre of the halo (Whitmore et al 1993;Goto et al 2003;Postman et al 2005;Rasmussen et al 2012;Wetzel et al 2012;Fasano et al 2015;Haines et al 2015). Therefore, in order to probe the environmentally driven aspects of galaxy evolution it is crucial to account for both the dependence on the host halo environment as well as the radial position within the group or cluster.…”

Galaxy Cluster Mass Reconstruction Project – II. Quantifying scatter and bias using contrasting mock catalogues

“…Statistical studies of the galaxy cluster population, in particular the cluster mass function, provide indispensable knowledge of cosmological model parameters (see Allen et al 2011 for a review, Tinker et al 2012), large scale structure (e.g., Bahcall 1988;Einasto et al 2001;Yang et al 2005b;Papovich 2008;Willis et al 2013) and galaxy evolution (e.g., Goto et al 2003;Postman et al 2005;Martínez et al 2008). However, deducing accurate masses of these gravitationally bound structures remains a fundamental challenge for current and future cosmological studies.…”