Why do elliptical galaxies generally have very low Star Formation Rates (SFRs)?
They have dispersed or heated their cold gas supply
Elliptical galaxies are frequently described as being 'red and dead' regarding star birth because they fundamentally lack the necessary raw materials for continuous stellar formation. The crucial fuel for star birth is vast reservoirs of cold gas and dust, specifically molecular hydrogen. In elliptical galaxies, this essential supply has either been entirely consumed over long cosmic timescales, or it has been heated to temperatures where it cannot condense or collapse under gravity efficiently. Without this cold, dense gas reservoir, star formation cannot be initiated, leading to very low or negligible SFRs, even though they typically possess a large central bulge composed of older, evolved stars.
