When Hubble discovered galaxies, they look like Universe-Islands, obviously individualized entities. When one looks at recent images of the deep sky, this picture is striking, even in the case of two galaxies in the process of merging. So the question of defining the object “galaxy” seems both odd and useless.
However, when galaxy formation is concerned, then things are not so clear, especially in the very first moments of the Universe when the density of matter was high.
There has been some debate in the recent literature on how define a galaxy. Is this really useful, and more importantly can we converge?
I think it is important, but I am also convinced that we cannot converge and agree on a robust and definitive answer. Essentially because our knowledge always improve.
There are four compulsory ingredients that we do observe: self-gravitation, stars, gas and dust. However, the very first objects were formed from a simple gas cloud. These very first objects could have been stars, or ensemble of stars. It is possible that already formed stars gathered into a self-graviting cluster, with or without gas.
Since astrocladistics aims at understanding the evolutionary relationships between galaxy species throughout the Universe evolution, then we must adopt a definition that is able to include the very first ancestors. We thus establish the following very general definition for a “galaxy”:
A galaxy is a self-graviting ensemble of stars, gas OR dust.
We could add black holes as another component, especially central massive black holes. Is such a black hole if isolated could be a kind of galaxy? Why not, but it would be interesting if and only if we can find evolutionary relationships with other kinds of galaxies. In addition, a huge black hole in isolation seems to me a very improbable event, but who knows?
Basically, our definition can be made a little bit more general:
A galaxy is a self-graviting ensemble of baryonic matter.
Why exclude Dark Matter? As explained in another post (Environment), Dark Matter shapes the gravitational field in which galaxies live. Its structure seems quite complex, with massive and large Dark Matter halos, together with many small halos much smaller than the sizes of galaxies. Currently, I do not see them to be part of galaxies, and can a Dark Matter halo without baryonic matter be considered as a “galaxy”? For the time being, our knowledge of Dark Matter is not enough to unambiguously answer this question, and I do not see the need to include it into the definition.
I indeed see the gravitational field as the environment of galaxies, on which they depend very much but on which they have some influence as well, like the microcosm for living organisms. Dark Matter halos can be seen as the gravitational field itself or as a competitor of galaxies sharing the same environment.
Hence, for astrocladistics at least, the two definitions given here are sufficiently general and precise.
Astrocladistics 