ALMA, currently the largest telescope array in the world, just took a look at the Hubble Space Telescope’s deepest-ever image, the Hubble Ultra Deep Field. Here’s what it found.
When the Hubble Space Telescope peered into a small region of space in the constellation Fornax, the image it took became an iconic example of the telescope’s abilities to reach into the farthest corners of the universe.
But Hubble isn’t the only telescope to have gazed at this little square of sky. The Hubble Ultra-Deep Field (HUDF) now has coverage across 28 wavebands, all the way from X-ray to far-infrared wavelengths.
Now the Atacama Large Millimeter/submillimeter Array (ALMA) is jumping on that train, sifting through some 10,000 galaxies to pick out the rare young galaxies that are rife with dust-enshrouded stars. The result is a new view of the Golden Age of star formation 10 billion years ago.
Star Formation Across the Ages
Young stars emit intense ultraviolet radiation. Hubble and other telescopes have excelled at picking out this light to pinpoint star-forming galaxies in the early universe. But before stars ignite nuclear fusion they’re enshrouded in their natal clouds of dusty gas, mostly hiding them from view.
That’s where ALMA comes in. This array of 66 dishes detects the millimeter-wavelength radiation that comes from these star-forming reservoirs of cold dust and gas, which glow at just a few dozen degrees above absolute zero.
ALMA looked at the HUDF in two distinct and complementary ways: first, it collected radiation emitted at wavelengths around 1.3 millimeters across the field’s entire 4 square arcminutes. Using these data, Jim Dunlop (University of Edinburgh, UK) and colleagues identified 16 sources that matched up with galaxies seen by Hubble, each one in the violent, dusty throes of starbirth.
A second, ongoing survey, titled “ALMA Spectroscopic Survey in the Hubble UDF” (or ASPECS), is measuring millimeter-wavelength spectra from the HUDF galaxies. By measuring emission lines associated with carbon monoxide molecules and ionized carbon, ALMA digs up a lot of dirt on any given galaxy, measuring both its distance (by the redshift of spectral lines) and its star-forming properties. The cool glow of carbon monoxide, for example, corresponds to reservoirs of gas available for star formation, while ionized carbon gives a measure of how active star formation is.
The first set of spectra around 1.3 mm comes from 11 sources, the other set (around 3 mm) comes from 10 sources. The project is still plugging along until September 2017, but for now the astronomers involved have published the first square arcminute of the survey.
That may not sound like much, but these few sources are enough to trace the history of hidden star formation across cosmic time. “We obtain a fully three-dimensional map of the cosmos to the earliest times,” says Chris Carilli (NRAO), a member of the ASPECS team. “That’s never really been done before to such depth.”
A Closer Look
ALMA is an array of telescopes that work together as one. Since the final image we see comes from multiple telescopes, both the sharpness of that image and the size scale of the things revealed in it depend on how far apart the telescopes are. The farther the dishes sit from one another, the smaller the features they can collectively resolve.
To survey galaxy-sized sources in the distant universe, the researchers set the telescopes up to 1.25 km apart. But ALMA’s dishes can separate as much as 15 km from each other, so in the future ALMA could even map out the structures within the brighter galaxies, Carilli says.
Dunlop, Jim et al. “A deep ALMA image of the Hubble Ultra Deep Field.” To appear in Monthly Notices of the Royal Astronomical Society. (Full text.)
Walter, Fabian et al. “ALMA Spectroscopic Survey in the Hubble Ultra Deep Field: Survey Description.” To appear in Astrophysical Journal. (Full text.)
NRAO Press release with links to additional ASPECS papers.