Universe Today

Most present-day large galaxies are spirals, presenting a disc surrounding a central bulge. Famous examples are our own Milky Way or the Andromeda Galaxy. When and how did these spiral galaxies form? Why do a great majority of them present a massive central bulge?

An international team of astronomers [1] presents new convincing answers to these fundamental questions. For this, they rely on an extensive dataset of observations of galaxies taken with several space- and ground-based telescopes. In particular, they used over a two-year period, several instruments on ESO’s Very Large Telescope.

Among others, their observations reveal that roughly half of the present-day stars were formed in the period between 8,000 million and 4,000 million years ago, mostly in episodic burst of intense star formation occurring in Luminous Infrared Galaxies.

From this and other evidence, the astronomers devised an innovative scenario, dubbed the “spiral rebuilding”. They claim that most present-day spiral galaxies are the results of one or several merger events. If confirmed, this new scenario could revolutionise the way astronomers think galaxies formed.

A fleet of instruments
How and when did galaxies form? How and when did stars form in these island universes? These questions are still posing a considerable challenge to present-day astronomers.

Front-line observational results obtained with a fleet of ground- and space-based telescopes by an international team of astronomers [1] provide new insights into these fundamental issues.

For this, they embarked on an ambitious long-term study at various wavelengths of 195 galaxies with a redshift [2] greater than 0.4, i.e. located more than 4000 million light-years away. These galaxies were studied using ESO’s Very Large Telescope, as well as the NASA/ESA Hubble Space Telescope, the ESA Infrared Space Observatory (ISO) satellite and the NRAO Very Large Array.

With the Very Large Telescope, observations were performed on Antu and Kueyen over a two-year period using the quasi-twin instruments FORS1 and FORS2 in the visible and ISAAC in the infrared. In both cases, it was essential to rely on the unique capabilities of the VLT to obtain high-quality spectra with the required resolution.

A fleet of results
From their extensive set of data, the astronomers could draw a number of important conclusions.

First, based on the near-infrared luminosities of the galaxies, they infer that most of the galaxies they studied contain between 30,000 million and 300,000 million times the mass of the Sun in the form of stars. This is roughly a factor 0.2 to 2 the amount of mass locked in stars in our own Milky Way.

Second, they discovered that contrary to the local Universe where so-called Luminous Infrared Galaxies (LIRGs; [3]) are very rare objects, at a redshift from 0.4 to 1, that is, 4,000 to 8,000 million years ago, roughly one sixth of bright galaxies were LIRGs.

Because this peculiar class of galaxies is believed to be going through a very active phase of star formation, with a doubling of the stellar mass occurring in less than 1,000 million years, the existence of such a large fraction of these LIRGs in the past Universe has important consequences on the total stellar formation rate.
As Fran?ois Hammer (Paris Observatory, France), leader of the team, states: “We are thus led to the conclusion that during the time span from roughly 8,000 million to 4,000 million years ago, intermediate mass galaxies converted about half of their total mass into stars. Moreover, this star formation must have taken place in very intense bursts when galaxies were emitting huge amount of infrared radiation and appeared as LIRGs.”

Another result could be secured using the spectra obtained with the Very Large Telescope: the astronomers measured the chemical abundances in several of the observed galaxies (PR Photo 02a/05). They find that galaxies with large redshifts show oxygen abundances two times lower than present-day spirals. As it is stars which produce oxygen in a galaxy, this again gives support to the fact that these galaxies have been actively forming stars in the period between 8,000 and 4,000 million years ago.

And because it is believed that galaxy collisions and mergers play an important role in triggering such phases of enhanced star-forming activity, these observations indicate that galaxy merging still occurred frequently less than 8,000 million years ago.

Spiral Rebuilding
The story revealed by these observations is in agreement with the so-called “hierarchical merging of galaxies” scenario, present in the literature since about 20 years. According to this model, small galaxies merge to build larger ones. As Fran?ois Hammer however points out: “In the current scenario, it was usually assumed that galaxy merging almost ceased 8,000 million years ago. Our complete set of observations show that this is far from being the case. In the following 4,000 million years, galaxies still merged to form the large spirals we observe in the local Universe.”

