Why are galaxies in clusters often red and gas-poor?

The dense cluster environment strips and heats their gas through processes like ram-pressure stripping and strangulation, cutting off the supply needed to form new stars. Without fresh star formation, their remaining stars age and the galaxies turn red.

What is the morphology-density relation?

It is the observed trend that elliptical and lenticular galaxies become more common, and spirals less common, in regions of higher galaxy density such as cluster cores. It shows that where a galaxy lives is closely tied to what kind of galaxy it is.

Environmental Effects on Galaxies

A galaxy's surroundings shape its fate: dense cluster environments strip gas, suppress star formation, and transform galaxy morphology.

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Definition

Environmental effects on galaxies are the processes by which a galaxy's surroundings, especially the dense, gas-rich, high-velocity environment of a cluster, alter its gas content, star formation, and morphology relative to galaxies in isolation.

Scope

This topic covers the morphology-density relation, the physical mechanisms by which the cluster environment affects galaxies including ram-pressure stripping, galaxy harassment, and strangulation, the quenching of star formation in dense regions, and the resulting differences between galaxies in clusters and in the field.

Core questions

How does galaxy morphology depend on local density?
What physical mechanisms strip gas from galaxies in clusters?
Why do galaxies in dense environments tend to stop forming stars?
How do environmental processes differ from internal evolution?

Key theories

The morphology-density relation: Dressler showed that the fraction of elliptical and lenticular galaxies rises with local galaxy density while spirals decline, demonstrating that environment correlates strongly with morphology.
Ram-pressure stripping: Gunn and Gott showed that as a galaxy moves through the hot intracluster gas, the resulting pressure can strip away its own interstellar gas, removing the fuel for star formation.
Galaxy harassment: Repeated rapid gravitational encounters between galaxies in a cluster can disturb disks and drive morphological transformation, a process termed harassment.

Clinical relevance

Environmental effects explain why cluster galaxies are predominantly gas-poor and red while field galaxies are more often star-forming spirals, making environment a key variable in galaxy evolution alongside a galaxy's own mass.

History

Gunn and Gott introduced ram-pressure stripping in 1972, and Dressler's 1980 morphology-density relation established environment as a driver of galaxy properties. Mechanisms such as harassment and strangulation were developed in the 1990s, and large surveys have since quantified how environment quenches star formation.

Key figures

Alan Dressler
James Gunn
J. Richard Gott
Ben Moore

Seminal works

gunngott1972
dressler1980
moore1996

Frequently asked questions

Why are galaxies in clusters often red and gas-poor?: The dense cluster environment strips and heats their gas through processes like ram-pressure stripping and strangulation, cutting off the supply needed to form new stars. Without fresh star formation, their remaining stars age and the galaxies turn red.
What is the morphology-density relation?: It is the observed trend that elliptical and lenticular galaxies become more common, and spirals less common, in regions of higher galaxy density such as cluster cores. It shows that where a galaxy lives is closely tied to what kind of galaxy it is.