Astronomers may finally have laid eyes on a population of enormous but elusive galaxies in the early universe.
These hefty, star-forming galaxies are shrouded in dust, which hid them from previous searches that used starlight. Now observations of radiation emitted by that interstellar dust have revealed dozens of massive, active galaxies from when the universe was younger than 2 billion years, researchers report online August 7 in Nature. These galaxies may be the long-sought precursors to heavyweight galaxies seen later in the universe’s history, as well as the most massive galaxies around today.
“Discovering massive galaxies at such early times is very exciting,” says Christina Williams, an astronomer at the University of Arizona in Tucson not involved in the work.
Big, inactive galaxies have been found dating back to a couple billion years after the Big Bang (SN Online: 3/14/14). But the formation of those gentle giants has remained mysterious. That’s because astronomers expect such massive, inactive galaxies to originate from big, star-forming powerhouses, and surveys of the earliest cosmos hadn’t uncovered a population of such star-forming progenitor galaxies.
Using the Atacama Large Millimeter/submillimeter Array, or ALMA, in Chile to examine distant galactic dust emissions, astronomers identified 39 star-forming galaxies from when the universe was about 1 billion to 2 billion years old. These galaxies, which boast an average mass of about 40 billion suns and form about 200 new suns per year, are about as common as the large, inactive galaxies seen slightly later in cosmic history.
Detection by dust
Because they tend to be thick with dust that masks their starlight, massive, star-forming galaxies from the very distant, very early universe are invisible to telescopes like Hubble (left). ALMA, on the other hand, peered at radiation emitted by the obscuring dust itself to pick out dozens of massive galaxies from the ancient universe (four shown at right).
“This is definitely a plausible population that could give rise to the quiescent galaxies,” says Karl Glazebrook, an astronomer at Swinburne University of Technology in Melbourne, Australia, not involved in the work.
The newly identified galaxies are also embedded in that early era’s most massive dark matter halos — blobs of invisible, unidentified particles that surround galaxies (SN: 3/3/18, p. 8). The finding suggests that those ancient galaxies are the ancestors of today’s biggest galaxies, which now sit in the most massive dark matter halos, says study coauthor Tao Wang, an astronomer at the University of Tokyo. These modern descendants may include behemoths like M87, home to the first black hole ever imaged (SN: 4/27/19, p. 6).
Astronomer Caitlin Casey at the University of Texas at Austin calls the new galaxy detections “exciting and tantalizing.” But she cautions that the current analysis of ALMA observations, in which she was not involved, gives only rough estimates of how far these galaxies date back in cosmic history. Further investigations with ALMA or NASA’s James Webb Space Telescope, launching in 2021, could help cinch the galaxies’ precise ages and roles in galactic evolution.
The discovery of such big, star-forming galaxies when the universe was less than 2 billion years old fits well with past observations of big, quiescent galaxies later in cosmic history. But these observations don’t jibe with current theories of galaxy formation. In computer simulations, the universe at 2 billion years old contains too few massive galaxies to explain ALMA’s observations, says Williams, the Arizona astronomer. “This is a surprise the universe had for us.”
Wang and colleagues now plan to take a larger census of ancient massive galaxies with ALMA. That work could give theorists more information about how to tweak cosmological simulations to match early-universe observations.