How did galaxies form? Complex question that in reality encompasses two different problems.
On one hand, how did objects that could be called “galaxies” ever appear in the history of the Universe?
On the other hand what is the origin of the galaxies we observe? In other words, how did the galaxies have been formed in the state we observe them?
Again the parallel with biology is enlightening: how did the living organisms form? is a double question: how did life ever appeared and how did dinosaurs appeared? The first part is very very general, since the appearance of life on Earth is intimately connected with life elsewhere in the Universe. The second part is obviously related to diversification, it is the history of living organisms.
Interestingly, the question of the origin of life or the origin of galaxies asks for a clear definition of these entity. And the more we investigate, the less we are able to find a sharp limit. The definition of a galaxy is not so simple as it looks.
When did galaxies form? Same thing as before: when did the first “galaxies” did ever appear and when elliptical or dwarf galaxies did ever appear? These are two different questions.
One one hand, the first appearance of a “galaxy” determines the start of the history of galaxies as a whole in the history of the Universe.
On the other hand, the now established evolution of galaxies implies that the transform themselves and yields new types of objects. The question here is to know when did a given type of galaxies first appear in the history of the galaxies and of the Universe.
Hence, “formation” of galaxies is quite vague a word since “how” and “when” are not synonymous in an evolutionary context. When looking at a given galaxy, we actually want to know when and how this kind of objects formed (were “built”).
The ambiguity of “formation” between how and when is somewhat present in to extreme scenarios of galaxy formation: the monolithic and the hierarchical models. In the first one, galaxies were assembled rather quickly at the beginning of the history of galaxies with rather little evolution. In this case, the formation means both the formation of all galaxies and the formation of some particular type. The “when” and the “how” are basically the same for all. In the hierarchical model, galaxies are supposed to be built up by accretion and merging of small entities appeared at the beginning of the Universe. The “when” is quite vague and the “how” is very general: is a galaxy simply the evolution of a small entity, in which case formation and evolution mean the same thing, or is a big galaxy formed from a smaller object, in which case there is no more room for evolution?
Naturally, it is now clear that none of these two scenarios is entirely true and the reality is a mixture of both. We also now recognize that the “how” and the “when” are specific of each galaxy. For instance, it seems that elliptical galaxies can form by monolithic collapse or by merger, more or less at anytime in the history of the Universe (although more certainly in the very first Giga years). It is also clear that galaxies change a lot. However, the hierarchical scenario is dictated by the hierarchical growth of dark matter halos in which galaxies fit. Interestingly enough, it is linked to the size and the mass of the objects, leading to a “traditional” classification à la Hubble.
The above ambiguity comes from the fact that no distinction is clearly made between an individual and the population, between the formation of and individual and the formation of the population, between the evolution of an individual and the evolution of the global population, and between different “species” of galaxies. There is no “species” among galaxies because no adequate classification exists. For that we need a description of a galactogenesis, one goal of astrocladistics.
How can we define “formation”? We adopt the following definition: the formation of galaxies is the formation of a galaxy or a class of galaxies in the state we observe it, at a given time. The formation is related to the first appearance of an object or a class of objects. Since this is the result of a long history, the outcome of a sequence of evolution and transformation processes, the concept of formation necessary includes evolution. In other words, when we say “formation”, we imply “modification”. When the modifications are important, we newly formed object can be sufficiently different that we decide it to belong to a different class. This is exactly how living species have “appeared” (we do not say they “formed”).
In the hierarchical scenario of galaxy “formation”, dwarf galaxies merge and accrete to “form” larger galaxies as we observe them today. Are the dwarf galaxies around us identical to the pristine ones? Did some large galaxies “form” early in the Universe as observations tend to show? Maybe, but this is only a very small part of the history of galaxies, the one related to one and only one property: the size or the mass. It leaves aside most of the physical and chemical history.
The concept of formation must thus include all modification events of all constituents that compose and characterize galaxies. These evolutionary events have occurred at different epochs with different strength for various lineages of galaxies. This is this complex history of galaxy “formation” that we want to infer.
In summary, the formation of galaxies is the sequence of evolutionary processes that yields the properties specific to each galaxy or to each species of galaxies.
The diversification is the ensemble of the events that have led throughout the history of the Universe to the diversity of objects, existing or having existed, that make the whole population of galaxies.