Two of NASA's "Great Observatories", the Hubble Space Telescope (HST) and the Spitzer Space Telescope (SST), have joined forces to discover nine of the smallest, faintest, most compact galaxies ever observed in the distant Universe. Shining with the luminosity of millions of stars, each of the newly discovered galaxies is far dwarfed by our Milky Way Galaxy (100,000 light years across).

The bottom row of pictures shows several of these clumps (the values are expressions of distance). Three of the galaxies appear to be slightly warped. The images of these galaxies are smeared into tadpole-like shapes, rather than being round blob-shaped objects.

This indicates that they may be undergoing interactions and mergers with nearby companion galaxies to form larger structures. The galaxies were observed in an intriguing long-exposure image, the Hubble Ultra Deep Field (HUDF), with Hubble's Advanced Camera for Surveys and the Near Infrared Camera and Multi-Object Spectrometer. Observations were also carried out with SST's Infrared Array Camera and the European Southern Observatory's Infrared Spectrometer and Array Camera.

"These are among the lowest mass galaxies ever directly observed in the early universe," said Nor Pirzkal of the Space Telescope Science Institute (STScI) and the European Space Agency (ESA).

The standard model for galaxy evolution predicts that small galaxies in the early Universe evolved into the massive galaxies of today by merging. These nine Lego-like primitive galaxies, initially detected by Hubble, were probably the building blocks of the observable Universe.

Pirzkal was surprised to find that the galaxies' computed masses turned out to be so small. The SST observations confirmed that these galaxies are some of the smallest building blocks of the Universe.

These young galaxies yield important new insights into the early Universe, just one billion years after the Big Bang. Hubble detected sapphire blue stars within the nine primordial galaxies. These stars, estimated to be only a few million years old, are very young by cosmic standards. They are converting Big Bang elements (primarily hydrogen and helium) into heavier elements. The stars have probably not yet begun to seed the surrounding space with chemical elements synthesized within their cores.

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

The galaxies were spotted by James Rhoads, Arizona State University; and Chun Xu, Shanghai Institute of Technical Physics in Shanghai, by their prominent and energetic light coming from glowing hydrogen.

Pirzkal's main collaborators were Malhotra, Rhoads, Xu, and the GRism ACS Program for Extragalactic Science (GRAPES) team.

Further Reading

HubbleSite
http://hubblesite.org/
Hubble/ESA Release
http://www.spacetelescope.org/news/html/heic0714.html
SST
http://www.spitzer.caltech.edu/

Aymen Mohamed Ibrahem
Senior Astronomy Specialist