High Redshift Research at Oxford

The past decade has seen an explosion of data and excitement in the area of high-redshift galaxy formation. Following the initial detection of z ∼ 2 − 4 Lyman break galaxies by Steidel and co-workers in the 1990s, distant sub-mm sources were detected at comparable redshifts by SCUBA, and most recently deep infrared data has revealed a quiescent population of massive ‘distant red galaxies’ at z > 2. Understanding how these very different denizens of the early Universe relate to one another is a key challenge. Does the co-existence of such diverse sources imply some form of ‘duty-cycle’ where one type evolves into another, and how do these sources relate to the most massive elliptical galaxies we see today? The key to progress lies in not merely addressing the global history of star formation (i.e., updating the Madau-Lilly plot with the same reliable Hα star formation indicator used over all redshifts) but also determining how star formation is distributed among galaxies of different mass as a function of cosmic time. Fundamental consistency checks are measuring the evolution of the stellar mass density (which should relate to the integral of past star formation activity) and determining the history of chemical enrichment of galaxies.

Independently of the above, the standard paradigm for mass assembly in galaxies has been challenged by the abundance of massive galaxies seen at early times. At redshifts z < 2, ‘downsizing’ - the early maturing of massive galaxies followed by the continued growth of lower mass systems – indicates the importance of physical processes in addition to those expected from gravitational instability governed by the merger of pure dark matter halos. Does this paradoxical behaviour extend to higher redshift as suggested by the detection of surprisingly massive galaxies at redshifts as high as z = 5 − 6?

Finally, the frontier beyond z ∼ 7 beckons. Gaining a glimpse of the earliest sources seen in the first 500 Myr with present facilities and future opportunities with JWST and ground-based Extremely Large Telescopes. The most basic questions might be answered with upcoming instruments. When did reionization occur? Were star forming galaxies primarily responsible? Did the dominant contribution of reionizing photons come from rare luminous or abundant sub-luminous sources?

The High Red Shift Universe