If you point a telescope near the constellations of Leo and Virgo, you might be able to catch a glimpse of the galactic monster in question: the Umbrella Galaxy, formally called NGC 4651.
This spiral galaxy — a twin of the Milky Way — is eating a smaller galaxy, and it gets its whimsical nickname from the wispy “parasol” that surrounds it.
When scientists discovered this umbrella in the 1950s, they interpreted it as a dwarf galaxy companion to the bigger galaxy. But recent research has suggested this parasol might actually be made up of crumbs from a leftover meal.
Astronomers have shown that our own Milky Way has fattened up by acquiring stars from other, smaller galaxies. They’ve found streams of star crumbs emanating from the nearby Sagittarius dwarf galaxy, which is being engulfed by the Milky Way.
What’s more, a study in 2010 that looked at eight spiral galaxies, including the Umbrella Galaxy, found that six of them had signs of mergers: shells, clouds and arcs of tidal debris.
Researchers led by Caroline Foster of the Australian Astronomical Observatory (AAO) have been studying the Umbrella Galaxy, and they learned that its distinctive arc is made up of the crumbs from a single dinner, rather than a series of meals.
“Through new techniques we have been able to measure the movements of the stars in the very distant, very faint, stellar stream in the Umbrella,” Foster explained in a statement from the AAO. “This allows us to reconstruct the history of the system, which we couldn’t before.”
The astronomers used observations from the Subaru and Keck telescopes in Hawaii, and they tracked the movement of the stars in the stream by looking at globular clusters, planetary nebulae and patches of hydrogen gas in the galaxy. (Its distance from Earth is not well established, but the researchers in the study estimated that it is 62 million light-years away.)
The study, which has been accepted for publication in the Monthly Notices of the Royal Astronomical Society, is free to read online at the preprint service arxiv.
Brilliant bursts of star formation in distant dwarf galaxies seen by NASA’s Hubble Space Telescope could reveal new information about the early history of the universe, scientists say.
Galaxies churn out new stars all the time, but most of the universe’s stars formed between two and six billion years after the Big Bang (which occurred 13.8 billion years ago). The new Hubble observations capture the prolific dwarf galaxies, which are known as “starburst galaxies,” during this dramatic epoch, researchers said. You can watch a video explaining the new dwarf galaxy observations .
“We already suspected that dwarf starbursting galaxies would contribute to the early wave of star formation, but this is the first time we’ve been able to measure the effect they actually had,”study lead author Hakim Atek, of the École Polytechnique Federale de Lausanne in Switzerland, said in a statement.
“They appear to have had a surprisingly significant role to play during the epoch where the universe formed most of its stars,” Atek added.
The distant dwarf galaxiesthat the Hubble telescope observed are forming stars so quickly that they can double the number of stars they hold in just 150 million years. Normal galaxies take 1 to 3 billion years to do this, researchers said.
Starburst galaxies are relatively rare; researchers think these galaxies generally require a powerful event, such as a supernova explosion or galaxy merger, to get kicked into star-forming gear.
Hubble observed the dwarf galaxies using its Wide Field Camera 3 (WFC3) instrument, which captures images in a wide range of wavelengths. In the new study, infrared light proved key to illuminating the faraway starburst galaxies.
WFC3 also has a prism, which splits light into its constituent wavelengths. The spectroscopy mode of the camera produced the images, in which each galaxy appears as a rainbow streak. Scientists analyze the galaxies’ spectra to estimate how far away they are from Earth and determine their chemical composition.
Previous studies of starburst galaxies had focused on nearby or large galaxies, leaving out the faraway, ancient dwarfs, which are more difficult to observe, researchers said.
The iconic Hubble Space Telescope has been snapping pictures of the universe since 1990 and is part of NASA’s “Great Observatories” project.
The universe is expanding — and it is doing so at the same rate in all directions, according to new measurements that appear to confirm the standard model of cosmology.
Astrophysicist Jeremy Darling of the University of Colorado Boulder came to this conclusion after employing a research strategy known as “real-time cosmology,” which seeks out the tiny changes in the universe that occur over human timescales.
The idea of “real-time cosmology” was proposed in two separate papers by Alan Sandage in 1962 and by Harvard astrophysicist Avi Loebin 1998. The possibility of seeing the redshifts of sources changing in real time is thus called the “Sandage-Loeb Test”. [The Universe: Big Bang to Now in 10 Easy Steps]
“Real-time cosmology offers new ways to observe the universe, including some observations and cosmological tests that cannot be made any other way,” Darling told Space.com via email.
Researchers discovered in 1998 that the universe is expanding at an accelerating rate — a surprising phenomenon believed to be due to a mysterious force called dark energy. Scientists don’t know much about dark energy, except that it may be a property of the vacuum. In a bid to understand dark energy, researchers are making a wide array of cosmological tests and building new telescopes and instruments.
“This work asks whether the expansion today — that is dominated by dark energy — is the same in all directions,” Darling said.
To make the measurements, Darling used data previously collected by other researchers on the motion of extra-galactic objects across the sky.
