Run of the Mill: Finding galactic building blocks in early universe

Hunters of distant galaxies, meet Joe Average.

LITTLE GUYS. A glow of hydrogen gas emanates from a population of low-mass, weakly star-forming galaxies believed to be the building blocks of bright, present-day galaxies. Rauch, et al.

For more than 3 decades, astronomers have scoured the skies for tiny, ultrafaint galaxies that could be the early building blocks of the massive galaxies common in the universe today. Now researchers report that they have found 27 remote galaxies that appear to fill the bill.

These galaxies have low rates of star formation and appear to be 20 times as numerous as other, larger galaxies previously found from the same early era, when the universe was just 2 billion years old. Their properties suggest that the newly discovered bodies are part of a long-sought population of average-size galaxies that merged to form larger galaxies like the Milky Way.

Using the European Southern Observatory’s Very Large Telescope (VLT) in Paranal, Chile, Michael Rauch of the Carnegie Observatories in Pasadena, Calif., and his colleagues studied a tiny patch of sky for an unprecedented 92 hours, recording extraordinarily faint levels of a particular wavelength of light. That wavelength, known as Lyman-alpha, is emitted when energetic radiation from newborn, massive stars bombards hydrogen gas within galaxies, causing the gas to glow.

As observed from Earth, the ultraviolet Lyman-alpha radiation is shifted to longer, or redder, wavelengths by the expansion of the universe. The more distant the galaxy, the greater the redshift. The redshifted Lyman-alpha radiation detected by Rauch and his colleagues indicates that the 27 galaxies reside nearly 12 billion light-years from Earth, the team will report in the March 1, 2008 Astrophysical Journal.

The weak Lyman-alpha emission indicates that these galaxies are forming stars at a sluggish rate, equaling a tenth of the sun’s mass every year. In addition, the density of the galaxies found in such a small area of sky suggests that the galaxies are about 20 times as common as a well-documented collection of brighter but equally remote galaxies that make stars more prodigiously. Those galaxies, known as Lyman-break, were found using a different detection technique. They are not only rarer than the new group but also more massive, Rauch says.

Hints of the newly found building blocks for larger galaxies emerged when astronomers began studying the detailed spectra of the brilliant beacons known as quasars. The spectra revealed that as the quasar light journeyed to Earth, some of the radiation was absorbed by intervening blobs of hydrogen gas. Rauch and his collaborators now suggest that those blobs, previously revealed only as shadows on the quasar light, are the small, Lyman-alpha-emitting galaxies his team has detected.

Images of another sky region, studied intensively with the Hubble Space Telescope and known as the Hubble Ultra Deep Field, may also show signs of these run-of-the-mill galaxies, notes Rychard Bouwens of the University of California, Santa Cruz.

The 92-hour VLT study, pieced together from odd corners of the night over several years, “is a heroic observation, and I hope that it represents the start of an era rather than something the world decides is too expensive to repeat,” comments David Weinberg of the Ohio State University in Columbus. Although the objects are detected at a low signal-to-noise ratio and the sample is small, the team “most likely is detecting star formation in small galaxies, mapping out the iceberg of which the previously known Lyman-break galaxies are the tip,” says Weinberg. “Knowing what is going on for these more run-of-the-mill systems will be valuable for testing theories of the [early] galaxy population.”

The observation bodes well for finding many more of the building blocks with ground-based telescopes, rather than having to conduct such studies in space, says Rauch.

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