To account for all these properties, the astronomers thus devised a new galaxy formation scenario, comprising three major phases: a merger event, a compact galaxy phase and a “growth of the disc” phase (see PR Photo 02b/05).

Because of the unique aspects of this scenario, where big galaxies get first disrupted by a major collision to be born again later as a present-day spiral galaxy, the astronomers rather logically dubbed their evolutionary sequence, the “spiral galaxy rebuilding”.

Although being at odds with standard views which assert that galaxy mergers produce elliptical galaxies instead of spiral ones, the astronomers stress that their scenario is consistent with the observed fractions of the different types of galaxies and can account for all the observations.

The new scenario can indeed account for the formation of about three quarters of the present-day spiral galaxies, those with massive central bulge. It would apply for example to the Andromeda Galaxy but not to our own Milky way. It seems that our Galaxy somehow escaped major collisions in the last thousands of million years.

Further observations, in particular with the FLAMES instrument on the VLT, will show if spiral galaxies are indeed relatively recent born-again systems created from major merger events.

More information
The research presented in this Press Release has been published in the leading astronomical journal Astronomy and Astrophysics, vol. 430(1). The paper (“Did most present-day spirals form during the last 8 Gyrs? – A formation history with violent episodes revealed by panchromatic observations” by F. Hammer et al.) is available in PDF format from the A&A web site.

Notes
[1]: The team is composed of Fran?ois Hammer and Hector Flores (Observatoire de Paris, Meudon, France), David Elbaz (CEA Saclay, France), Xian-Zhong Zheng (Observatoire de Paris, Meudon, France and Max-Planck Instiut f?r Astronomie, Germany), Yan-Chun Liang (Observatoire de Paris, Meudon, France and National Astronomical Observatories, China) and Catherine Cesarsky (ESO, Garching, Germany).

[2]: In astronomy, the redshift denotes the fraction by which the lines in the spectrum of an object are shifted towards longer wavelengths. The observed redshift of a remote galaxy provides an estimate of its distance. The distances and ages indicated in the present text are based on an age of the Universe of 13,700 million years.

[3]: Luminous Infrared Galaxies (LIRGs) are a subset of galaxies whose infrared luminosity is larger than 100,000 million time the luminosity of our Sun. They were first discovered as a class by the ESA ISO satellite and are believed to be galaxies undergoing enhanced stellar formation.

Original Source: ESO News Release

New, very high-resolution (false-color) images of a dying star IRAS16342-3814 (hereafter the Water-Fountain Nebula) taken with the Keck II Telescope equipped with adaptive optics, at the W. M. Keck Observatory on Mauna Kea, Hawaii, are helping astronomers understand the extraordinary deaths of ordinary Sun-like stars. These results are being presented today to the 205th American Astronomical Society meeting in San Diego, California, by Raghvendra Sahai of the Jet Propulsion Laboratory (JPL), California Institute of Technology, Pasadena; D. Le Mignant, R.D. Campbell, F.H. Chaffee of W. M. Keck Observatory, Mauna Kea, Hawaii; and C. S?nchez Contreras of the California Institute of Technology.

Sun-like stars shine sedately for billions of years, but die in spectacular fashion, creating intricate and beautiful gaseous shrouds around them in the relatively short period of about a thousand years or less. These shrouds, called planetary nebulae, come in a wide variety of beautiful non-spherical shapes, in striking contrast to the round shapes of their progenitor stars. The answer to the question of how planetary nebulae acquire their diverse shapes has long eluded astronomers.

The images of the Water-Fountain Nebula (which lies at an estimated distance of 6500 light years in the direction of Scorpius) shown here, were acquired using the adaptive optics (AO) technique, at two near-infrared wavelengths (using filters centered at wavelengths of 2.1 and 3.8 microns). The AO technique removes the blurring effect of Earth’s atmosphere and allows astronomers to take full advantage of large ground-based telescopes like the W. M. Keck Telescope, revealing important details which were hidden even to the sharp eyes of the Hubble Space Telescope (HST). The images show two lobes, which are cavities (each of size about 2000 Astronomical Units) in an extended cloud of gas and dust, illuminated by light from a central star which lies between the two lobes, but is hidden from our view behind a dense, dust lane that separates the two lobes. These near-infrared AO images probe much deeper than HST into the two lobes of the Water-Fountain Nebula, showing a remarkable corkscrew-shaped structure (marked by dashed lines) apparently etched into the lobe walls.

