“Star formation in High Redshift Galaxies"

The Hubble Space Telescope has opened up a view of star formation in young galaxies that has never been possible before. Because the most distant galaxies appear as they were when light left them long ago, we can see the various steps of galaxy formation throughout time. Our work in the last three years has concentrated on the nature of star formation in these galaxies, most of which are peculiar by the standards of our own neighborhood. The dominant peculiarity is the presence of enormous young clusters and star complexes in the disk systems. These complexes are 1000 times more massive than any star-forming regions in galaxies today, and yet they appear to form by standard processes involving the localized collapse of disk gas. The average radial distribution of the star-forming patches is exponential, even though each disk is highly irregular. This implies that spiral galaxy disks form by the dispersal and dissolution of giant star complexes, which form by gravitational instabilities in thick and turbulent gas layers. Clumpy disk star formation precedes the formation of spirals and ellipticals, going back to the bandshifting limit of the ACS camera on HST, which is z~5. Thus most star formation in the Universe seems to begin in patchy disk-like systems, and from these, all of today's Hubble types eventually form. Numerical simulations illustrate the buildup of exponential disks in the manner observed.