Astronomy
Elliptical Galaxies and the Role of Mergers in Their Evolution
Quick fact
Most elliptical galaxies contain almost no cold gas and dust, which means they have stopped forming new stars. They are often called 'red and dead' galaxies because their remaining stars are old, cool, and reddish.
Why this is interesting
You've seen gorgeous spiral galaxies with swirling arms, but some galaxies look like fuzzy, featureless blobs. What makes these elliptical giants so different? Their secret lies in a violent history of cosmic collisions.
Read the full explanation
Understanding Elliptical Galaxies and the Role of Mergers in Their Evolution
Imagine a spiral galaxy as a spinning pizza dough with arms of stars and gas. Now, imagine two pizzas crashing into each other. The collision scrambles their orderly shapes and triggers a burst of star formation as clouds of gas compress. Eventually, the merged dough becomes a shapeless, puffy ball — an elliptical galaxy. In these mergers, gas is either consumed in the star burst or blown away by supernova explosions, leaving the galaxy with little fuel to make new stars. The resulting elliptical is filled with old, low-mass stars that give it a smooth, red appearance. This is why ellipticals are most common in dense clusters, where galaxies frequently collide.
A deeper explanation
Elliptical galaxies are formed through the merger of smaller galaxies, a process driven by gravity. When galaxies merge, their orbital energy is dissipated through dynamical friction, causing them to sink toward the center and eventually coalesce. The merger scrambles the orbits of stars, leading to the random, elliptical orbits characteristic of ellipticals. The merger also compresses gas clouds, triggering rapid star formation, but the resulting supernovae and active galactic nuclei drive out the remaining gas, quenching star formation. Hence, ellipticals are often red and dead, lacking the cold gas needed for new stars. This explains their smooth brightness profiles and old stellar populations. The most massive ellipticals likely formed through multiple major mergers, while smaller ones may result from minor mergers. This hierarchical merging is a key prediction of the cold dark matter model, where galaxies grow by accreting smaller systems.