ALMA Pans for Galactic Gold in the Hubble Ultra Deep Field

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.

ALMA image of Hubble Ultra Deep Field

ALMA captured a gold mine of galaxies rich in carbon monoxide, indicating star-forming potential (gold) in the Hubble Ultra Deep Field. The blue features are Hubble-imaged galaxies. This image from the ASPECS survey covers one-sixth of the full Hubble Ultra Deep Field.

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 close-up in HUDF

This close-up image reveals one galaxy rich in carbon monoxide (orange) and primed for star formation.

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.

5 thoughts on “ALMA Pans for Galactic Gold in the Hubble Ultra Deep Field

  1. Anthony BarreiroAnthony Barreiro

    This is very interesting and exciting work. I’m trying to understand these findings. Please let me know what I’ve got right, correct my misunderstandings, and answer a question:

    A galaxy that shows up broadly in mm-band light is dusty and warm. We’re seeing the black-body thermal radiation from the proto-stars’ dusty cocoons.

    If we see CO and ionized carbon emission lines, we can measure their redshifts, which gives us distance.

    What does the presence of CO and ionized carbon tell us about the star-forming properties of a galaxy?


    1. Monica YoungMonica Young Post author

      Great question, Anthony. You’ve got it mostly right except that I would probably call the dust cool rather than warm – it’s only a couple dozen degrees above absolute zero. Carbon monoxide traces molecular hydrogen, which is otherwise essentially invisible in the interstellar medium. So the carbon monoxide glow gives us a sense of how much star-forming gas is present. Ionized carbon is the spectral line that cools gas, so it works as a tracer of active star formation.

      1. Anthony BarreiroAnthony Barreiro

        Ah, I see, we don’t really care about the CO itself, except that where there’s CO there’s H2, and where there’s H2 there’s gonna be stars.

        The last sentence in your reply doesn’t completely make sense to me. How does a spectral line cool gas? I get that very hot stars could be emitting enough radiation to ionize carbon, but I don’t get the cooling gas bit.

        Please forgive my obtuseness, but I find it fascinating that we’re able to understand so much about things that are so far away and so ancient, and I just want to understand a bit more.

        1. Monica YoungMonica Young Post author

          Not at all, these are good, probing questions, and I went back to Dr. Carilli to improve my own understanding. So here’s the general way this works: newborn stars produce intense ultraviolet radiation. These photons hit dust grains in the cloud around them and ionize atoms, releasing their electrons. Those electrons bounce around and collisionally excite other atoms, such as carbon. When the electron hits the carbon atom, it loses its thermal energy in the form of a 158-micron photon (this produces the emission line from ionized carbon, or C II). So the CII light that you’re seeing in that emission line comes (indirectly) is part of the cooling process of the cloud around the forming star, and therefore traces active star formation.

          1. Anthony BarreiroAnthony Barreiro

            Thanks very much. It makes sense. Radiation being emitted from the nebula would carry energy away from the nebula, so the nebula would get cooler.

            That’s enough physics for me for today.

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