Observations of the High Redshift UniverseRichard S EllisarXiv:astro-ph/0701024v1 1 Jan 2007Observations of the High Redshift UniverseRichard S Ellis1Astronomy Department, California Institute of [email protected] this series of lectures, aimed for non-specialists, I review the consider-able progress that has been made in the past decade in understanding howgalaxies form and evolve. Complementing the presentations of my theoreticalcolleagues, I focus primarily on the impressive achievements of observationalastronomers. A credible framework, the ΛCDM model, now exists for inter-preting these observations: this is a universe with dominant dark energy whosestructure grows slowly from the gravitational clumping of dark matter halosin which baryonic gas cools and forms stars. The standard model fares wellin matching the detailed properties of local galaxies, and is addressing thegrowing body of detailed multi-wavelength data at high redshift. Both thestar formation history and the assembly of stellar mass can now be empir-ically traced from redshifts z ≃6 to the present day, but how the variousdistant populations relate to one another and precisely how stellar assemblyis regulated by feedback and environmental processes remains unclear. In thelatter part of my lectures, I discuss how these studies are being extended to lo-cate and characterize the earliest sources beyond z ≃6. Did early star-forminggalaxies contribute significantly to the reionization process and over what pe-riod did this occur? Neither theory nor observations are well-developed in thisfrontier topic but the first results are exciting and provide important guidanceon how we might use more powerful future facilities to fill in the details.2 Richard S Ellis1 Role of Observations in Cosmology & GalaxyFormation1.1 The Observational RenaissanceThese are exciting times in the field of cosmology and galaxy formation! Tojustify this claim it is useful to review the dramatic progress made in thesubject over the past ≃25 years. I remember vividly the first distant galaxyconference I attended: the IAU Symposium 92 Objects of High Redshift, held inLos Angeles in 1979. Although the motivation was strong and many observerswere pushing their 4 meter telescopes to new limits, most imaging detectorswere still photographic plates with efficiencies of a few percent and there wasno significant population of sources beyond a redshift of z=0.5, other thansome radio galaxies to z ≃ 1 and more distant quasars.In fact, the present landscape in the subject would have been barely rec-ognizable even in 1990. In the cosmological arena, convincing angular fluc-tuations had not yet been detected in the cosmic microwave background norwas there any consensus on the total energy density ΩT OT. Although therole of dark matter in galaxy formation was fairly well appreciated, neitherits amount nor its power spectrum were particularly well-constrained. Thepresence of dark energy had not been uncovered and controversy still reignedover one of the most basic parameters of the Universe: the current expansionrate as measured by Hubble’s constant. In galaxy formation, although evolu-tion was frequently claimed in the counts and colors of galaxies, the physicalinterpretation was confused. In particular, there was little synergy betweenobservations of faint galaxies and models of structure formation.In the present series of lectures, aimed for non-specialists, I hope to showthat we stand at a truly remarkable time in the history of our subject, largely(but clearly not exclusively) by virtue of a growth in observational capabili-ties. By the standards of all but the most accurate laboratory physicist, wehave ‘precise’ measures of the form and energy content of our Universe and adetailed physical understanding of how structures grow and evolve. We havesuccessfully charted and studied the distribution and properties of hundredsof thousands of nearby galaxies in controlled surveys and probed their lumi-nous precursors out to redshift z ≃6 - corresponding to a period only 1 Gyrafter the Big Bang. Most importantly, a standard model has emerged which,through detailed numerical simulations, is capable of detailed predictions andinterpretation of observables. Many puzzles remain, as we will see, but theprogress is truly impressive.This gives us confidence to begin addressing the final frontier in galaxyevolution: the earliest stellar systems and their influence on the intergalacticmedium. When did the first substantial stellar systems begin to shine? Werethey responsible for reionizing hydrogen in intergalactic space and what phys-ical processes occurring during these early times influenced the subsequentevolution of normal galaxies?Observations of the High Redshift Universe 3Let’s begin by considering a crude measure of our recent progress. Figure1 shows the rapid pace of discovery in terms of the relative fraction of therefereed astronomical literature in two North American journals pertainingto studies of galaxy evolution and cosmology. These are cast alongside somemilestones in the history of optical facilities and the provision of widely-useddatasets. The figure raises the interesting question of whether more publica-tions in a given field means most of the key questions are being answered.Certainly, we can conclude that more researchers are being drawn to work inthe area. But some might argue that new students should move into other,less well-developed, fields. Indeed, the progress in cosmology, in particular, isso rapid that some have raised the specter that the subject may soon reachingsome form of natural conclusion (c.f. Horgan 1998).Fig. 1. Fraction of the refereed astronomical literature in two North American jour-nals related to galaxy evolution and the cosmological parameters. The survey impliesmore than a d oubling in fractional share over the past 15 years. Some possibly-associated milestones in the provision of unique facilities and datasets are marked(courtesy: J. Brinchmann).I believe, however, that the rapid growth in the share of publications islargely a reflection of new-found observational capabilities. We are witnessingan expansion of exploration which will most likely be followed with a more4 Richard S Ellisdetailed physical phase where we will be concerned with understanding howgalaxies form and evolve.1.2 Observations Lead to SurprisesIt’s worth emphasizing that many of the key