To obtain these images, Michael Garrett of the Joint Institute for Very Long Based Interferometry in Dwingeloo, the Netherlands, and his colleagues focused on a small region of sky devoid of bright, nearby stars or local galaxies -- a sort of window on the distant universe.

Ever since the Hubble Space Telescope peered at this same region, this otherwise unremarkable patch of sky has become famous as the Hubble Deep Field and is now known to contain thousands of galaxies.

Three radio sources were detected in an area of sky no bigger than that covered by a grain of sand held up to the night sky. The results appear in the latest issue of the European journal, Astronomy and Astrophysics.

Garrett said the team had not expected to detect this many radio sources.

"Even this is a large area of sky for a high-resolution radio instrument like the EVN -- imaging such a large field so deeply and at such high resolution is something of a first for VLBI," Garrett said, "and detecting three very faint radio sources is a bit like going shopping in the winter sales, and getting three items for the price of one!"

The diversity of the galaxies hosting the radio sources also surprised the researchers.

"Our three radio sources are identified with a broad range of galaxy type: an elliptical galaxy, a spiral galaxy and a very distant, dust-obscured starburst galaxy," Garrett said.

Simon Garrington and Tom Muxlow, both with MERLIN/Jodrell Bank, said the real surprise is that the sources are so small (less than 600 light-years across).

"This clearly suggests that the radio emission is generated by a powerful central engine, associated with a supermassive black hole," Garrington said.

Astronomers had previously thought that the radio emission from these distant galaxies would come from the numerous remnants of short-lived massive stars. Indeed, this rapid churning of material in the galaxy produces cosmic dust that glows at infrared wavelengths but is impenetrable to all but radio and sub-millimeter telescopes.

"These high-resolution radio observations are crucial in distinguishing between galaxies, which contain massive black holes and those that are powered purely by star-formation processes" said Ken Kellermann of the National Radio Astronomy Observatory, United States, another member of the team.

Data for these images at each of the radio telescopes were archived on high-speed magnetic tape recorders, generating almost 25,000 gigabytes of data in total. Using a supercomputer operated by the National Radio Astronomy Observatory in Socorro, New Mexico, the magnetic tapes were later played back and combined together to form a supersensitive, giant radio telescope of continental dimensions.