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Introduction

The galaxies were not identified as worlds apart, as the philosopher Immanuel Kant foresaw, until the 1920s, just over a century ago. Before then, astronomers had observed nebulae, but had not distinguished between a cloud of emitting gas, such as the Orion Nebula, and outer galaxies, such as the Andromeda Nebula. At the beginning of the 20th Century, a great debate took place to know the size of our world (the Milky Way), and the distance of the various stars in the sky. In 1924, Edwin Hubble observed the variable stars in Andromeda of the Cepheids, which Henrietta Leavitt in 1909 had shown to be a good indicator of distance. Thus, it was shown that Andromeda was a galaxy “outside” ours, located about 2 million light-years away.

Progress since then has been meteoric. We now know millions of galaxies, and determine their distance because of the expansion of the Universe and the corresponding redshift. Due to the finite speed of light, we can go back in time, observing distant galaxies in their youth. We thus observe galaxies as far as the edge of our Universe, at the limit of our horizon, which allows us to reconstruct their history.

In this book, we will first describe the various morphologies and categories of galaxies, which are essential for a better understanding of their formation and evolution. There are several classifications, depending on whether we consider the various stellar components: bulge, disc, spheroid, photometry and more or less blue colors according to the rate of star formation, or the kinematics of stars, that is to say a kinetic energy either dominated by rotation, or by the velocity of dispersion, or the fraction of gas. Disc galaxies generally contain spiral or barred structures, which are the engine of evolution. These structures give rise to resonances which will be the source of rings, or pseudo-rings, very useful to know the speed of spirals and bars.

Our Galaxy, the Milky Way, the best known and most familiar, has a barred spiral structure, yet it took a long time to be identified, because we are in its plane: it appears edge-on, obscured by dust lanes. Its structure appears more clearly in the near infrared. It consists of a thin disc, where the gas and young stars are located, and a thick disc, which dates back billions of years. It has a pseudobulge, mainly due to the bar, and its vertical resonance. There is also a more or less spherical, diffuse halo of stars, whose formation would be essentially due to the accretion of small satellite galaxies. These are destroyed by tidal interaction, and deploy in a multitude of stellar streams. We can go back to the history of the formation of the different components, by galactic archaeology, by determining the age and metallicity of stars. The GAIA astrometric satellite has recently made enormous progress in specifying the distances and proper motions of a large number of webs.

Galaxies can be divided into two main categories: disc galaxies or “late-type” and elliptical galaxies or “early-type”. In our Local Universe, the majority of galaxies are disc galaxies and dominate in number, but since elliptical galaxies are more massive, they dominate in mass in the Universe. Early-type galaxies have a spheroidal morphology, and even if they sometimes have a stellar disc, it is subdominant in front of the massive bulge. Their stellar populations are old, and they have little or no gas. For a very long time, astronomers thought that the flattening of stellar spheroids was due to rotation, but when it became possible to measure their velocities, it was realized that the shapes of these spheroids were instead due to the anisotropy of the velocity dispersion: ellipticals have no or very little rotation. It is then difficult to deproject these systems, which can be triaxial. They are classified as slow or fast rotators. The origin of these differences comes from their history of formation by merger of spiral galaxies.

Late-type disc galaxies are also called spiral galaxies. Their structure of open spiral arms has long been a mystery, because the differential rotation of the disc should wind up the arms around very quickly, making them disappear. But they are actually density waves and not material arms. The arms are quite transient and several waves follow one another; on the other hand, the bars are more robust structures, which allow the galaxy to assemble its mass and evacuate the angular momentum of the accreted gas. The speed of the bar wave can be determined by the different resonances in the plane (forming rings) and perpendicular (forming peanuts or boxes). The discs of the spirals evolve essentially in a secular way, but from time to time an interaction or merger with another galaxy can increase the mass of the bulge and the mass concentration. Star formation is enriched by the accretion of gas from the cosmic filaments, which keeps the galaxy blue and active. On the other hand, the environment of a group or cluster of galaxies can dry up the gas reservoir and stop star formation. The galaxy then becomes red and passive. Among the millions of known galaxies, a clear bimodality has been observed between the red sequence and the blue cloud.

Interactions between galaxies contribute to the enrichment of galaxies with gas, which can create a starburst. Interactions are visible even at large distances because of tidal arms and morphological perturbations of galaxies. Of course, these effects are all the more visible in groups and clusters of galaxies. In the latter, galaxies are swept away from their gas by the ram pressure. Looking back in time, we can see that the number of interacting galaxies was greater in the past, in the first part of the age of the Universe. This could explain part of the history of cosmic star formation.

In today’s galaxy clusters, we observe a morphological segregation: the spirals that dominate in number in the rest of the Universe are gradually disappearing to make way for lenticulars and ellipticals. Galaxies in clusters no longer form many stars. But this was not the case in the past. Already the first observers of distant clusters had noticed blue galaxies, the so-called Butcher-Oemler effect. There must even have been a time in the Universe, at the time of cluster formation, when interactions between galaxies make clusters or proto-clusters richer in star formation than elsewhere in the field. The evolution of galaxies and the history of their star formation have made a lot of progress in recent years, due in particular to several infrared satellites, which have revealed galaxies obscured by dust, where star formation is hidden in the visible, but emerges only in the infrared radiation of dust heated by young stars. The spectral distribution of the radiation from galaxies also makes it possible to distinguish the amount of star formation, and to separate the heating that is due to the energy of super-massive black holes from that due to the energy of stars. The efficiency of star formation varies over time, due to the fraction of gas in galaxies, which was much larger in the past, and to the feedback phenomena of active nuclei, corresponding to the rapid growth of black holes.

The enormous progress made in our knowledge of galaxies should not make us forget all that remains to be discovered, especially since most of the mass of a galaxy is made of exotic dark matter, the nature of which we do not know!