Galaxies

Galaxies are vast ensembles of stars, gas and dust, embedded in darkmatter halos. They are the basic building blocks of the Universe,gathered in groups, clusters and super-clusters. They exist in manyforms, either as spheroids or disks. Classifications, such as the Hubblesequence (based on mass concentration and gas fraction) and the colormagnitude diagram (which separates a blue cloud from a red sequence)help to understand their formation and evolution. Galaxies spend a largepart of their lives in the blue cloud, forming stars as spiral or dwarfgalaxies. Then, via a mechanism that is still unclear, they stop formingstars and quietly end in the red sequence, as spheroids. Thistransformation may be due to galaxy interactions, or because of thefeedback of active nuclei, through the energy released by their centralsuper-massive black holes. These mechanisms could explain the historyof cosmic star formation, the rate of which was far greater in the first halfof the Universe s life.Galaxies delves into all of these surrounding subjects in six chapterswritten by dedicated, specialist astronomers and researchers in the field,from their numerical simulations to their evolutions.