Why can life only arise in spiral galaxies? I have heard this said but never explained.

 

Whether elliptical galaxies can host life doesn’t matter for us much now, as they are too far away, but if our biological or robotic descendants still exist billions of years from now, they will reside in an elliptical galaxy that will form after the collision of the Milky Way with Andromeda.

In spiral galaxies, stars can have well-defined orbits that keep them at roughly the same distance from the center. This provides a stable environment for the evolution of life, characterized by a lower likelihood of nearby supernova explosions or radiation from high-density stars in the center of galaxies, as well as the emission of radiation from supermassive black holes’ accretion disks or relativistic jets.

In elliptical galaxies stars have radomly oriented radial orbits that can take them for millions of years to the densest parts where there is a high chance of something dramatic to happen, like a nearby supernova explosion or active galactic nucleus when supermassive black holes feed for a while on more matter than usually, releasing a lot of deadly radiation or even relativistic jets that can sterilize planets like powerful lasers.

There are also other types of galaxies besides elliptical and spiral. Irregular ones typically retain this shape for a while after colliding with another galaxy, and living plants could survive with some luck until the situation in the galaxy stabilizes. Some satellite galaxies assume this shape due to the tidal effects of the massive host galaxy and nearby other satellite galaxies. Such irregular galaxies can only make a few orbits before they are absorbed, though, and the survival of life on planets then depends on where they end up in the galaxy after their system is absorbed.

Dwarf non-spiral galaxies can be too small to have safe areas where life could emerge uninterrupted without nearby supernova explosions. A supernova that detonates anywhere in a tiny galaxy can affect a large part of the galaxy.

All of this suggests that spiral galaxies, like the Milky Way, are thought to be more likely to host life and technological civilizations than other types of galaxies.

The question was: Why can life only arise in spiral galaxies? I have heard this said but never explained.

What is a satellite galaxy?

 

We live in the Milky Way Galaxy, which is above average in size, mass, and number of stars. Satellite galaxies are smaller in these three measures. They used to be independent, but they found themselves attracted to the gravity of the Milky Way and became its satellite. Galaxies are strongly affected by tidal effects and cannot orbit their hosts for as many orbits as planets can. Each such revolution can take hundreds of millions of years, and their ultimate fate is to be absorbed; their stars to become part of their master galaxy.

Some that are already close to the Milky Way and orbit at an angle can pierce through the disk and come out of the other side on their last few revolutions.

Until about a year ago, we knew about 59 satellite galaxies of the Milky Way, which contains 200 to 400 billion stars. The most massive is the Large Magellanic Cloud, which looks quite misformed. It used to be a pretty spiral galaxy, but now its interaction with the Milky Way and other satellite galaxies like the Small Magellanic Cloud has made it look wonky. It contains about 30 billion stars and might be 10 to 20% as massive as the Milky Way.

The least massive satellite galaxies can have millions of stars, but we still discover new ones. A recent research paper proposed that galaxies like ours should have up to hundreds of satellite galaxies and found an additional tens on top of 59 already known.

Some are difficult to spot because they are faint, and some we cannot see well because they are behind the bright central bulge and thick disk of our galaxy. The brightness of stars, their density, and the presence of dust make it difficult to see what’s behind. One of the objectives of the space mission of the European Space Agency, Gaia, was to find more satellite galaxies via peering through the central bulge and the disk, and it found some indeed.