Galaxies are vast ensembles of stars, gas and dust, embedded in dark
matter halos. They are the basic building blocks of the Universe,
gathered in groups, clusters and super-clusters. They exist in many
forms, either as spheroids or disks. Classifications, such as the Hubble
sequence (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 large
part of their lives in the blue cloud, forming stars as spiral or dwarf
galaxies. Then, via a mechanism that is still unclear, they stop forming
stars and quietly end in the red sequence, as spheroids. This
transformation may be due to galaxy interactions, or because of the
feedback of active nuclei, through the energy released by their central
super-massive black holes. These mechanisms could explain the history
of cosmic star formation, the rate of which was far greater in the first half
of the Universe s life.
Galaxies delves into all of these surrounding subjects in six chapters
written by dedicated, specialist astronomers and researchers in the field,
from their numerical simulations to their evolutions.
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Francoise Combes is a Professor at College de France on the Chair of
Galaxies and Cosmology. She is a specialist of galaxy evolution through
bars and spiral waves, and has uncovered dynamical mechanisms to
fuel central black holes in active galaxy nuclei.
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