Galaxies are the most obvious tracers of our Universe. But not the only one fortunately. Indeed, we now know that the baryonic matter represents only 4% in energy of the Universe. 70% comes from the Dark Energy and 26% from the Dark Matter. We do not know what they are (hence “dark”) but the former is responsible for the expansion of the Universe and the latter appears to be purely gravitational and is of extreme importance for galaxies.

After the Big Bang, a very short inflationary phase, baryonic matter recombined and light was then free. In other words, the Universe became transparent and yes, we can see it at this time ! This is the Cosmic Microwave Background (CMB) that is the thermal emission of the Universe 13.7 giga years ago and gives us the precise shape of the gravitational field at this epoch. What do we see ? Tiny, very tiny heterogeneities over a very uniform field. Since light and matted were so intimately coupled just before, we infer that these heterogeneities are tiny overdensities of the gravitational field.

Since gravitation is attractive, the more massive the more you attract and the more you increase your mass. Thus, these overdensities, called halos, began to grow by merging. This is the hierarchical growth of Dark Matter Halos. But wait, since the Universe is expanding, not all halos merge into a single monstrous one. Rather, merging occur sporadically during the 13.7 Gyears depending on the gravitational encounters. This point is important because one should not forget that small Dark Matter halos still exist.

What about the baryonic matter galaxies are made of ? Well, being 6 times less than Dark Matter, it just follows. This is a very common paradigm nowadays that stipulates that galaxies are invariably at the center of a Dark Matter halo. This is a paradigm, very convenient in numerical simulations. But observations tend to show that this is not entirely true. And logic tells a somewhat different situation.

Baryonic and Dark Matter are exposed to gravitation, there are shaping the gravitational field in which they move. But Dark Matter is dominant, so it dictates the gravitational field to the baryonic matter. It is like a microcosm where the environment influences the species but the species act the environment to live and evolve.  And with gravitation, nothing is static, everything moves, and for a galaxy to stay at the center of a Dark Matter halo, either it had to be here from the start or dissipate some of its kinetic energy and angular momentum to stop right at the center. I personally prefer the picture of galaxies moving into a sea of Dark Matter.

This question has some importance because Dark Matter halos are not constituents of galaxies. Their respective evolutions can be linked, but they are not the same object. Halos are the environment in which galaxies move and evolve.  Galaxies are self-gravitating objects, they do not require Dark Matter to hold since baryonic matter is capable of shaping the very local gravitational field.

There another simple reason to distinguish halos from galaxies. If we think about the first moments after recombinations, we would reasonably think that the baryonic matter is trapped into the Dark Matter halos. Yes, but there is not enough baryonic matter to fill all halos. As a consequence, there should be somewhat empty Dark Matter halos. Must we call them galaxies ? Galaxies without any baryonic matter ? Why not, after all this is merely a question of definition. But our current knowledge of Dark Matter makes this solution quite adventurous.

We see that when we try to track the history of galaxy diversification, we are confronted to the nature of the first objects in the Universe. We know the story : gas concentrates into the halos and after a while (the Dark Age where no light is emitted) it forms the very first stars which reionise the Universe with their ultraviolet radiation. First stars, first light, we are not far from a galaxy. Or do we already have galaxies ?

Well, we need a clear definition of what we call a galaxy.

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