Models based on cold dark matter and dark energy predict many
more satellite halos than existing dwarf galaxies in the Local
Group. This "missing satellites problem" may be resolved by
considering inefficient cooling of gas in small dark matter
halos. Observations show that many dwarf galaxies exhibit
extended star formation histories continuing well after hydrogen
reionization. It remains a challenge for the models to
reproduce these star formation histories in detail.
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Globular star clusters are remnants of early bursts of star
formation in galaxies. They form in giant molecular clouds,
following gas-rich galactic mergers. Only massive clusters
survive the dynamical evolution to be observed now, while
low-mass clusters gradually dissolve. It remains a challenge
for a self-consistent model to reproduce the observed spatial
and metallicity distributions of globular clusters.
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Galaxies never stop to change. Their properties evolve
continuously as a result of star formation, stellar evolution,
merging and interactions with their neighbors. The latter
effect is particularly strong in galaxy clusters, where galaxies
transform from disk to spheroidal types. It remains a challenge
to quantify the evolution of galaxies due to the effects of
tidal interactions, ram pressure of intracluster gas, and
stellar and AGN feedback.
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A code for calculating the contraction of dark matter halos
in response to condensation of baryons.
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