Hubble's infrared camera goes the distance.

Astronomers have done their level best to chronicle the early history of galaxies, but there's a glaring gap in the cosmic baby album they've assembled: The first few pages are blank. Using the Hubble Space Telescope, researchers have now begun filling in the gap, identifying 26 galaxies that may be the youngest and most distant known. The colors of the galaxies, which are ablaze with the blue light of hot, massive stars, suggest that they lie about 13 billion light-years from Earth and were imaged as they appeared soon after birth.

Rodger I. Thompson of the University of Arizona in Tucson reported the findings last week at the annual symposium of the Space Telescope Science Institute in Baltimore. He and his colleagues based their findings on an analysis of an arresting array of images known as the Hubble Ultra Deep Field (UDF). These near-infrared and visible-light pictures represent the deepest look at a patch of the cosmos ever taken (SN: 3/13/04, p. 164).

To identify the oldest galaxies in the UDF, Thompson's team looked for ones that show up in infrared images but disappear in visible-light pictures. That pattern is an indication that the galaxies have a huge volume of hydrogen gas between them and Earth and are therefore extremely distant.

Thompson says that eight galaxies among the several thousand in the Hubble UDF clearly meet this distance criterion and that another that 18 are likely candidates. Indeed, these 26 galaxies could be the most remote known, but because they show up only in the near-infrared images, no existing detector can directly measure their distance. That will require the more-sensitive instruments of the proposed successor to Hubble, the James Webb Space Telescope, which is scheduled for launch in about 2011.

For galaxies that far away, light that arrives at Hubble at infrared wavelengths started its journey as blue light. This shift toward longer, or redder, wavelengths is due to the expansion of the universe.

The researchers found that most of the 26 galaxies they identified in their search were about twice as bright at the shorter of two near-infrared wavelengths that Hubble's Near Infrared Camera and Multiobject Spectrograph can record. The relative intensities of the detected infrared wavelengths indicate that the galaxies are populated by some of the bluest stars ever observed.

Blue light is a hallmark of young, massive stars, which fizzle rapidly. If that's the case, the galaxies must have been born only about 50 million years before the long-ago era that Hubble has recorded, Thompson's team estimates.

Although finding the bluest, brightest galaxies is intriguing, it's not a surprise, Thompson adds, because such galaxies would be the ones most easily detected by the infrared camera. Even so, he emphasizes that since his team has had the Hubble data for only 6 weeks, any interpretation is preliminary. "We're working right up against the limit of detection" of the infrared camera, he notes, so the team members have to check all the ways in which they might be fooled.

Sangeeta Malhotra, an astronomer at the Space Telescope Science Institute who is analyzing visible-light images from the Hubble UDF, agrees that the infrared observations aren't easy to interpret. She calls the area of study "kind of frontier country." Interpreting the early history of galaxies from the images may become clearer once other research groups complete their own analyses of the Hubble data, Malhotra adds.