Universe Today

Robert’s Quartet. Image credit: ESO. Click to enlarge.
ESO PR Photo 34a/05 shows in amazing details a group of galaxies known as Robert’s Quartet [1]. The image is based on data collected with the FORS2 multi-mode instrument on ESO’s Very Large Telescope.

Robert’s Quartet is a family of four very different galaxies, located at a distance of about 160 million light-years, close to the centre of the southern constellation of the Phoenix. Its members are NGC 87, NGC 88, NGC 89 and NGC 92, discovered by John Herschel in the 1830s. NGC 87 (upper right) is an irregular galaxy similar to the satellites of our Milky Way, the Magellanic Clouds. NGC 88 (centre) is a spiral galaxy with an external diffuse envelope, most probably composed of gas. NGC 89 (lower middle) is another spiral galaxy with two large spiral arms. The largest member of the system, NGC 92 (left), is a spiral Sa galaxy with an unusual appearance. One of its arms, about 100,000 light-years long, has been distorted by interactions and contains a large quantity of dust.

The quartet is one of the finest examples of compact groups of galaxies. Because such groups contain four to eight galaxies in a very small region, they are excellent laboratories for the study of galaxy interactions and their effects, in particular on the formation of stars.

Using another set of VLT data also obtained with FORS2, astronomers [2] were able to study the properties of regions of active star formation (“HII regions” [3]) in the sister members of Robert’s Quartet. They found more than 200 of such regions in NGC 92, with a size between 500 and 1,500 light-years. For NGC 87, they detected 56 HII regions, while the two other galaxies appear to have far fewer of them. For NGC 88, however, they found two plume-like features, while NGC 89 presents a ring of enhanced stellar activity. The system is thus clearly showing increased star formation activity, most probably as the result of the interaction between its members. The sisters clearly belong to a perturbed family.

The quartet has a total visual magnitude of almost 13, i.e. it is about 600 times fainter than the faintest object that can be seen with the unaided eye. The brightest member of the group has a magnitude of about 14. On the sky, the four galaxies are all within a circle of radius of 1.6 arcmin, corresponding to about 75,000 light-years.

Notes
[1]: The group of galaxies was known as a Compact Group since 1977 by J.A. Rose, under the designation Rose 34. Robert’s Quartet is also known under the less poetic name of AM 0018-485 from the Catalogue of Southern Peculiar Galaxies and Associations, compiled in 1987 by astronomers Halton “Chip” Arp and Barry Madore. But who is Robert then? As discovered by Australian amateur astronomer Mike Kerr, Arp and Madore named Robert’s Quartet after Robert Freedman who generated many of the updated positions of galaxies in the catalogue. The astronomers clearly had a very good sense of humour as the catalogue also contains a system of galaxies called Wendy (ESO 147- 8; for Wendy Freedman) and another called the Conjugal galaxy (ESO 384- 53)!

[2]: The astronomers are S. Temporin (University of Innsbruck, Austria), S. Ciroi and P. Rafanelli (University of Padova, Italy), A. Iovino (INAF-Brera Astronomical Observatory, Italy), E. Pompei (ESO), and M. Radovich (INAF-Capodimonte Astronomical Observatory, Italy). (The article describing this result is available in PDF format at http://www.ast.cam.ac.uk/%7Esb2004/posters/files/Temporin.pdf)

[3]: The radiation of young hot stars embedded in an interstellar cloud is able to heat the surrounding gas, resulting in the apparition of an emission nebula that shines mostly in the light of ionized hydrogen (H) atoms. Such nebulae are therefore often referred to as “HII regions”. The well-known Orion Nebula is an outstanding example of that type of nebula.

Original Source: ESO News Release

Cloudshine in L1448. Image credit: CfA. Click to enlarge.
Hubble’s iconic images include many shots of cosmic clouds of gas and dust called nebulae. For example, the famous “Pillars of Creation” mark the birthplace of new stars within the Eagle Nebula. Yet despite their beauty, visible-light images show only the nebulae surfaces. Baby stars may hide beneath, invisible even to Hubble’s powerful gaze.

Harvard astronomers have pioneered a new way to peer below the surface using near-infrared light that is invisible to the human eye. The resulting images are both beautiful and scientifically valuable because they can be used to map the structure of interstellar matter.

“We can now see the structure of gigantic star-forming regions over vast distances with a resolution 50 times better than before,” said Alyssa Goodman of the Harvard-Smithsonian Center for Astrophysics (CfA). “This technique will revolutionize the way we map stellar birthplaces.”

While Hubble’s NICMOS instrument and NASA’s Spitzer Space Telescope also use infrared light to study nebular interiors, ground-based images at near-infrared wavelengths provide an unparalleled combination of wide-field coverage and high resolution.

“Images like these will give astronomers new insight into what those giant complexes of gas and dust really look like,” added Jonathan Foster, a graduate student at Harvard University and the paper’s first author.

