Why are elliptical galaxies red?

They are dominated by old stars and contain little cold gas, so they have largely stopped forming new, hot, blue stars. The remaining long-lived stars are cooler and redder, giving ellipticals their characteristic red color.

What is a lenticular galaxy?

A lenticular, or S0, galaxy has a smooth disk and a large central bulge but lacks the spiral arms and active star formation of spirals. It sits at the transition between ellipticals and spirals on the Hubble sequence.

Elliptical and Lenticular Galaxies

Elliptical and lenticular galaxies are gas-poor, pressure-supported systems of mostly old stars, representing the early-type end of the galaxy population.

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Definition

Elliptical galaxies are smooth, featureless ellipsoids supported mainly by the random motions of old stars, while lenticular galaxies are intermediate systems with a prominent bulge and a smooth disk but little spiral structure or active star formation.

Scope

This topic covers the smooth, ellipsoidal structure of elliptical galaxies, the disk-plus-bulge nature of lenticulars, the dynamical support by random stellar motions rather than rotation, the scaling relations such as the Faber-Jackson relation and the fundamental plane, and the largely old, metal-rich stellar populations these galaxies contain.

Core questions

How are elliptical galaxies structured and dynamically supported?
What distinguishes lenticular galaxies from both ellipticals and spirals?
What scaling relations connect the sizes, luminosities, and velocity dispersions of early-type galaxies?
What stellar populations and gas content characterize these systems?

Key theories

Pressure support and anisotropy: Ellipticals are held up against gravity by the random velocities of their stars, and their flattening reflects anisotropic velocity distributions rather than rotation.
The Faber-Jackson relation: An elliptical galaxy's luminosity scales with its central stellar velocity dispersion, a relation that links mass to light and underlies distance estimation for early-type galaxies.
The fundamental plane: Elliptical galaxies occupy a tight plane relating their size, surface brightness, and velocity dispersion, reflecting regularities in their mass-to-light ratios and dynamical structure.

Clinical relevance

Early-type galaxies dominate the massive end of the galaxy population and the cores of clusters; their tight scaling relations serve as distance indicators and as records of the merger-driven assembly of massive galaxies.

History

The 1976 Faber-Jackson relation gave ellipticals their first major scaling law. In 1987 the discovery of the fundamental plane tightened the description of their structure, and integral-field spectroscopy later revealed that many ellipticals are slow rotators while others retain significant rotation, refining the early-type picture.

Key figures

Sandra Faber
Robert Jackson
George Djorgovski
John Kormendy

Seminal works

faber1976
djorgovski1987
binney2008

Frequently asked questions

Why are elliptical galaxies red?: They are dominated by old stars and contain little cold gas, so they have largely stopped forming new, hot, blue stars. The remaining long-lived stars are cooler and redder, giving ellipticals their characteristic red color.
What is a lenticular galaxy?: A lenticular, or S0, galaxy has a smooth disk and a large central bulge but lacks the spiral arms and active star formation of spirals. It sits at the transition between ellipticals and spirals on the Hubble sequence.