According to JPL Research Scientist Dr. Sahai, ” The corkscrew structure seen here is the proverbial writing on the wall signature of an underlying high-speed jet of matter which has changed its direction in a regular fashion (called precession). These images of the Water-Fountain Nebula thus show direct evidence for a jet actively carving out a bipolar nebula, providing unambiguous support for our recently proposed hypothesis that the shaping of most planetary nebulae is carried out by such jets”.

The discovery of the corskcrew pattern resulting from a precessing jet in the Water-Fountain Nebula is an exciting addition to our knowledge of jets in dying stars as well as astrophysical jets in general. The jets in dying stars are thought to operate for a very short period of time (few hundred years). Finding direct evidence for these jet-like outflows has been generally very difficult, because they are compact, not always active, and it is difficult to see them against the bright nebular background. A detailed comparison of the images of the Water-Fountain Nebula taken with filters of different colors allows scientists to determine the physical properties of the nebula. New AO imaging in a few years from now will enable Dr. Sahai and collaborators to measure the physical motion of matter in the corkscrew pattern, and provide strong constraints on the nebular shaping process.

When Sun-like stars get old, they become cooler and redder, increasing their sizes and energy output tremendously: they are called red giants. Most of the carbon (the basis of life) and particulate matter (crucial building blocks of solar systems like ours) in the universe is manufactured and dispersed by red giant stars. Preplanetary nebulae are formed when the red giant star has ejected most of its outer layers. As the very hot core (six or more times hotter than the Sun) gets further exposed, the cloud of ejected material is bathed with ultraviolet light, making it glow; the object is then called a planetary nebula.

Original Source: Keck News Release

An astronomer studying small irregular galaxies has discovered a remarkable feature in one of them that may provide key clues to understanding how galaxies form and the relationship between the gas and the stars within galaxies.

Liese van Zee of Indiana University Bloomington, using the National Science Foundation’s Very Large Array radio telescope in New Mexico, found that a small galaxy 16 million light-years from Earth is surrounded by a huge disk of hydrogen gas that has not been involved in the galaxy’s star-formation processes and may be primordial material left over from the galaxy’s formation. “If that’s the case, then we may have found a nearby sample similar to the stuff of the early universe,” van Zee said.

“Why the gas in the disk has remained so undisturbed, without stars forming, is somewhat perplexing. When we figure out how this happened, we’ll undoubtedly learn more about how galaxies form,” she said.

She presented her findings on Wednesday (Jan. 12) at the national meeting of the American Astronomical Society in San Diego, Calif.

The galaxy van Zee studied, called UGC 5288, had been regarded as just one ordinary example of a numerous type called dwarf irregular galaxies. As part of a study of such galaxies, she had earlier made a visible-light image of it at Kitt Peak National Observatory in Arizona.

When she observed the galaxy later using the radio telescope, she found that it is embedded in a huge disk of atomic hydrogen gas. In visible light, the elongated galaxy is about 6,000 by 4,000 light-years, but the hydrogen-gas disk, seen with the VLA, is about 41,000 by 28,000 light-years. “The gas disk is more than seven times bigger than the galaxy we see in visible light,” she said.

The hydrogen disk can be seen by radio telescopes because hydrogen atoms emit and absorb radio waves at a frequency of 1420 MHz, a wavelength of about 21 centimeters.

A few other dwarf galaxies have large gas disks, but unlike these, UGC 5288’s disk shows no signs that the gas was either blown out of the galaxy by furious star formation or pulled out by a close encounter with another galaxy. “This gas disk is rotating quite peacefully around the galaxy,” van Zee explained. That means, she said, that the gas around UGC 5288 most likely is pristine material that has never been “polluted” by the heavier elements produced in stars.

