The Wilds of the Local Group

PR Image eso1610a

The WLM galaxy on the edge of the Local Group 

PR Image eso1610b

The dwarf galaxy WLM in the constellation of Cetus

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Wide-field view of the sky around the dwarf galaxy WLM

Videos

PR Video eso1610a

Zooming in on the dwarf galaxy WLM

PR Video eso1610b

The WLM galaxy on the edge of the Local Group

This scene, captured by ESO’s OmegaCAM on the VLT Survey Telescope, shows a lonely galaxy known as Wolf-Lundmark-Melotte, or WLM for short. Although considered part of our Local Group of dozens of galaxies, WLM stands alone at the group’s outer edges as one of its most remote members. In fact, the galaxy is so small and secluded that it may never have interacted with any other Local Group galaxy — or perhaps even any other galaxy in the history of the Universe.

Rather like an uncontacted tribe living deep in the Amazon rainforest or on an island in Oceania, WLM offers a rare insight into the primordial nature of galaxies that have been little disturbed by their environment.

WLM was discovered in 1909 by German astronomer Max Wolf, and identified as a galaxy some fifteen years later by astronomers Knut Lundmark and Philibert Jacques Melotte — explaining the galaxy’s unusual moniker. The dim galaxy is located in the constellation of Cetus (The Sea Monster) about three million light-years away from the Milky Way, which is one of the three dominant spiral galaxies in the Local Group.

WLM is quite small and lacks structure, hence its classification as a dwarf irregular galaxy. WLM spans about 8000 light-years at its greatest extent, a measurement that includes a halo of extremely old stars discovered in 1996 (eso9633).

Astronomers think that comparatively small primeval galaxies gravitationally interacted with each other and in many cases merged, building up into larger composite galaxies. Over billions of years, this merging process assembled the large spiral and elliptical galaxies that now appear to be common in the modern Universe. 

Galaxies congregating in this manner is similar to the way in which human populations have shifted over thousands of years and intermixed into larger settlements, eventually giving rise to today’s megacities.

WLM has instead developed on its own, away from the influence of other galaxies and their stellar populations. Accordingly, like a hidden human population with limited contact with outsiders, WLM represents a relatively unperturbed “state of nature”, where any changes occurring over its lifetime have taken place largely independent of activity elsewhere.

This small galaxy features an extended halo of very dim red stars, which stretches out into the inky blackness of the surrounding space. This reddish hue is indicative of advanced stellar age. It is likely that the halo dates back to the original formation of the galaxy itself, helpfully offering clues about the mechanisms that spawned the very first galaxies.

The stars at the centre of WLM, meanwhile, appear younger and bluer in colour. In this image, pinkish clouds highlight areas where the intense light from young stars has ionised ambient hydrogen gas, making it glow in a characteristic shade of red.

This detailed image was captured by the OmegaCAM wide-field imager, a huge camera mounted on ESO’s VLT Survey Telescope (VST) in Chile — a 2.6-metre telescope exclusively designed to survey the night sky in visible light. OmegaCAM’s 32 CCD detectors create 256-megapixel images, offering a very detailed wide-field view of the cosmos.

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ESO is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive ground-based astronomical observatory by far. It is supported by 16 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world’s largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is a major partner in ALMA, the largest astronomical project in existence. And on Cerro Armazones, close to Paranal, ESO is building the 39-metre European Extremely Large Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.

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Contacts:

Richard Hook
ESO education and Public Outreach Department
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591
Email: rhook@eso.org 

Source: ESO

Astronomers Discover How Lowly Dwarf Galaxy Becomes Star-Forming Powerhouse

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)

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.

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[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

Source:  National Radio Astronomy Observatory (NRAO)