How do galaxies form and evolve?

We know very little about the nature of dark matter and dark energy, and we are more familiar with ’ordinary matter’, made of protons, neutrons, electrons, etc. Yet, much remains to be done to fully map the evolution of this baryonic component of the Universe.

The finite volume accessible to astronomical observations is bounded by the sphere at a redshift of ~1000 emitting the microwave background, beyond which the Universe is fully opaque to radiation. Within this volume, three quarters of all the baryons we could, in principle, detect lie between redshifts of 7 and 1000. These are the Dark Ages, between the epoch of recombination (~400000 years after the Big Bang) and the most distant galaxy so far detected (~750 million years after the Big Bang). No direct evidence has yet been gathered for any kind of event in the Dark Ages, in spite of density fluctuations having grown by many orders of magnitude during this critical half billion years in the life of the Universe.

Nevertheless, our first glimpses at redshift seven reveal that the young Universe was by then almost completely re-ionized, while stars, galaxies, and quasars had begun to form and shine, many with the metal-rich signatures of even earlier generations of star formation. Understanding this rapid build up of stars, metals (elements heavier than boron), galaxies, and supermassive black holes, as well as the subsequent transformation of these young objects to the present day Hubble sequence of galaxies is a major challenge.

As the Universe was re-ionized, most baryons were heated by stellar radiation and mechanical energy input from exploding stars (supernovae) as well as from active galactic nuclei powered by supermassive black holes. As a result, over 90% of the baryons were left in a diffuse intergalactic medium. Nearly half of even the local intergalactic baryons have yet to be detected. Understanding the formation and evolution of galaxies, and the role of the intergalactic baryons and metals, will require a clever combination of observational and theoretical approaches.

Key questions to be addressed are:

• How can we peer into the Dark Ages, and map the growth of matter density fluctuations from their tiny size at redshift one thousand to the formation of the first stars and galaxies?
• What are the dominant sources for re-ionization of the Universe: Star light, black hole powered active galactic nuclei, or even decaying supersymmetric particles? How long did the process take? How did the structure of the cosmic web of galaxies and intergalactic gas evolve?
• What are the histories of the production and distribution of the metals in the Universe, within and between the galaxies?
• How was the present-day Hubble sequence of galaxies assembled, as traced by the build up of their mass, gas, stars, metals, and magnetic fields?
• What is the detailed history of the formation and evolution of our own Galaxy, and what lessons does it hold for the formation and evolution of galaxies generally?