What’s surprising, said Martha Haynes, an astronomer at Cornell University in Ithaca, N.Y., is that the huge gas disk seems to be completely uninvolved in the small galaxy’s star-formation processes. “You need the gas to make the stars, so we might have thought the two would be better correlated. This means we really don’t understand how the star-forming gas and the stars themselves are related,” Haynes said.

It’s exciting to find such a large reservoir of apparently unprocessed matter, Haynes said. “This object and others like it could be the targets for studying pristine material in the universe,” she said.

Haynes was amused that a galaxy that looked “boring” to some in visible-light images showed such a remarkable feature when viewed with a radio telescope.

“This shows that you can’t judge an object by its appearance at only one wavelength. What seems boring at one wavelength may be very exciting at another,” Haynes said.

Original Source: Indiana University

NASA’s Spitzer Space Telescope has uncovered a hatchery for massive stars.

A new striking image from the infrared telescope shows a vibrant cloud called the Trifid Nebula dotted with glowing stellar “incubators.” Tucked deep inside these incubators are rapidly growing embryonic stars, whose warmth Spitzer was able to see for the first time with its powerful heat-seeking eyes.

The new view offers a rare glimpse at the earliest stages of massive star formation ? a time when developing stars are about to burst into existence.

“Massive stars develop in very dark regions so quickly that is hard to catch them forming,” said Dr. Jeonghee Rho of the Spitzer Science Center, California Institute of Technology, Pasadena, Calif., principal investigator of the recent observations. “With Spitzer, it’s like having an ultrasound for stars. We can see into dust cocoons and visualize how many embryos are in each of them.”

The new false-color image can be found at http://www.spitzer.caltech.edu/Media. It was presented today at the 205th meeting of the American Astronomical Society in San Diego, Calif.

The Trifid Nebula is a giant star-forming cloud of gas and dust located 5,400 light-years away in the constellation Sagittarius. Previous images taken by the Institute for Radioastronomy millimeter telescope in Spain show that the nebula contains four cold knots, or cores, of dust. Such cores are “incubators” where stars are born. Astronomers thought the ones in the Trifid Nebula were not yet ripe for stars. But, when Spitzer set its infrared eyes on all four cores, it found that they had already begun to develop warm stellar embryos.

“Spitzer can see the material from the dark cores falling onto the surfaces of the embryonic stars, because the material gets hotter as gravity draws it in,” said Dr. William T. Reach of the Spitzer Science Center, co-author of this new research. “By measuring the infrared brightness, we can not only see the individual embryos but determine their growth rate.”

The Trifid Nebula is unique in that it is dominated by one massive central star, 300,000 years old. Radiation and winds emanating from the star have sculpted the Trifid cloud into its current cavernous shape. These winds have also acted like shock waves to compress gas and dust into dark cores, whose gravity caused more material to fall inward until embryonic stars were formed. In time, the growing embryos will accumulate enough mass to ignite and explode out of their cores like baby birds busting out of their eggs.

Because the Trifid Nebula is home to just one massive star, it provides astronomers a rare chance to study an isolated family unit. All of the newfound stellar embryos are descended from the nebula’s main star. Said Rho, “Looking at the image, you know exactly where the embryos came from. We use their colors to determine how old they are. It’s like studying the family tree for a generation of stars.”

Spitzer discovered 30 embryonic stars in the Trifid Nebula’s four cores and dark clouds. Multiple embryos were found inside two massive cores, while a sole embryo was seen in each of the other two. This is one of the first times that clusters of embryos have been observed in single cores at this early stage of stellar development.

“In the cores with multiple embryos, we are seeing that the most massive and brightest of the bunch is near the center. This implies that the developing stars are competing for materials, and that the embryo with the most material will grow to be the largest star,” said Dr. Bertrand Lefloch of Observatoire de Grenoble, France, co-author of the new research.

Spitzer also uncovered about 120 small baby stars buried inside the outer clouds of the nebula. These newborns were probably formed around the same time as the main massive star and are its smaller siblings.

