ALMA discovers an unexpected population of compact interstellar clouds inside the dwarf irregular galaxy WLM. These star-forming clouds provide the necessary nurturing environment to form star clusters. As seen in relation to an optical image of the galaxy taken with the Blanco 4-meter telescope, (box upper left) an overlaying blanket of hydrogen gas (red) imaged with NRAO’s VLA telescope provides the pressure necessary to concentrate molecules of carbon monoxide (yellow) as seen with ALMA. These regions correspond to dense cores capable of forming clusters like those found in the Milky Way and other large galaxies. Credit: B. Saxton (NRAO/AUI/NSF); M. Rubio et al., Universidad de Chile, ALMA (NRAO/ESO/NAOJ); D. Hunter and A. Schruba, VLA (NRAO/AUI/NSF); P. Massey/Lowell Observatory and K. Olsen (NOAO/AURA/NSF)
Using ALMA, astronomers have discovered an unexpected population of compact interstellar clouds hidden within the nearby dwarf irregular galaxy Wolf–Lundmark–Melotte, more commonly known as WLM. These clouds, nestled within a heavy blanket of interstellar material, help explain how dense star clusters 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 astronomer Monica Rubio. “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 CO molecules, typically associated with star-forming interstellar clouds. “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. “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,” said Bruce Elmegreen
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.
https://public.nrao.edu/news/pressreleases/wlm-star-cluster
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