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Impact des fusions majeures sur l'évolution des galaxies spirales et naines

Abstract : The discovery of the expansion of the Universe in 1929 by Edwin Hubble and the study of cosmological models have removed the static and infinite view of the Universe ; the Universe has been evolving for 13 Gyrs, since the Big Bang. The standard hierarchical ΛCDM cosmological model predict that, during this evolution, the dark matter haloes would have mainly accreted their mass by successive mergers. The baryonic evolution would have followed that of the dark matter one because baryons are in minority compared to the dark matter. Two kind of mergers would have structured the evolution of galaxies : the minor and major mergers. Moreover, a continuous accretion of cold gas, similar to several minor mergers, could also be an important mechanism to gather galaxy masses. The minor mergers and the gas accretion lead to a soft galaxy evolution. On the other hand, the major mergers suddenly modify the morphology as well as the the kinematic of merger galaxies and thus form new galaxies. A last type of evolution can appear when a galaxy is isolated or during a period between two mergers: the secular evolution. The morphology and the kinematic of a galaxy can thus change via internal perturbations or generated by the last merger. The secular evolution does not add mass to the galaxy ; alone, it is inefficient to create a galaxy. In order to better constrain the galaxy evolution, I first work on the galaxy evolution during the last eight Gyrs. In this topic, I have worked on observational data of the IMAGES (Intermediate MAss Galaxies Evolution Sequence) project, a study, based on 63 galaxies at intermediate redshift (z ∼ 0.6), whose the main objective is to draw up a picture of the galaxy state at intermediate redshift and to understand the evolution mechanisms. I have mainly used the methods developed on the IMAGES sample for 12 new galaxies who have a mean redshift slightly higher than that of the previous sample (z ∼ 0.7 instead of 0.6). With HST data from GOODS, I have morphologically classified the galaxies of the new sample. Then, using the data of the multi-object spectrographe GIRAFFE, I have determined their kinematic. I have confirmed, for a smaller statistic, the results of the IMAGES project : the important fraction of the peculiar galaxies which represent more than 50 % of the intermediate mass galaxies at intermediate red- shifts at the cost of the spiral ones; a correlation between the morphological spiral class and the kinematical rotating disk one; a trend for the peculiar galaxies to have complex or perturbed kinematics. These results imply that galaxies have changed their morphology between z = 0.7 and z = 0. The galaxies which have a complex or perturbed kinematic on large scales (> 5 kpc) require mechanisms which can deeply change the motion of their gas. The major merger is likely the mechanism to explain these kinematics rather than the soft gas accretion or the minor merger. The peculiar galaxies had to evolve into spiral galaxies because the elliptical ones were already in place at z > 1. My second research work was to test the scenario of rebuilding spiral galaxies after a major merger. The gas fraction in galaxies which is higher in the past (> 50 % at z ∼ 1 − 2) has a prominent role in the rebuilding process. A part of the gas fall by cooling into the galaxy potential after a merger, keeping its angular momentum which make possible the rebuilding of a disk. Hammer et al. (2005a) explain the stellar formation during the last eight Gyrs as well as the morphological evolution and abundance of the galaxies by short major merger phases followed by disk rebuilding phases. According to this scenario, several spiral galaxies of the nearby universe would be the result of major mergers. The Andromeda galaxy seems to be a good candidate for this kind of phenomenon. It has a number of globular clusters and dwarf galaxies close to twice that of the Milky Way, several stellar streams whose the Giant Stream and especially a classical bulge. I have participated to the M31 rebuilding scenario by a merger major via numerical simulations in order to test this hypothesis. A major merger with a mass ratio ∼ 3, gas fractions over 60 % and described by a first passage occurring 8-9 Gyrs ago followed by a second passage 5-6 Gyrs ago, reproduce the main morphological and kinematical structures of M31 (bulge, thick and thin disks, Giant Stream). This supports the M31 disk rebuilding scenario after a major merger. My last work deals with the consequences of major mergers on their environment. Indeed, the matter ejected by a major merger can reach masses of more than 15 % of the total baryonic mass of merger galaxies. The main part of this matter could be ejected at large distances due to the tidal tail formation during the merger. Inside these tidal tails, new galaxies can be formed, the tidal dwarf galaxies. Thus, a major merger can be at the origin of new forming galaxies. If the main part of the spiral galaxies have been formed after a major merger, the consequences of this evolution process can not be neglected. More specifically, the major merger which would be at the origin of the Andromeda galaxy could have populated the Local Group with tidal dwarf galaxies. The Milky Way dwarf galaxies have two particularities : a spatial distribution which form a thick plan, named the VPOS (Vast Polar Structure), and the presence of two dwarf irregular galaxies, the Magellanic Clouds (MC), very close to the Milky Way (< 60 kpc). My work has consisted in testing the hypothesis that a tidal tail, ejected by the M31 merger major, could have formed the companion dwarf galaxies of the Milky Way. This scenario is convincing to reproduce their spatial and angular momentum distribution. However, it seems in contradiction with the high mass-to-light ratio measured by velocity dispersions in the dwarf spheroidal galaxies. These ratios could be explained by the presence of large amounts of dark matter, whereas the tidal dwarf galaxies have no dark matter by nature. This apparent contradiction could be explained if the hypothesis of the internal stability of the dwarf galaxies is discarded. More generally, this study revive the question about the dwarf galaxy origin. Are they relics of the primordial galaxies of the Universe or the result of major mergers? If the major merger phenomenon is confirmed as the main mechanism to form spiral galaxies and if studies prove that a major merger can create several tidal dwarf galaxies, the research on the formation, the number and the spatial distribution of the dwarf galaxies will have to be re-examined.
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Submitted on : Monday, April 7, 2014 - 6:44:10 PM
Last modification on : Wednesday, November 17, 2021 - 12:29:13 PM
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Sylvain Fouquet. Impact des fusions majeures sur l'évolution des galaxies spirales et naines. Astrophysique galactique [astro-ph.GA]. Université Paris-Diderot - Paris VII, 2013. Français. ⟨NNT : ⟩. ⟨tel-00975096⟩

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