The Relation between Galaxy Morphology and Environment in the Local Universe: An RC3-SDSS Picture

We present results of an analysis of the local (z ~ 0) morphology-environment relation for 911 bright (M_B < -19) galaxies, based on matching classical RC3 morphologies with the Sloan Digital Sky Survey based group catalog of Yang et al., which includes halo mass estimates. This allows us to study how the relative fractions of spirals, lenticulars, and ellipticals depend on halo mass over a range of 10^11.7-10^14.8 h ^-1 M _⊙, from isolated single-galaxy halos to massive groups and low-mass clusters. We pay particular attention to how morphology relates to central versus satellite status (where "central" galaxies are the most massive within their halo). The fraction of galaxies which are elliptical is a strong function of stellar mass; it is also a strong function of halo mass, but only for central galaxies. We interpret this as evidence for a scenario where elliptical galaxies are always formed, probably via mergers, as central galaxies within their halos, with satellite ellipticals being previously central galaxies accreted onto a larger halo. The overall fraction of galaxies which are S0 increases strongly with halo mass, from ~10% to ~70%. Here, too, we find striking differences between the central and satellite populations. 20% ± 2% of central galaxies with stellar masses M _* > 10^10.5 M _⊙ are S0 regardless of halo mass, but satellite S0 galaxies are only found in massive (>10¹³ h ^-1 M _⊙) halos, where they are 69% ± 4% of the M _* > 10^10.5 M _⊙ satellite population. This suggests two channels for forming S0 galaxies: one which operates for central galaxies and another which transforms lower-mass ( M _* <~ 10¹¹ M _⊙) accreted spirals into satellite S0 galaxies in massive halos. Analysis of finer morphological structure (bars and rings in disk galaxies) shows some trends with stellar mass, but none with halo mass; this is consistent with other recent studies which indicate that bars are not strongly influenced by galaxy environment. Radio sources in high-mass central galaxies are common, similarly so for elliptical and S0 galaxies, with a frequency that increases with the halo mass. Emission-line active galactic nuclei (mostly LINERs) are more common in S0s, but show no strong trends with environment.