Credit: NRAO/AUI/NSF; D. Berry
To correct for the effects of gravitational lensing in
these galaxies, the ALMA data (left panel) is compared to a
lensing-distorted model image (second panel from left). The difference
is shown in the third panel from the left. The structure of the galaxy,
after removing the lensing effect, is shown at right. This image loops
through the different velocity ranges within the galaxy, which appear at
different frequencies to ALMA due to the Doppler effect.Credit: ALMA (ESO/NAOJ/NRAO); D. Marrone et al.
Astronomers expect that the first galaxies, those that formed just a few hundred million years after the Big Bang,
would share many similarities with some of the dwarf galaxies we see in
the nearby universe today. These early agglomerations of a few billion
stars would then become the building blocks of the larger galaxies that
came to dominate the universe after the first few billion years.
Ongoing observations with the Atacama Large Millimeter/submillimeter Array
(ALMA), however, have discovered surprising examples of massive,
star-filled galaxies seen when the cosmos was less than a billion years
old. This suggests that smaller galactic building blocks were able to
assemble into large galaxies quite quickly.
The latest ALMA observations push back this epoch of massive-galaxy
formation even further by identifying two giant galaxies seen when the
universe was only 780 million years old, or about 5 percent its current
age. ALMA also revealed that these uncommonly large galaxies are nestled
inside an even-more-massive cosmic structure, a halo of dark matter several trillion times more massive than the sun.
The two galaxies are in such close proximity — less than the distance
from the Earth to the center of our galaxy — that they will shortly
merge to form the largest galaxy ever observed at that period in cosmic
history. This discovery provides new details about the emergence of
large galaxies and the role that dark matter plays in assembling the
most massive structures in the universe.
The researchers report their findings in the journal Nature.
“With these exquisite ALMA observations, astronomers are seeing the
most massive galaxy known in the first billion years of the universe in
the process of assembling itself,” said Dan Marrone, associate professor
of astronomy at the University of Arizona in Tucson and lead author on
the paper.
Astronomers are seeing these galaxies during a period of cosmic
history known as the Epoch of Reionization, when most of intergalactic
space was suffused with an obscuring fog of cold hydrogen gas. As more
stars and galaxies formed, their energy eventually ionized the hydrogen
between the galaxies, revealing the universe as we see it today.
“We usually view that as the time of little galaxies working hard to
chew away at the neutral intergalactic medium,” said Marrone. “Mounting
observational evidence with ALMA, however, has helped to reshape that
story and continues to push back the time at which truly massive
galaxies first emerged in the universe.”
The galaxies that Marrone and his team studied, collectively known as
SPT0311-58, were originally identified as a single source by the South Pole Telescope.
These first observations indicated that this object was very distant
and glowing brightly in infrared light, meaning that it was extremely
dusty and likely going through a burst of star formation. Subsequent
observations with ALMA revealed the distance and dual nature of the
object, clearly resolving the pair of interacting galaxies.
To make this observation, ALMA had some help from a gravitational lens,
which provided an observing boost to the telescope. Gravitational
lenses form when an intervening massive object, like a galaxy or galaxy
cluster, bends the light from more distant galaxies. They do, however,
distort the appearance of the object being studied, requiring
sophisticated computer models to reconstruct the image as it would
appear in its unaltered state.
This “de-lensing” process provided intriguing details about the
galaxies, showing that the larger of the two is forming stars at a rate
of 2,900 solar masses
per year. It also contains about 270 billion times the mass of our sun
in gas and nearly 3 billion times the mass of our sun in dust. “That’s a
whopping large quantity of dust, considering the young age of the
system,” noted Justin Spilker, a recent graduate of the University of
Arizona and now a postdoctoral fellow at the University of Texas at
Austin.
The astronomers determined that this galaxy’s rapid star formation
was likely triggered by a close encounter with its slightly smaller
companion, which already hosts about 35 billion solar masses of stars
and is increasing its rate of starburst at the breakneck pace of 540
solar masses per year.
The researchers note that galaxies of this era are “messier” than the
ones we see in the nearby universe. Their more jumbled shapes would be
due to the vast stores of gas raining down on them and their ongoing
interactions and mergers with their neighbors.
The new observations also allowed the researchers to infer the
presence of a truly massive dark matter halo surrounding both galaxies.
Dark matter provides the pull of gravity that causes the universe to
collapse into structures (galaxies, groups and clusters of galaxies,
etc.).
“If you want to see if a galaxy makes sense in our current
understanding of cosmology, you want to look at the dark matter halo —
the collapsed dark matter structure — in which it resides,” said Chris
Hayward, associate research scientist at the Center for Computational
Astrophysics at the Flatiron Institute in New York City. “Fortunately,
we know very well the ratio between dark matter and normal matter in the
universe, so we can estimate what the dark matter halo mass must be.”
By comparing their calculations with current cosmological
predictions, the researchers found that this halo is one of the most
massive that should exist at that time.
“There are more galaxies discovered with the South Pole Telescope
that we’re following up on,” said Joaquin Vieira of the University of
Illinois at Urbana-Champaign, “and there is a lot more survey data that
we are just starting to analyze. Our hope is to find more objects like
this, possibly even more distant ones, to better understand this
population of extreme dusty galaxies and especially their relation to
the bulk population of galaxies at this epoch.”
“In any case, our next round of ALMA observations should help us
understand how quickly these galaxies came together and improve our
understanding of massive galaxy formation during reionization,” added
Marrone.
The National Radio Astronomy Observatory is a facility of the
National Science Foundation, operated under cooperative agreement by
Associated Universities, Inc.
This research is presented in a paper titled “’Galaxy growth in a
massive halo in the first billion years of cosmic history,” by D.
Marrone, et al., which appears in Advance Online Publication for Nature. [http://www.nature.com/nature].
The Atacama Large Millimeter/submillimeter Array (ALMA), an
international astronomy facility, is a partnership of the European
Organisation for Astronomical Research in the Southern Hemisphere (ESO),
the U.S. National Science Foundation (NSF) and the National Institutes
of Natural Sciences (NINS) of Japan in cooperation with the Republic of
Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in
cooperation with the National Research Council of Canada (NRC) and the
Ministry of Science and Technology (MOST) in Taiwan and by NINS in
cooperation with the Academia Sinica (AS) in Taiwan and the Korea
Astronomy and Space Science Institute (KASI).
ALMA construction and operations are led by ESO on behalf of its
Member States; by the National Radio Astronomy Observatory (NRAO),
managed by Associated Universities, Inc. (AUI), on behalf of North
America; and by the National Astronomical Observatory of Japan (NAOJ) on
behalf of East Asia. The Joint ALMA Observatory (JAO) provides the
unified leadership and management of the construction, commissioning and
operation of ALMA.
Contact:
Charles Blue, Public Information Officer
(434) 296-0314;
cblue@nrao.edu
