The evolution of Milky Way satellite galaxies

More than a dozen low-mass galaxies are orbiting the Milky Way as satellite dwarf galaxies (SDGs). The current understanding within the concordance cosmological model of Cold Dark Matter (CDM) requests that the progenitors of these dwarf galaxies were the subunits which assembled by hierarchical merging to the more massive galaxies. This means that the still existing SDGs in the Milky Way environment represent the survivors of these cosmological building blocks. Although the observed present infall of a few SDGs into the Milky Way supports the picture that their accumulation continued since the early epochs, there are only a few stellar streamers observed until now which could witness this accretion effect. In addition to this lack of expected kinematical signatures, the present-day SDGs show chemical compositions substantially different from the one observed in the Milky Way, indicating that the process of star formation and chemical enrichment must have been quite different in these small objects. Because comprehensive models of their formation and evolution are still lacking, on the other hand, their contribution to the stellar populations in the Milky Way halo, in its bulge as well as in the thick disk is not only unclear but far from any clue and therefore a field of many speculations. Also significant differences among the SDGs are difficult to explain and do not allow to place them into a uniform and consistent formation and evolution picture. Not only their low masses are responsible for heir complex properties, moreover, their vicinity to a major galaxy and the evolution of this should have determined the SDGs` formation and evolution accordingly. This proximity, on the other hand, allows us to study in detail and with reasonable accuracy their chemical composition, their star-formation histories, their density structure and gas-to- star composition, and their internal dynamics. The herewith proposed project aims at investigating the evolution of SDGs by numerical models and, by this, at filling the gap of understanding the causes for peculiarities but also diversities of the Milky Way SDGs. Chemo- dynamical modeling can shed light on their evolution and clarify the chemical abundances and abundance ratios of their stellar populations. For this study both, well-observed Local Group dwarf galaxies will be used as a benchmarks, and we perform numerical models in which the most relevant physical processes are included, in order to understand if and under which conditions one can reproduce objects similar to the observed SDGs of the Milky Way. For this purpose accurate chemo-dynamical simulations are required taking the internal processes of stars and gas into account but also allowing for external conditions in the environment of a massive galaxy. To identify the ruling physical mechanisms and the initial conditions able to differentiate the evolution of the SDGs and to produce SF histories and chemical patterns similar to the observed ones will provide a deep insight into the fundamental understanding of galaxy evolution.

In the cosmological structure-formation scenario of the universe the dominant population of Cold Dark Matter (CDM) accumulations possess much lower masses than our Milky Way (MW) which consists of a trillion of sun masses in DM and 200 billion as stars and gas. While the cosmology predicts thousands of small subunits orbiting massive galaxies, observations so far have only detected 22 around the Milky Way and about 25 around our neighbour, the Andromeda galaxy M31. This fact is called the Missing Satellite Problem and requires an urgent solution because of its fundamental relevance for our cosmological picture. Moreover, the observed satellite systems of the MW and M31 have peculiar properties, as e.g. the coincidence of orbit and rotation directions, old stellar populations, high stellar velocities, etc. Most of them are gas free and their star-formation history finished earlier the closer they orbit the MW. Because of their very low masses in the range of 10000 to 100 Mio. times solar with the exception of the more massive Magellanic Clouds, these satellites are too faint to be also observed around massive galaxies outside the Local Group. This means that their general existence is uncertain, because it must be questioned, how low-mass galaxies survive the energetic impact of their stars. Therefore, their numerical modeling is one of the most important challenges of modern Astrophysics to understand these extreme objects. We have therefore started an extensive exploration of how numerical simulations the satellite evolution in the influential field of a massive parent galaxy like the MW can match their observed properties. For this purpose, the development of a new highly sophisticated numerical 3D code with adaptive-mesh refinement was necessary based on the FLASH code, that allows for the simultaneous treatment of the various galactic components, DM, stars, and different gases, and of their mutual interactions, e.g. by star formation and stellar deaths. Moreover, stellar release of chemical elements is included to trace the chemodynamical evolution, therefore called cdFLASH. The physical processes affecting the evolution of satellite dwarf galaxies are of high complexity and act on different lengths and timescales. This, on the other hand, requires detailed investigations on different levels: 1) gas clouds passing through the hot gas surrounding a massive galaxy with respect to their dynamical disruption, thermal heating, and magnetic fields; 2) the response of single dwarf galaxies around their parent one to the stretching by the tidal field and to the hot halo gas; 3) finally, the evolution of a whole system of dwarf satellite galaxies around a central massive galaxy starting from cosmological structure simulations. In addition to the successful development of the advanced code, the evolutionary studies of the various sub-problems yielded the following most spectacular results out of several: Massive gas clouds are not disrupted on their fast motion through hot gas! Satellite galaxies even without dark matter survive in the strange environment of a massive parent galaxy! Of the system of satellite galaxies in a CDM cosmology a much larger number of objects should exist and be visible than observed!