The researchers took long-exposure photographs of a star-forming region in the constellation Perseus and were surprised to see something they had never seen before. Just as earthly clouds shine orange at night as they reflect light from streetlights below, they discovered that clouds in outer space show a similar effect. In space, otherwise “dark” clouds of dust and gas are illuminated by faint starlight washing over them.

Goodman and Foster dubbed the new celestial phenomenon “cloudshine.” Their long-exposure, near-infrared images uncovered the faintly shining billows of material. Recent advances in infrared detectors, combined with longer than usual imaging times, led to the discovery.

“Other astronomers have seen hints of cloudshine in their images, but our new photographs are the most spectacular evidence of cloudshine to date,” said Goodman.

Reflection nebulae such as the wisps surrounding the Pleiades star cluster have been observed for decades. Importantly, the Pleiades and other famous “nebulae” are illuminated from within, by the stars associated with them, as a cloud is when fireworks explode inside of it. Cloudshine is the result of the illumination of otherwise “dark” clouds from “without,” by the faint, and nearly uniform, ambient light produced by the sum of all the stars outside the cloud. Simple modeling in Foster & Goodman’s paper demonstrates that there is enough of this faint ambient light to illuminate the clouds at the levels observed.

The cloudshine images were obtained as part of the COMPLETE survey (Coordinated Molecular Probe Line Extinction Thermal Emission) of star-forming regions. COMPLETE involves making wide-field, high-resolution studies of three nearby star-forming regions. COMPLETE will allow for detailed analysis and understanding of the physics of star formation on scales ranging from one-hundredth of a light-year up to 30 light-years.

A companion paper by astronomer Paolo Padoan (UC San Diego) and colleagues describes theoretical modeling of the cloudshine effect in turbulent clouds of gas. They showed that the near-infrared “color” of a nebula correlates to the nebula’s density, and can therefore be used to map its structure.

“By using cloudshine, astronomers can study star-forming regions at a very small scale,” said Padoan. “We will be able to learn much more about the physics of star formation.”

Foster and Goodman anticipate gathering many additional images of cloudshine as the COMPLETE survey continues.

“We can cover wide areas of the sky at high resolution relatively quickly,” said Foster. “We expect that this will become the best technique for mapping the density of `dark’ clouds with very high resolution.”

Foster and Goodman’s paper reporting the cloudshine observations has been submitted for publication to The Astrophysical Journal Letters and is available online at http://arxiv.org/abs/astro-ph/0510624.

A paper on the theory of cloudshine by Padoan, Mika Juvela and Veli-Matti Pelkonen (University of Helsinki) also has been submitted for publication to The Astrophysical Journal Letters and is available online at http://arxiv.org/abs/astro-ph/0510600.

Foster and Goodman’s work, and the COMPLETE Survey, are supported by the National Science Foundation, NASA, and Harvard University.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

Original Source: CfA News Release

The Milky Way’s nucleus. Sagittarius A* is the bright white dot at center. Image credit: NRAO/AUI/NSF, Jun-Hui Zhao, W.M. Goss. Click to enlarge.
Astronomers have gotten their deepest glimpse into the heart of our Milky Way Galaxy, peering closer to the supermassive black hole at the Galaxy’s core then ever before. Using the National Science Foundation’s continent-wide Very Long Baseline Array (VLBA), they found that a radio-wave-emitting object at the Galaxy’s center would nearly fit between the Earth and the Sun. This is half the size measured in any previous observation.

“We’re getting tantalizingly close to being able to see an unmistakable signature that would provide the first concrete proof of a supermassive black hole at a galaxy’s center,” said Zhi-Qiang Shen, of the Shanghai Astronomical Observatory and the Chinese Academy of Sciences. A black hole is a concentration of mass so dense that not even light can escape its powerful gravitational pull.

The astronomers used the VLBA to measure the size of an object called Sagittarius A* (pronounced “A-star”) that marks the exact center of our Galaxy. Last year, a different team announced that their measurements showed the object would fit inside the complete circle of Earth’s orbit around the Sun. Shen and his team, by observing at a higher radio frequency, measured Sagittarius A* as half that size.

A mass equal to four million Suns is known to lie within Sagittarius A*, and the new measurement makes the case for a black hole even more compelling than it was previously. Scientists simply don’t know of any long-lasting object other than a black hole that could contain this much mass in such a small area. However, they would like to see even stronger proof of a black hole.

“The extremely strong gravitational pull of a black hole has several effects that would produce a distinctive ‘shadow’ that we think we could see if we can image details about half as small as those in our latest images,” said Fred K.Y. Lo, Director of the National Radio Astronomy Observatory and another member of the research team. “Seeing that shadow would be the final proof that a supermassive black hole is at the center of our Galaxy,” Lo added.