Other authors of this work include Dr. Giovanni Fazio, Smithsonian Astrophysical Observatory, Cambridge, Mass.

NASA’s Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA’s Science Mission Directorate, Washington, D.C. Science operations are conducted at the Spitzer Science Center, Pasadena, Calif. JPL is a division of Caltech.

The new Spitzer image is a combination of data from the telescope’s infrared array camera and multiband imaging photometer. The infrared array camera was built by NASA Goddard Space Flight Center, Greenbelt, Md.; its development was led by Fazio. The multiband imaging photometer was built by Ball Aerospace Corporation, Boulder, Colo., the University of Arizona, Tucson, and Boeing North American, Canoga Park, Calif. The instrument’s development was led by Dr.George Rieke, University of Arizona.

Additional information about the Spitzer Space Telescope is available at http://www.spitzer.caltech.edu.

Original Source: NASA/JPL News Release

A dense globular star cluster near the center of our Milky Way Galaxy holds a buzzing beehive of rapidly-spinning millisecond pulsars, according to astronomers who discovered 21 new pulsars in the cluster using the National Science Foundation’s 100-meter Robert C. Byrd Green Bank Telescope (GBT) in West Virginia. The cluster, called Terzan 5, now holds the record for pulsars, with 24, including three known before the GBT observations.

“We hit the jackpot when we looked at this cluster,” said Scott Ransom, an astronomer at the National Radio Astronomy Observatory in Charlottesville, VA. “Not only does this cluster have a lot of pulsars — and we still expect to find more in it — but the pulsars in it are very interesting. They include at least 13 in binary systems, two of which are eclipsing, and the four fastest-rotating pulsars known in any globular cluster, with the fastest two rotating nearly 600 times per second, roughly as fast as a household blender,” Ransom added. Ransom and his colleagues reported their findings to the American Astronomical Society’s meeting in San Diego, CA, and in the online journal Science Express.

The star cluster’s numerous pulsars are expected to yield a bonanza of new information about not only the pulsars themselves, but also about the dense stellar environment in which they reside and probably even about nuclear physics, according to the scientists. For example, preliminary measurements indicate that two of the pulsars are more massive than some theoretical models would allow. “All these exotic pulsars will keep us busy for years to come,” said Jason Hessels, a Ph.D student at McGill University in Montreal.

Globular clusters are dense agglomerations of up to millions of stars, all of which formed at about the same time. Pulsars are spinning, superdense neutron stars that whirl “lighthouse beams” of radio waves or light around as they spin. A neutron star is what is left after a massive star explodes as a supernova at the end of its life.

The pulsars in Terzan 5 are the product of a complex history. The stars in the cluster formed about 10 billion years ago, the astronomers say. Some of the most massive stars in the cluster exploded and left the neutron stars as their remnants after only a few million years. Normally, these neutron stars would no longer be seen as swiftly-rotating pulsars: their spin would have slowed because of the “drag” of their intense magnetic fields until the “lighthouse” effect is no longer observable.

However, the dense concentration of stars in the cluster gave new life to the pulsars. In the core of a globular cluster, as many as a million stars may be packed into a volume that would fit easily between the Sun and our nearest neighbor star. In such close quarters, stars can pass near enough to form new binary pairs, split apart such pairs, and binary systems even can trade partners, like an elaborate cosmic square dance. When a neutron star pairs up with a “normal” companion star, its strong gravitational pull can draw material off the companion onto the neutron star. This also transfers some of the companion’s spin, or angular momentum, to the neutron star, thereby “recycling” the neutron star into a rapidly-rotating millisecond pulsar. In Terzan 5, all the pulsars discovered are rotating rapidly as a result of this process.

Astronomers previously had discovered three pulsars in Terzan 5, some 28,000 light-years distant in the constellation Sagittarius, but suspected there were more. On July 17, 2004, Ransom and his colleagues used the GBT, and, in a 6-hour observation, found 14 new pulsars, the most ever found in a single observation.

