The largest science initiative Steve carried out since coming to the STScI is the Hubble Ultra Deep Field, currently the deepest optical image of the universe ever taken. It is a 1 million second exposure of a small patch of the southern sky taken with the Advanced Camera for Surveys by a team of scientists Steve led in the fall of 2003. The HUDF was released as a public dataset on March 9, 2004. The data are available on the internet as well as a detailed description of the observations: www.stsci.edu/hst/udf.

Since Galileo first pointed a telescope to the sky in the 17th century, astronomy has progressed by discovering properties of the universe at ever increasing distances from Earth. Extending our knowledge from the Solar System to the Milky Way, then to the nearby galaxies, and finally to distant galaxies and quasars has been an ongoing quest over nearly four centuries and involving multiple advances in technology. The recent culmination of the search for ever more distant objects at visual wavelengths was the Hubble Deep Field (HDF), a project created by Robert Williams, past Director of the Space Telescope Science Institute, to get a deep image of the universe using the Wide Field Planetary Camera 2 aboard the Hubble Space Telescope.

One major advance of the HDF was to reveal the universe at a time when typical galaxies looked different from the way they do today. Prior to the HDF, only a handful of HST images at intermediate depth had been available, and they hinted at a population of amorphous galaxies at high redshift. However, these first studies lacked the depth and angular resolution needed to characterize the objects. The HDF overcame this limitation by providing multi-color images of the most distant galaxies observed with sufficient resolution to discern their structures.

Several advances since the HDF suggested that even deeper observations would reveal important aspects of the way that galaxies were created. We believe the atoms in the universe were neutral following initial cooling about 100,000 years after the Bib Bang, but the universe heated up again when it was approximately 1 billion years old by the first generation of stars and black holes. The exact determination of the epoch of heating that reionized the atoms remains one of the holy grails of extragalactic astronomy.

Installation of the Advanced Camera for Surveys (ACS) on the fourth servicing mission to the Hubble Space Telescope in April 2002 provided an opportunity to make even deeper images than the HDF and simultaneously get better resolution because of the more advanced detectors. Our team realized early on that it would be possible to go four to five times fainter than the HDF with the Wide Field Channel (WFC) of ACS, and that additional depth could reveal a population of galaxies at a time when the universe was less than 1 billion years old, possibly beyond the reionization boundary. The great success of the HDF in stimulating the study of the early universe motivated us to design a program that would push back the observational boundaries to redshifts greater than 6 to see if we could reach the reionization epoch for the first time with direct images.

A brief review of the HUDF project appeared in the Fall 2003 STScI Newsletter (PDF).