NASA's Hubble and Spitzer space telescopes have joined forces to discover nine of the smallest, faintest, most compact galaxies ever observed in the distant universe. Blazing with the brilliance of millions of stars, each of the newly discovered galaxies is 100 to 1,000 times smaller than our Milky Way galaxy.

"These are among the lowest mass galaxies ever directly observed in the early universe," said Nor Pirzkal of the Space Telescope Science Institute in Baltimore, Md.

The conventional model for galaxy evolution predicts that small galaxies in the early universe evolved into the massive galaxies of today by coalescing. These nine Lego-like "building block" galaxies initially detected by Hubble likely contributed to the construction of the universe as we know it.

Pirzkal was surprised to find that the galaxies' estimated masses were so small. Hubble's cousin observatory, Spitzer, was called upon to make precise determinations of their masses. The Spitzer observations confirmed that these galaxies are some of the smallest building blocks of the universe.

These young galaxies offer important new insights into the universe's formative years, just one billion years after the Big Bang. Hubble detected sapphire blue stars residing within the nine pristine galaxies. The youthful stars are just a few million years old and are in the process of turning Big Bang elements, primarily hydrogen and helium, into heavier elements. The stars have probably not yet begun to pollute the surrounding space with elemental products forged within their cores.

"While blue light seen by Hubble shows the presence of young stars, it is the absence of red light in the sensitive Spitzer images that was conclusive in showing that these are truly young galaxies without an earlier generation of stars," says Sangeeta Malhotra of Arizona State University in Tempe, Ariz., one of the investigators.

The galaxies were first identified by James Rhoads of Arizona State University and Chun Xu of the Shanghai Institute of Technical Physics in Shanghai, China, by their prominent and energetic light coming from glowing hydrogen. Three of the galaxies appear to be slightly disrupted; rather than being shaped like rounded blobs, they appear stretched into tadpole-like shapes. This is a sign that they may be interacting and merging with neighboring galaxies to form larger, cohesive structures.

The galaxies were observed in the well-known Hubble Ultra Deep Field with Hubble's Advanced Camera for Surveys and the Near Infrared Camera and Multi-object Spectrometer. Observations were also done with Spitzer's Infrared Array Camera and the European Southern Observatory's Infrared Spectrometer and Array Camera.

Pirzkal's main collaborators were Malhotra, Rhoads, Xu and the Grism Advanced Camera for Surveys Program for Extragalactic Science team.

For images and additional information about these galaxies, visit: http://spitzer.caltech.edu/spitzer and http://hubblesite.org/news/2007/31 .

NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology, also in Pasadena. Caltech manages JPL for NASA.

The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. The Space Telescope Science Institute conducts Hubble science operations. The institute is operated for NASA by the Association of Universities for Research in Astronomy, Inc., Washington.