An eye on an ancient starburst

A tiny region of sky in the constellation Ursa Major affords perhaps our best view of the early universe. The Hubble Deep Field, a composite image of that region at visible and near-visible wavelengths, reveals some 3000 galaxies dating as far back as a few hundred million years after the Big Bang (see Physics Today, April 1997, page 24). Unseen in the image, however, are a handful of massive galaxies whose starlight is obscured by enveloping dust. Although detectable at submillimeter wavelengths, even the brightest among them, HDF 850.1, has been difficult to locate or date with precision. Now, an international team led by Fabian Walter (Max Planck Institute for Astronomy, Heidelberg, Germany) has gleaned HDF 850.1’s cosmic age and other key traits from observations at the IRAM millimeter-wave antenna array in the French Alps (see the picture). The team identified four spectral lines corresponding to various carbon monoxide and carbon transitions; each was redshifted by a factor of 5.2. That means we see the galaxy as it was 12.5 billion years ago, a mere billion years after the Big Bang. Older galaxies have been discovered, but this one is unusual in that it appears to be churning out new stars at a rate of 850 solar masses per year, nearly 1000 times faster than our own Milky Way. Also, it occupies an atypically crowded portion of cosmic real estate: It’s densely clustered with a dozen other galaxies—a rare instance of large-scale structure in the infant universe. (F. Walter et al., Nature 486, 233, 2012.)—Ashley G. Smart