Summary Of Key Findings From The Hubble Deep Field

Press Release downloaded from STScI HDF Key Findings

Overview

The Hubble Space Telescope spent 10 days in December 1995 observing a single tiny patch of sky near the Big Dipper. These observations resulted in the deepest image of the sky, called the Hubble Deep Field (HDF), revealing galaxies fainter than had ever been seen before. The striking full-color image of the distant universe was unveiled at the American Astronomical Society Meeting in January 1996, and for the last year has been the subject of intense study worldwide. Here is an overview of the current findings.

Small Galaxies in the Early Universe

The HDF is apparently dominated by galaxies smaller than the typical giant galaxies in the present universe (this was also inferred from the earlier Hubble Medium-Deep Survey). The HDF shows that galaxies stay small even at faint magnitudes. In theory the angular diameters of galaxies should stop decreasing significantly beyond 8 billion light years (redshift of 1). For a closed universe, galaxies even get bigger at high redshifts.

Open Versus Closed Universe

Open universe models work best for matching both the number of galaxies and their colors (used to asses stellar populations) in the HDF. Alternatively, closed universe models with an enhanced population of dwarf galaxies at redshifts between 0.5 and 2 might work as well. These dwarfs at moderate redshifts would either have to merge to become part of bright galaxies seen today or fade away so that they are no longer visible in the local universe. There are difficulties with both possibilities, but neither is firmly ruled out.

Disturbed Galaxies

A surprisingly large fraction of galaxies show disturbed or peculiar morphologies, even among the brighter galaxies in the HDF (whose sizes and numbers are reasonably consistent with the quiescent evolution of galaxies). Nevertheless, it is not clear how much of this is due to real evolution and how much is due to the fact that ultraviolet light from distant galaxies tends to highlight the peculiarities of even normal galaxies.

Stellar Baby Boom

Star formation in galaxies peaked at between 7 and 9 billion years ago (redshift of 1 and 2). This is based on colors and redshift surveys which provide an estimate of the total stellar output of galaxies at redshifts greater than 2.5 (very few galaxies were known above redshifts of 1 prior to the HDF).

In Search of Hidden Stars

Much of the star formation in the universe may be taking place in dusty galaxies. Calculations based only on optical measurements may be missing much of the starbirth action. (This claim is disputed and has yet to be published.)

Missing Mass -- Still Missing

The HDF results rule out low-mass, hydrogen-burning stars as a major source of dark matter in the Milky Way, because there are only a handful of red stars in the HDF field.