Growing up fast in the very early universe

An artist’s impression of a very dusty galaxy in the early universe that already exhibits signs of a rotating disc. Reds represent gast while blues and browns represent dust as seen by the ALMA radio telescope array. Image: B. Saxton NRAO/AUI/NSF, ESO, NASA/STScI; NAOJ/Subaru

One might expect galaxies forming in the very early universe to be relatively free of dust and the heavy elements cooked up when successive generations of massive stars run out of nuclear fuel and explode in supernova blasts. That process takes time, and most infant galaxies could be expected to experience rapid growth spurts in the eventual transition between the “primordial” and “mature” stages in their development.

But in a survey of 118 young galaxies dating back to within 1 billion to 1.5 billion years after the Big Bang, astronomers were surprised to find many more mature galaxies than expected.

“We didn’t expect to see so much dust and heavy elements in these distant galaxies,” said Andreas Faisst of the Infrared Processing and Analysis Center (IPAC) at the California Institute of Technology.

In fact, about 20 percent of the galaxies sampled in the survey “are already very dusty and a significant fraction of the ultraviolet light from newborn stars is already hidden by this dust,” said Daniel Schaerer of the University of Geneva.

Two galaxies in the early universe as imaged by ALMA in radio waves. Both are considered more “mature” than “primordial” based on the amounts of dust present (seen in yellow). Gas, seen in red, is used to measure obscured star formation and motion. Image: B. Saxton NRAO/AUI/NSF, ALMA (ESO/NAOJ/NRAO), ALPINE team

The ALMA Large Program to Investigate C+ at Early Times, or ALPINE, survey is the largest multi-wavelength study of galaxies in the early universe, utilising optical observations by ground- and space-based telescopes, including Keck, Subaru, the Very Large Telescope and the Hubble and Spitzer space telescopes and radio observations using the Atacama Large Millimetre/submillimetre Array, or ALMA.

The ALMA observations allowed researchers to detect star formation hidden by thick dust that blocks optical and infrared wavelengths and to follow the motion of gas associated with star-forming regions, finding “Hubble-dark galaxies” that even the space telescope cannot see.

“We want to see exactly where the dust is and how the gas moves around,” said Paolo Cassata of the University of Padua in Italy. “We also want to compare the dusty galaxies to others at the same distance and figure out if there might be something special about their environments.”