Video explains how an irregular dwarf galaxy is able to form star clusters similar to those found in larger galaxies. Artist animation revealing an emerging star cluster in the WLM galaxy. The optical image of the galaxy was taken with the Blanco 4-meter telescope. ALMA data reveal the presence of dense clouds of star-forming dust and gas. The zoom-in illustrates how a collection of stars would appear within one such cloud. Credit: Animation by B. Saxton (NRAO/AUI/NSF), editing by J. Hellerman (NRAO/AUI/NSF); Optical data: P. Massey/Lowell Observatory and K. Olsen (NOAO/AURA/NSF); ALMA data: M. Rubio et al., Universidad de Chile, ALMA (NRAO/ESO/NAOJ); Fly-in animation: B. Kent (NRAO/AUI/NSF)
The ALMA telescope as seen at night. Its superior resolution and sensitivity allow it to detect and image the faint millimeter-wavelength light emitted by molecules in space. Credit: C. Padilla (NRAO/AUI/NSF)
A nearby dwarf galaxy poses an intriguing mystery: How is it able to form brilliant star clusters without the dusty, gas-rich environments found in larger galaxies? The answer, astronomers believe, lies in densely packed and previously unrecognized nuggets of star-forming material sprinkled throughout the galaxy.
An international team of astronomers [1] using the Atacama Large Millimeter/submillimeter Array (ALMA) has discovered an unexpected population of compact interstellar clouds hidden within the nearby dwarf irregular galaxy [2] Wolf—Lundmark—Melotte, more commonly known as WLM.
These clouds, which are nestled within a heavy blanket of interstellar material, help explain how dense star clusters [3] are able to form in the tenuous environs of a galaxy thousands of times smaller and far more diffuse than our own Milky Way.
"For
many reasons, dwarf irregular galaxies like WLM are poorly equipped to
form star clusters," noted Monica Rubio, an astronomer with the
University of Chile and lead author on a paper to appear in the
scientific journal Nature. "These galaxies are fluffy with very
low densities. They also lack the heavy elements that contribute to
star formation. Such galaxies should only form dispersed stars rather
than concentrated clusters, but that is clearly not the case."
By
studying this galaxy with ALMA, the astronomers were able to locate,
for the first time, compact regions that appear able to emulate the
nurturing environments found in larger galaxies.
These regions
were discovered by pinpointing the almost imperceptible and highly
localized millimeter wavelength light emitted by carbon monoxide (CO)
molecules, which are typically associated with star-forming interstellar
clouds.
Earlier, an affiliated team of astronomers led by
Deidre Hunter at the Lowell Observatory in Flagstaff, Ariz., first
detected CO in the WLM galaxy with the single-dish Atacama Pathfinder
Experiment (APEX) telescope [4].
These initial, low-resolution observations could not resolve where the
molecules reside, but they did confirm that WLM contains the lowest
abundance of CO ever detected in any galaxy. This lack of CO and other
heavy elements should put a serious damper on star formation, the
astronomers note.
"Molecules, and carbon monoxide in particular,
play an important role in star formation," said Rubio. "As gas clouds
begin to collapse, temperatures and densities rise, pushing back against
gravity. That's where these molecules and dust particles come to the
rescue by absorbing some of the heat through collisions and radiating it
into space at infrared and submillimeter wavelengths." This cooling
effect enables gravity to continue the collapse until a star forms.
The
problem previously was that in WLM and similar galaxies with very low
abundances of heavy elements, astronomers simply didn't see enough of
this material to account for the new star clusters they observed.
The
reason the CO was initially so difficult to see, the researchers
discovered, is that unlike in normal galaxies, the WLM clouds are very
tiny compared to their overlying envelopes of molecular and atomic gas.
To
become viable star factories, the concentrated CO clouds need these
enormous envelopes of transitional gas to bear down on them, giving the
cores of CO a high enough density to allow them to form a normal cluster
of stars.
"Like a diver being squeezed at the bottom of a deep
abyss, these bundles of star-forming gas are under tremendous pressure,
even though the surrounding ocean of interstellar gas is much more
shallow," said Bruce Elmegreen, a co-author on the paper and researcher
at the IBM T.J. Watson Research Center in Yorktown Heights, N.Y. "By
discovering that the carbon monoxide is confined to highly concentrated
regions within a vast expanse of transitional gas, we could finally
understand the mechanisms that led to the impressive stellar
neighborhoods we see in the galaxy today."
Further studies with
ALMA will also help determine the conditions that formed the globular
clusters found in the halo of the Milky Way. Astronomers believe these
much larger clusters may have originally formed in dwarf galaxies and
later migrated to the halo after their host dwarf galaxies dispersed.
WLM
is a relatively isolated dwarf galaxy located approximately 3 million
light-years away on the outer edges of the Local Group: the collection
of galaxies that includes the Milky Way, the Magellanic Clouds,
Andromeda, M33, and dozens of smaller galaxies.
The National
Radio Astronomy Observatory is a facility of the National Science
Foundation, operated under cooperative agreement by Associated
Universities, Inc.
# # #
[1] Collaborators in
the present study include Monica Rubio, Universidad de Chile, Santiago;
Bruce G. Elmegreen, IBM T.J. Watson Research Center, Yorktown Heights,
N.Y.; Deidre A. Hunter, Lowell Observatory, Flagstaff, Ariz; Elias
Brinks, University of Hertfordshire, UK; Juan R. Cortes, Joint ALMA
Observatory and National Radio Astronomy Observatory, Santiago, Chile;
and Phil Cigan, New Mexico Institute of Mining and Technology, Socorro.
[2]
Irregular galaxies lack the distinctive shapes of spiral and elliptical
galaxies. Dwarf irregulars, like WLM, are hundreds of times smaller
than the larger variety and contain only a few hundred million stars
instead of tens of billions. Though small, some are now known to harbor
massive black holes at their centers.
[3] Star
clusters, like the Pleiades found in our own Milky Way galaxy, are made
up of hundreds of stars. Others, like globular clusters, can contain
hundreds of thousands to a few million stars. Though many stars in the
Milky Way originally form in clusters, some – like the Sun – drift away
from their stellar nurseries and move freely throughout their home
galaxy. Stars in the largest and densest clusters, like those observed
in WLM, remain relatively close together.
[4] The
APEX team was led by Deidre Hunter at the Lowell Observatory in
Flagstaff, Ariz., and Elias Brinks at the University of Hertfordshire,
U.K. It also included Monica Rubio; Bruce Elmegreen; Andreas Schruba,
California Institute of Technology, Pasadena, Calif.; and Celia Verdugo,
University of Chile.
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 National Science
Council of Taiwan (NSC) 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
(434) 296-0314;
Email: cblue@nrao.edu
