The Hubble Ultra-Deep Field shows galaxies as they were when the Universe was young. Those with the highest estimated redshifts (numbers and insets) were born more than 13 billion years ago, soon after the Big Bang. Image: NASA, ESA, R. Ellis (Caltech), UDF 2012 Team
For one sleepless week in early September 2009, Garth Illingworth and his team had the early Universe all to themselves. At NASA's request, Illingworth, Rychard Bouwens and Pascal Oesch had just spent the previous week staring into their computer screens at the University of California, Santa Cruz, scanning through hundreds of black-and-white portraits of faint galaxies recorded in a multi-day time exposure by a newly installed infrared camera on the Hubble Space Telescope. NASA simply wanted the three astronomers to preview the images and make sure that the camera was working correctly, before the agency released the data more widely.
But Illingworth, Bouwens and Oesch were hoping that they would find more — that at least some of those smudges of light would prove to be among the first galaxies to form in the Universe, less than 1 billion years after the Big Bang. Even a faint glimpse of such objects could provide fresh insights into some of the biggest questions in cosmology, ranging from the nature of the first stars to the tumultuous beginnings of galaxy formation.
That week, the astronomers began to focus on two dozen tiny candidate images — each so dim and grainy that they might easily be noise in the camera's digital sensors. But as their analysis proceeded, it became clear that these patches of light had the right color, appearing only in the camera's reddest filters — exactly what would be expected of newborn galaxies seen at a very great distance and very high redshift. And when the three colleagues started digitally adding together exposures of each candidate, says Illingworth, “suddenly there they were” — fuzzy, but undeniable images of galaxies. “That week in September was one of the most exciting times of my career!”
By the week's end, he, Bouwens and Oesch had posted two draft papers to the arXiv preprint server, detailing their first-ever collection of more than 20 galaxies from the age of galaxy formation, some 13 billion years ago, when the cosmos was only 600 million to 800 million years old. Since then, other researchers have made further observations of the same small patch of sky, known as the Hubble Ultra-Deep Field (HUDF), and four other larger regions. They have expanded that initial roster to some 1,400 young galaxies, from the same era.
The data from this growing catalogue are already hinting at a still-unseen time — an infant Universe thronged with countless small galaxies and lit by primordial stars so massive that they burned out and blew up in a cosmic eye-blink. And a new generation of instruments promises to bring that era into clear view. They include the Atacama Large Millimeter/submillimeter Array (ALMA) of radio telescopes in Chile, which is already beginning such observations; and Hubble's successor, the infrared James Webb Space Telescope (JWST), which is set for launch in late 2018.
It's a heady time for early-Universe astronomers, says cosmologist Avi Loeb of Harvard University in Cambridge, Massachusetts. “We're looking at our origins,” he says. “The first galaxies were the building blocks of the Milky Way, and the desire to understand them is a search for our roots.”
Over the past few decades, observers have developed a general storyline describing how galaxies formed (see 'Dawn's early light'). Astronomers know, for example, that the raw material was a hot, ionized plasma of hydrogen and helium that emerged from the Big Bang, then rapidly cooled as the Universe expanded. Once its temperature had fallen far enough, about 370,000 years after the Big Bang, protons and electrons combined to make neutral atoms and created a light-absorbing haze that plunged the Universe into a cosmic 'dark ages'.