“This was possible because of the great sensitivity of the GBT and the new capabilities of our backend processor,” said Ingrid Stairs, a professor at the University of British Columbia in Vancouver. The processor, named, appropriately, the Pulsar Spigot, was built in a collaboration between the NRAO and the California Institute of Technology. The processor, which generates almost 100 GigaBytes of data per hour, allowed the astronomers to gather and analyze radio waves over a wide range of frequencies (1650-2250 MegaHertz), adding to the sensitivity of their system.

Eight more observations between July and November of 2004 discovered seven additional pulsars in Terzan 5. In addition, the astronomers’ data show evidence for several more pulsars that still need to be confirmed.

Future studies of the pulsars in Terzan 5 will help scientists understand the nature of the cluster and the complex interactions of the stars at its dense core. Also, several of the pulsars offer a rich yield of new scientific information. The scientists suspect that one pulsar, which shows strange eclipses of its radio emission, has recently traded its original binary companion for another, and two others have white-dwarf companions that they believe may have been produced by the collision of a neutron star and a red-giant star. Subtle effects seen in these two systems can be explained by Einstein’s general relativistic theory of gravity, and indicate that the neutron stars are more massive than some theories allow. The material in a neutron star is as dense as that in an atomic nucleus, so that fact has implications for nuclear physics as well as astrophysics.

“Finding all these pulsars has been extremely exciting, but the excitement really has just begun,” Ransom said. “Now we can start to use them as a rich and valuable cosmic laboratory,” he added.

In addition to Ransom, Hessels and Stairs, the research team included Paulo Freire of Arecibo Observatory in Puerto Rico, Fernando Camilo of Columbia University, Victoria Kaspi of McGill University, and David Kaplan of the Massachusetts Institute of Technology.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc. The pulsar research also was supported by the Canada Foundation for Innovation, Science and Engineering Research Canada, the Quebec Foundation for Research on Nature and Technology, the Canadian Institute for Advanced Research, Canada Research Chairs Program, and the National Science Foundation.

Original Source: NRAO News Release

Hubble astronomers have uncovered, for the first time, a population of infant stars in the Milky Way satellite galaxy, the Small Magellanic Cloud (SMC, visible to the naked eye in the southern constellation Tucana), located 210,000 light-years away.

Hubble’s exquisite sharpness plucked out an underlying population of infant stars embedded in the nebula NGC 346 that are still forming from gravitationally collapsing gas clouds. They have not yet ignited their hydrogen fuel to sustain nuclear fusion. The smallest of these infant stars is only half the mass of our Sun.

Although star birth is common within the disk of our galaxy, this smaller companion galaxy is more primeval in that it lacks a large percentage of the heavier elements that are forged in successive generations of stars through nuclear fusion.

Fragmentary galaxies like the SMC are considered primitive building blocks of larger galaxies. Most of these types of galaxies existed far away, when the universe was much younger. The SMC offers a unique nearby laboratory for understanding how stars arose in the early universe. Nestled among other starburst regions with the small galaxy, the nebula NGC 346 alone contains more than 2,500 infant stars.

The Hubble images, taken with the Advanced Camera for Surveys, identify three stellar populations in the SMC and in the region of the NGC 346 nebula ? a total of 70,000 stars. The oldest population is 4.5 billion years, roughly the age of our Sun. The younger population arose only 5 million years ago (about the time Earth’s first hominids began to walk on two feet). Lower-mass stars take longer to ignite and become full-fledged stars, so the protostellar population is 5 million years old. Curiously, the infant stars are strung along two intersecting lanes in the nebula, resembling a “T” pattern in the Hubble plot.

The observations, by Antonella Nota of the European Space Agency (ESA) and the Space Telescope Science Institute (STScI), Baltimore, Md., are being presented today at the meeting of the American Astronomical Society in San Diego, Calif.

The other science team members are: M. Sirianni (STScI/ESA), E. Sabbi (Univ. of Bologna), M. Tosi (INAF – Bologna Observ.), J.S. Gallagher (Univ. of Wisconsin), M. Meixner (STScI), M. Clampin (GSFC), S. Oey (Univ. of Michigan), A. Pasquali (ETH Zurich), L. Smith (Univ. College London), and R. Walterbos (New Mexico State Univ.).

Original Source: Hubble News Release