Many galaxies are believed to have supermassive black holes at their centers, and many of these are much more massive than the Milky Way’s black hole. The Milky Way’s central black hole is much less active than that of many other galaxies, presumably because it has less nearby material to “eat.” Astronomers believe that the radio waves they see coming from Sagittarius A* are either generated by particle jets that have been detected in many more-active galaxies or from accretion flows that are spiraling into the central black hole. By observing the object at higher radio frequencies, scientists have detected a region of radiation ever closer to the black hole. The results announced last year were based on observations at 43 GigaHertz (GHz), and the latest observations were made at 86 GHz.

“We believe that if we can double the frequency again, we will see the black-hole shadow produced by effects of Einstein’s General Relativity theory,” Lo said.

In a few years, when the Atacama Large Millimeter Array (ALMA) comes on line, it may be used in conjunction with other millimeter-wave telescopes to make the higher-frequency observations that will reveal the telltale black-hole shadow.

At a distance of 26,000 light-years, the Milky Way’s central black hole is the closest such supermassive object. That makes it the most likely one to finally reveal the concrete evidence for a black hole that astronomers have sought for years.

Shen and Lo worked with Mao-Chang Liang of Caltech, Paul Ho of the Harvard-Smithsonian Center for Astrophysics (CfA) and the Institute of Astronomy & Astrophysics of the Academia Sinica in Taiwan, and Jun-Hui Zhao of CfA. The astronomers published their findings in the November 3 issue of the scientific journal Nature.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

Original Source: NRAO News Release

Artist illustration of the early Universe. Image credit: NASA/JPL-Caltech/R. Hurt (SSC). Click to enlarge.
Scientists using NASA’s Spitzer Space Telescope say they have detected light that may be from the earliest objects in the universe. If confirmed, the observation provides a glimpse of an era more than 13 billion years ago when, after the fading embers of the theorized Big Bang gave way to millions of years of pervasive darkness, the universe came alive.

This light could be from the very first stars or perhaps from hot gas falling into the first black holes. The science team, based at NASA Goddard Space Flight Center in Greenbelt, Md., describes the observation as seeing the glow of a distant city at night from an airplane. The light is too distant and feeble to resolve individual objects.

“We think we are seeing the collective light from millions of the first objects to form in the universe,” said Dr. Alexander Kashlinsky, Science Systems and Applications scientist and lead author on the Nature article that appeared in the Nov. 3 issue. “The objects disappeared eons ago, yet their light is still traveling across the universe.”

Scientists theorize that space, time and matter originated 13.7 billion years ago in a Big Bang. Another 200 million years would pass before the era of first starlight. A 10-hour observation by Spitzer’s infrared array camera in the constellation Draco captured a diffuse glow of infrared light, lower in energy than optical light and invisible to us. The Goddard team says that this glow is likely from Population III stars, a hypothesized class of stars thought to have formed before all others. (Population I and II stars, named by order of their discovery, comprise the familiar types of stars we see at night.)

Theorists say the first stars were likely over a hundred times more massive than Earth’s sun and extremely hot, bright, and short-lived, each one burning for only a few million years. The ultraviolet light that Population III stars emitted would be redshifted, or stretched to lower energies, by the universe’s expansion. That light should now be detectable in the infrared.

“This deep observation was filled with familiar-looking stars and galaxies,” said Dr. John Mather, senior project scientist for JWST and a co-author on the Nature article. “We removed everything we knew—all the stars and galaxies both near and far. We were left with a picture of part of the sky with no stars or galaxies, but it still had this infrared glow with giant blobs that we think could be the glow from the very first stars.”

This new Spitzer discovery agrees with observations from the NASA Cosmic Background Explorer (COBE) satellite from the 1990s that suggested there may be an infrared background that could not be attributed to known stars. It also supports observations from the NASA Wilkinson Microwave Anisotropy Probe from 2003, which estimated that stars first ignited 200 million to 400 million years after the Big Bang.

“This difficult measurement pushes the instrument to performance limits that were not anticipated in its design,” said team member Dr. S. Harvey Moseley, instrument scientist for Spitzer. “We have worked very hard to rule out other sources for the signal we observed.”

The low noise and high resolution of Spitzer’s infrared array camera enabled the team to remove the fog of foreground galaxies, made of later stellar populations, until the cumulative light from the first light dominated the signal on large angular scales. The team, which also includes Dr. Richard Arendt, Science Systems and Applications scientist, noted that future missions, such as NASA’s James Webb Space Telescope, will find the first individual clumps of these stars or the individual exploding stars that might have made the first black holes.

This analysis was partially funded through the National Science Foundation. The Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer mission for NASA. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology in Pasadena. NASA Goddard built Spitzer’s infrared array camera which took the observations. The instrument’s principal investigator is Dr. Giovanni Fazio, Smithsonian Astrophysical Observatory, Cambridge, Mass.

Original Source: Spitzer News Release