### Question --- What is the morphology-density relation for galaxies? Give several possible explanations for this trend. ### Answer --- The **Morphology-Density Relation** is an empirical trend between a galaxy's [[Galaxy Classification#Hubble Morphological Sequence|morphological types]] and the environment they are found in. It shows that... - [[Galaxy Classification#Elliptical Galaxy|Elliptical]] and [[Galaxy Classification#Lenticular Galaxy|Lenticular]] galaxies are more likely to be found in denser environments, like [[Galaxy Group|galaxy groups]] and [[Galaxy Cluster|galaxy clusters]]. - Conversely, [[Galaxy Classification#Spiral Galaxy|Spiral]] galaxies are more likely to be found in less dense environments, like the field (i.e. isolation) Illustrated below, where ${\rm S0}$ is another name for the [[Galaxy Classification#Lenticular Galaxy|lenticular galaxies]]. ![[morphology-density-relation.png|align:center|650]] ![[morphology_density.png|align:center|400]] **Potential Explanations:** Strong evidence suggest that star formation is suppressed when galaxies enter high density environments such a clusters. (not well understood) - Denser environments lead to more galaxy mergers ("galaxy harassment"), [[Fluid Mechanics#Ram Pressure|ram pressure]] stripping, and galaxy strangulation - These processes disrupt spiral structure formation, leading to higher populations of elliptical and lenticular galaxies (which tend to be more gas-poor) in these dense regions