Hubble sees early building blocks of today's galaxies

17 Jun 2002

New Hubble Space Telescope images reveal what may be galaxies under construction in the early universe, out of a long sought ancient population of "galactic building blocks."

Hubble's detailed images, taken with the Wide Field Planetary Camera 2, reveal a grouping of 18 gigantic star clusters that appear to be the same distance from Earth, and close enough to each other that they will eventually merge into a few galaxy-sized objects. They are so far away, 11 billion light-years, that they existed during the epoch when it is commonly believed galaxies started to form.

These results add weight to a leading theory that galaxies grew by starting out as clumps of stars, which, through a complex series of encounters, consolidated into larger assemblages that we see as fully-formed galaxies today. Astronomers at Arizona State University, Tempe, AZ, and the University of Alabama at Tuscaloosa found 18 of these cosmic building blocks packed into an area about two-million light-years across.

The building blocks seen by Hubble consist of about a billion young stars each, and Hubble shows star formation is underway through the presence of many blue stars and glowing gases. The objects typically measure only 2,000 light-years across (1/10th of an arc second). These were seen in a two-day (67-orbit) exposure by Hubble of a small region of sky in the northern part of the Hercules constellation near the border with Draco.

The researchers used an optical filter precisely tuned to detect the ultraviolet emission from glowing hydrogen gas heated by newborn stars that formed early in the universe, but shifted to longer visual wavelengths by the universal expansion. Follow-up spectroscopic observations with the Multi-Mirror Telescope at Mt. Hopkins, AZ, showed at least five of the clumps are all at the same distance from Earth. The team confirmed that another five objects were at the same distance by imaging another redshifted hydrogen line in the near infrared with NASA's Infrared Telescope Facility, and through spectroscopic follow-up at the 10 meter W. M. Keck Telescope, both on Mauna Kea, Hawaii. The amount of redshift corresponds to a distance of 11 billion light-years -- far enough to probe the early universe during the period where many of the giant galaxies were being assembled.