What is the recipe for starburst? Astronomers studied NGC 253 with ALMA to find out. These new ALMA data reveal a diffuse envelope of carbon monoxide gas (shown in red), which surrounds stellar nurseries -- regions of active star formation (in yellow). By dissecting these regions with ALMA, astronomers are uncovering clues to the processes and conditions that drive furious star formation. The ALMA data are superimposed on a Hubble image that covers part of the same region. Credit: B. Saxton (NRAO/AUI/NSF); ALMA (NRAO/ESO/NAOJ); A. Leroy; STScI/NASA, ST-ECF/ESA, CADC/NRC/CSA
Animation of ALMA data reveals a diffuse envelope of carbon monoxide gas (shown in red), which surrounds stellar nurseries -- regions of active star formation (in yellow). By dissecting these regions with ALMA, astronomers are uncovering clues to the processes and conditions that drive furious star formation. The ALMA data are superimposed on a Hubble image that covers part of the same region. Credit: B. Saxton (NRAO/AUI/NSF); ALMA (NRAO/ESO/NAOJ); A. Leroy; STScI/NASA, ST-ECF/ESA, CADC/NRC/CSA
Starburst galaxies transmute gas into new stars at a dizzying pace – up
to 1,000 times faster than typical spiral galaxies like the Milky Way.
To help understand why some galaxies "burst" while others do not, an
international team of astronomers used the Atacama Large Millimeter/submillimeter Array (ALMA) to dissect a cluster of star-forming clouds at the heart of NGC 253, one of the nearest starburst galaxies to the Milky Way.
"All
stars form in dense clouds of dust and gas," said Adam Leroy, an
astronomer formerly with the National Radio Astronomy Observatory (NRAO)
in Charlottesville, Virginia, and now with The Ohio State University in
Columbus. "Until now, however, scientists struggled to see exactly what
was going on inside starburst galaxies that distinguished them from
other star-forming regions."
ALMA changes that by offering the
power to resolve individual star-forming structures, even in distant
systems. As an early demonstration of this capability, Leroy and his
colleagues mapped the distributions and motions of multiple molecules in
clouds at the core of NGC 253, also known as the Sculptor Galaxy.
Sculptor,
a disk-shape galaxy currently undergoing intense starburst, is located
approximately 11.5 million light-years from Earth, which is remarkably
nearby for such an energetic star factory. This proximity makes Sculptor
an excellent target for detailed study.
“There is a class of
galaxies and parts of galaxies, we call them starbursts, where we know
that gas is just plain better at forming stars,” noted Leroy. “To
understand why, we took one of the nearest such regions and pulled it
apart – layer by layer – to see what makes the gas in these places so
much more efficient at star formation.”
ALMA’s exceptional
resolution and sensitivity allowed the researchers to first identify ten
distinct stellar nurseries inside the heart of Sculptor, something that
was remarkably hard to accomplish with earlier telescopes, which
blurred the different regions together.
The team then mapped the
distribution of about 40 millimeter-wavelength “signatures” from
different molecules inside the center of the galaxy. This was critically
important since different molecules correspond to different conditions
in and around star-forming clouds. For example, carbon monoxide (CO)
corresponds to massive envelopes of less dense gas that surround stellar
nurseries. Other molecules, like hydrogen cyanide (HCN), reveal dense
areas of active star formation. Still rarer molecules, like H13CN and
H13CO+, indicate even denser regions.
By comparing the
concentration, distribution, and motion of these molecules, the
researchers were able to peel apart the star-forming clouds in Sculptor,
revealing that they are much more massive, ten times denser, and far
more turbulent than similar clouds in normal spiral galaxies.
These
stark differences suggest that it’s not just the number of stellar
nurseries that sets the throttle for a galaxy to create new stars, but
also what kind of stellar nurseries are present. Because the
star-forming clouds in Sculptor pack so much material into such a small
space, they are simply better at forming stars than the clouds in a
galaxy like the Milky Way. Starburst galaxies, therefore, show real
physical changes in the star-formation process, not just a one-to-one
scaling of star formation with the available reservoir of material.
“These
differences have wide-ranging implications for how galaxies grow and
evolve,” concluded Leroy. “What we would ultimately like to know is
whether a starburst like Sculptor produces not just more stars, but
different types of stars than a galaxy like the Milky Way. ALMA is
bringing us much closer to that goal.”
These results are accepted for publication in the Astrophysical Journal
and are being presented February 15, 2015, at a news conference at the
American Association for the Advancement of Science (AAAS) meeting in
San Jose, California.
# # #
The
National Radio Astronomy Observatory is a facility of the National
Science Foundation, operated under cooperative agreement by Associated
Universities, Inc.
The Atacama Large Millimeter/submillimeter
Array (ALMA), an international astronomy facility, is a partnership of
Europe, North America and East Asia in cooperation with the Republic of
Chile. ALMA is funded in Europe by the European Organization for
Astronomical Research in the Southern Hemisphere (ESO), in North America
by the U.S. National Science Foundation (NSF) in cooperation with the
National Research Council of Canada (NRC) and the National Science
Council of Taiwan (NSC) and in East Asia by the National Institutes of
Natural Sciences (NINS) of Japan in cooperation with the Academia Sinica
(AS) in Taiwan and the Korea Astronomy and Space Science Institute
(KASI).
ALMA construction and operations are led on behalf of
Europe by ESO, on behalf of North America by the National Radio
Astronomy Observatory (NRAO), which is managed by Associated
Universities, Inc. (AUI) and on behalf of East Asia by the National
Astronomical Observatory of Japan (NAOJ). The Joint ALMA Observatory
(JAO) provides the unified leadership and management of the
construction, commissioning and operation of ALMA.
Contact:
Charles E. Blue,
NRAO Public Information Officer
434-296-0314;
Email: cblue@nrao.edu
