The Hubble Deep Field North reveals over 2500, some more than 12 billion light years away, galaxies in an almost empty portion of the sky near Ursa Major. Distances to many of these faint galaxies was estimated by comparing Deep Field exposures taken through different color filters. Many galaxies seen in visible light abruptly vanish when viewed in ultraviolet light, because hydrogen gas absorbs UV light. The more distant a galaxy is the more hydrogen gas is present between it and the observer. The cumulative effect of the absorption is to make these galaxies essentially disappear in the UV. Using the color analysis method, Astronomers have identified a faint galaxy more than 11.5 billion light years away.
The Hubble Deep Field shows a suprising number of peculiar shaped galaxies, even some of the brightest. The relationship between these peculiar galaxies and the present-day normal galaxies is not well understood. However, one possible explanation for the apparent evolution in the shapes of galaxies may be that the Wide Field Camera sees only ultraviolet light from distant galaxies that has been redshifted to visible wavelengths.
The Near Infrared and Multi-Object Spectrometer (NICMOS) can observe visible light that has been redshifted into the infrared portion of the spectrum from the very distant galaxies in the Deep Fields. Some galaxies that exhibit irregular morphologies when view in UV light appear to have normal shapes when viewed in the infrared. However, some galaxies retain their peculiar morphologies at longer, infrared wavelengths, implying that they are really irregular objects.
The observation of small irregular galaxies seem to fit a model in which objects evolved from the bottom up. Galaxies begin as small groups of stars, and these merge into ever-larger structures.
Visible light observations of the Deep Fields reveals that star formation peaked between 7 and 9 billion years ago. This result is consistent with earlier studies of the chemical composition of the intergalactic medium, in which gas clouds along the line of sight to distant quasars is analyzed. However, the rare occurance of bright star-forming galaxies (starburst galaxies) at large distances is quite suprising. Studies of nearby elliptical galaxies suggest they formed their stars very rapidly when the universe was just 1 to 3 billion years old. During the star formation phase they should have been very bright and numerous. The Deep Field show a few bright star-forming galaxies, but not nearly enough. It might be that visible light observations are missing much of the star birth action.
Followup observations in the infrared will help to determine whether dust obscures ultraviolet and visible light from galaxies, hiding them from view in observations like the Deep Field. Infrared and radio telescopes along with new telescopes capable of observing light in submillimeter-wavelengths have identified several objects with faint or invisible optical counterparts that may be very distant, dusty "starburst" galaxies.
The new Hubble Deep Field South images are taken in the constellation Tucana, near the south celestial pole. The image clearly shows the presence of many spiral galaxies which are similar to our own Milky Way galaxy. The image also reveals a variety of peculiar-shaped galaxies that are merging with companion galaxies. Elliptical galaxies appear as reddish blobs. The colors in the image are a natural representation of the galaxies' stellar population. Blue corresponds to young hot stars and red may indicate older stars, starlight scattered by dust, or very distant starlight that has been shifted to redder wavelengths by the expansion of the universe.
