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Is there a beginning and an end to a galaxy's life?

I'm a year 8 student and have to find out about the lifecycle of a galaxy. Is there actually a beginning and an end to the life of a galaxy or does it just keep going and joining up with other galaxies?

You ask a good question. Though details of galaxy formation and evolution are still widely debated, the general theory of how they form is now generally accepted.

Structures in the Universe form by "hierarchical clustering": regions of the very young Universe collapse under their own gravity during the evolution of the Universe, in such a way that "little" structures like galaxies form before "big" structures like clusters. This can be explained physically if you assume that the dark matter in the Universe is non-relativistic. People call dark particles like this "cold"; thus, many astronomers call this theory of galaxy evolution "the cold dark matter paradigm".

In the cold dark matter paradigm, "halos" of dark matter first become gravitationally bound and "break away" from the general expansion of the Universe; that is, instead of expanding with the Universe, the Universe expands around the halos. Each halo (of galaxy size) will end up as a galaxy at some point: so in a sense, these primordial halos represent represent the "birth" of a galaxy. Once a halo is formed, normal (or "baryonic") matter falls towards its centre, because of the gravitational pull that the halo exerts. During this infall the baryons must conserve angular momentum, and so they begin to spin faster around the centre of the halo as they fall deeper into it. This process yields a disk of star-forming, normal matter inside a halo or dark matter, as we seem to observe in spiral galaxies today.

This "passive" evolution described above is complicated by galaxy collisions, however: numerical simulations have recently revealed that collisions influence the appearance (or "morphology") of an object. As a general rule, the morphology of a galaxy seems to be governed by the types of collisions it has undergone throughout its evolution: major mergers (collisions between two galaxies of comparable mass) produce elliptical galaxies, and minor mergers (collision between a galaxies of different masses) don't disturb the galaxy's evolution towards a thin disk. In other words, elliptical galaxies have recently undergone major mergers, very thin disk galaxies have not collided with anything much in a long time, and other galaxy types are somewhere in between. This evolution via merging is not supported by all observations of galaxies of different types, but the observations do seem to be broadly consistent with the idea. A galaxy's "life" therefore consists of a sequence of star forming events and collisions that it might undergo, depending on its environment and on the properties of the baryons which formed the disk in the first place. Defining the "end" of a galaxy's life is a bit more complicated...

Suppose that a galaxy doesn't collide with any others after a given amount of time: its evolution is then governed by star formation in the disk. Though the latter is cyclic (gas makes stars, stars die and return most of their gas to the interstellar medium), a fraction of the available gas becomes locked up in low-mass stellar remnants in each star-forming cycle, so that eventually no gas will be available to form new stars. As the stellar remnants dim, so will the galaxy, until it no longer emits much light at all. However, the dark matter halo of the galaxy is still ever-present. So if you define galaxy birth by the formation of a halo, then an isolated galaxy never "dies" in the sense that the halo remains even after star formation has ceased.

In reality, of course, we know that most galaxies reside in groups and clusters and hence undergo many collisions. When two galaxies collide, their dark matter halos are believed to coalesce into a single, larger one. One can then look at a minor merger as the "death" of the small galaxy (and the growth of the larger one), and a major merger as the "death" of two galaxies followed by their "reincarnation" as one larger one.

Given sufficient time, will all galaxies collide with each other, and hence merge into one big halo? This depends, actually, on the rate at which the Universe expands. We think today that universal expansion is accelerating with time, in such a way that distinct groups and clusters of galaxies will never merge with one another. So in the future, the members of a given group or cluster may coalesce into a single, monstrous "super-galaxy", but the super-galaxies themselves will not collide with each other. The halos of these objects would then last forever (assuming that the dark matter does not interact with anything at all), giving them ultimate immortality over everything else in the Universe.

October 2002, Kristine Spekkens (more by Kristine Spekkens) (Like this Answer)

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