FROM YOUNG TO OLD

SPECIAL ISSUE 1999

Course: High-redshift galaxies in the HDF and elsewhere

Prof. Mark E. Dickinson
Space Telescope Science Institute (STScI)
EEUU

IR image of the HDF.
Courtesy of Huble Space Telescope, NICMOS and STIS

Systematic redshift surveys of large numbers of galaxies are producing an increasingly good picture of galaxy evolution at high redshift, that is, of the early Universe. The analysis of the Hubble Deep Field (HDF) data has largely contributed to this scenario. The HDF images have been produced during a period of great progress towards understanding the evolution of galaxies over cosmic time, providing a deep insight into the distant Universe. The available evidence is that the largest and most massive galaxies were largely in place by z=0.8, but how different are galaxies at z=3 as compared to those at z=0? Has the HDF completed he galaxy population census? Prof. Mark Dickinson, of the Space Telescope Scientific Institute, answers in the following interview to questions like these and comments on the HDF results at high redshifts.

"There are interesting similarities and differences. The galaxies which we observe at z=3 have a space density comparable to that of L* galaxies today, and most interestingly they appear to cluster together with comparable correlation strength as do bright galaxies locally. We find "walls" of large scale structure at z=3 just as we do at z=0, and a similar correlation length. On the other hand, the sizes and morphologies of the galaxies are quite different than those of L* galaxies today — they are smaller, and we do not see forms which fit neatly into the classical Hubble Sequence of spirals and ellipticals. They are also forming stars more rapidly than do typical, bright galaxies today. In many ways their spectral properties are similar to those of present-day starburst galaxies. Their clustering leads us to believe that these are the predecessors of today’s massive galaxies, but the evolutionary path from z=3 to z=0 is not yet clear."

"Certainly at z=1 (when the Universe was somewhere between 1/2 and 1/3 its present age) the galaxy population was substantially different than it is today, with a much greater global star formation rate, and with many more luminous irregular galaxies present. There is reasonably good evidence for evolution even in the last few billion years of cosmic history, e.g. out at z=0.3 or 0.4 — e.g., the "Butcher-Oemler effect" in galaxy clusters, the greater abundance of blue, star forming galaxies, and also apparently rapid evolution in the ultraluminous infrared galaxy population. At the same time, out to z=1 we *do* find "classical" Hubble sequence giant spirals and ellipticals in the universe, very similar to those around us today."

"We now have good pictures of the distant universe and a partial census of its galaxy population, but at present our understanding of the formation and evolution of those galaxies is still fairly rudimentary. The work now consists of "connecting the dots" — i.e., of drawing evolutionary connections between the galaxy populations we see at different epochs. Much of this work will come from large ground-based surveys, especially those which let us connect mass with light: e.g., via measurements of galaxy kinematics at high redshift, or the study of galaxy clustering and its evolution. HST will continue to play an important role, most especially through larger surveys that will be done when the Advanced Camera for Surveys is installed late in 2000. We can expect significant new developments from mid-infrared and sub-millimeter studies of distant galaxies, both from space (SIRTF and eventually NGST) and with millimeter telescopes on the ground (SCUBA at the JCMT now, and the MMA/LSA in the future). NGST should lead to tremendous advances in our ability to study the spectral properties and kinematics of galaxies out to z=5, and should be capable of seeing the first star forming objects in the universe at substantially larger redshifts if they are not wholly obscured by dust."

"At this point, the basic paradigm of a Big Bang cosmology and structure formation by hierarchical clustering (CDM or some variant thereon) seems surprisingly robust. Galaxy formation is probably considerably more complex than has been modeled thus far by CDM-type models, and many of the details will probably have to change (or at least, additional physics will have to be incorporated in the future), but there are also many confirmations of the basic picture. In particular, the existing data on galaxy clustering at z=3 is in excellent agreement with the basic picture of biased galaxy formation in hierarchical models."

Born on November 24, 1962, in Connecticut (USA), MARK EVERETT DICKINSON was awarded his PhD in Astronomy by the University of California in Berkeley (1994), with a thesis entitled "The Cluster Environment of Distant Radio Galaxies" supervised by Hyron Spinrad.

He has ever since worked for the Johns Hopkins University and the Space Telescope Science Institute, were he has been an Assistant Astronomer since 1998.

His scientific interests are focused on subjects such as optical, infrared and X-ray observational astronomy, objects at high redschift, clusters of galaxies, large scale structure, QSO absorption line systems, radio galaxies and galaxy evolution and observational cosmology.