ALMA finds large numbers of massive galaxies in the early universe
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Imagine that you’re on an open field in the middle of the night. In the distance is the edge of a dark
forest. Using your binoculars, you see dozens of fireflies buzzing around between the trees. Great,
that tells you what kind of animals live out there, right?
But then you pick up your night goggles. In addition to visible light, they are also sensitive to the body
heat of living organisms. Now, you suddenly see other animals, too: mice, hedgehogs, birds, and even
the occasional swine or deer. These animals are both larger and more numerous than the fireflies.
Astronomers found themselves in a similar situation. They used the Hubble Space Telescope to study
the dark, distant universe. Sure enough, they detected many remote galaxies. These galaxies poured
out a lot of visible light because they were forming huge numbers of new stars. Such galaxies are
known as starburst galaxies.
But now, ALMA has studied the same part of the sky. Not at the visible wavelengths that Hubble can
see, but at millimeter and submillimeter wavelengths. It’s as if astronomer put their night goggles on.
And to their surprise, they found many large, dark galaxies that Hubble could not see.
The newly detected galaxies are also producing large numbers of new stars, although not as much as
the starburst galaxies that Hubble found. However, in the case of the ALMA galaxies, the starlight is
absorbed by massive amounts of dust. As a result, the galaxies can only be seen by telescopes that
observe at infrared or submillimeter wavelengths.
The galaxies studied by Hubble and ALMA are extremely far away. Their light takes some 10 billion
years to reach Earth. Therefore, we see them as they were ten billion years ago, when the universe
was still relatively young. Over time, the massive, dark ALMA galaxies will probably evolve into the
large, heavy-weight elliptical galaxies that we can see all around us today.
But there’s a puzzle. Astronomers didn’t expect that, ten billion years ago, there would already be so
many of these large, massive galaxies. Apparently, they form much faster and in greater numbers
than anyone imagined. So maybe we have to rethink our ideas about the origin and early evolution of
galaxies. That’s why this discovery is so important.
What?
The massive, dark galaxies that ALMA found are located in the so-called CANDELS field – a region in
the sky that has been studied in much detail. The Hubble Space Telescope discovered many distant
starburst galaxies in the CANDELS field. Later, the Spitzer Space Telescope, which observes infrared
radiation, detected 63 objects that were nog visible in the Hubble images. However, the Spitzer
images were not detailed enough to reveal their true nature. ALMA, with its superb sensitivity and
eagle-eyed vision, has now discovered submillimeter radiation from 39 of these 63 objects. They turn
out to be large, massive, dark galaxies. They produce about a hundred times more new stars than our
own Milky Way galaxy, but most of the starlight is absorbed by dust. Based on the number that
ALMA has detected, astronomers estimate that there may be over 20 million of these dark, massive
galaxies across the whole sky.
Who?
The ALMA observations of the distant dark, massive galaxies were carried out by Tao Wang of the
University of Tokyo (Japan) and a large international group of colleagues. The discovery was
important enough to end up in Nature, a prestigious professional weekly magazine on all topics of
scientific research.