The data allowed him to conclude that the cosmic expansion is indeed isotropic — in other words, the same in all directions — with a margin of error of 7 percent.
“The constraints will get better with forthcoming data from the Gaia mission,” said Loeb, who was not involved in the study.
The European Space Agency’s Gaia probe, which launched last December, is designed to create a three-dimensional map of Earth’s Milky Way galaxy, mapping the motions of about 1 billion objects. This work should dramatically expand the sample size currently available to Darling and other researchers.
Traditionally, most cosmological observations treat the universe as frozen in time: with a fixed age, fixed distances and fixed properties. So, to see the history of the universe, scientists must look at similar objects at different distances.
Since the speed of light is finite, observers see more-distant objects as they existed at earlier cosmological times. The traditional strategy is thus to develop a statistical sample of cosmological “probes” across time to study how everything in the universe changes and evolves.
There is one exception to this statistical approach, though: the cosmic microwave background (CMB), the so-called “first light” left over from the Big Bang that created the universe 13.8 billion years ago.
“It arises from a single time that shows a fairly complete snapshot of the universe at that instant,” Darling said. “But the CMB is also treated as static.” [Cosmic Microwave Background: Big Bang Relic Explained (Infographic)]
Real-time cosmology, however, takes a different tack, relying on the idea that “now” is a changing time.
“If we lived long enough, we would see objects receding away from us, growing smaller and fainter with distance, and accelerating,” Darling said. “We would see the CMB roiling as new parts of the last scattering surface — the light horizon — receded. We would see gravity at work, causing large structures of galaxies and galaxy clusters to collapse and voids to expand.
“Basically, any observable property should change in real time if we could watch for a very long time, or [if] we could measure things (positions, velocities) extremely precisely.”
Real-time cosmology measurements are “raw” observations that don’t rely on models or statistical samples.
“I could pick my favorite galaxy and watch it accelerate, shrink and dim as it recedes, directly revealing the dynamic aspect of the universe,” Darling said.
And, he added, real-time cosmology could help find answers to the most basic but important questions about the cosmos, such as whether or not the universe is rotating, the nature of dark energy and the masses of large-scale structures in the universe.
While future instruments should give real-time cosmology a big boost, it’s already possible to make major discoveries using this strategy, Darling said.
“Precision astrometry — measuring the position of objects in the sky — can be done now both at radio wavelengths, with very long baseline interferometry, especially the Very Long Baseline Array, and optical wavelengths — the Gaia mission,” he said. “The acceleration can be directly measured with 30-meter [98 feet] optical telescopes and with the future radio Square Kilometer Array [in Australian and South Africa].” [10 Biggest Telescopes on Earth: How They Measure Up]
And, he added, the International Celestial Reference Frame data already offer an early way to test the models and theories of real-time cosmology. The ICRF is the framework researchers use to calibrate their measurements of where objects are in the sky.
“It is used to monitor the Earth’s rotation and its wobbles and glitches,” said Darling. “A network of radio-bright quasars are monitored regularly using very long baseline interferometry, and have been for decades. The measurements of the positions of these quasars are so precise that they can be used for all sorts of ancillary science.”
There are certain limitations, though — such as precision in redshift and astrometry, and the systematic errors associated with precise measurement.
“Accelerations are expected to be less than 1 centimeter per second per year, and astrometry needs to be about 1 microarcsecond per year,” Darling said. “The next step is to reach the limit of current VLBI [very-long-baseline interferometry] and then start using Gaia, which is likely to happen in three to five years.”
Loeb calls the work “original and interesting.” However, he says, “within the broader context of cosmological data, one can use the cosmic microwave background to conclude that the expansion is isotropic to better than 0.001 percent, or else we would see temperature variations across the sky that are larger than observed.”
Darling acknowledges that this is true, but argues that the CMB temperature as observed depends only on the total amount of expansion that occurred between when the light was emitted and when it was observed.
“It cannot be used to measure the expansion rate today (or at any other time),” Darling said. “It can, however, tell us about the overall, history-integrated anisotropy (i.e., did the universe grow more in one dimension than another since it was 300,000 years old, making one part of the sky appear colder/redder than another).”
Most importantly, Darling said, real-time cosmology allows researchers to make “new measurements of the universe to test our theoretical understanding. This is how new discoveries are made. It is good to see that the universe as observed today behaves itself and supports the current cosmological paradigm and causes no conflict with the CMB.”
His research has been accepted for publication in the journal Monthly Notices of the Royal Astronomical Society.
But author and astrophysicist Jeffrey Bennett believes relativity should be something anyone can understand. He explains what relativity is and how it causes a slew of bizarre effects in his new book, “What is Relativity?” (Columbia University Press, 2014).
In an excerpt from the book’s first chapter, Bennett takes readers on a voyage to a black hole, a mysterious realm in which the full strangeness of relativity is on display.
Part 1: Introduction
Chapter 1: Voyage to a Black Hole
Imagine that the sun magically collapsed, retaining the same mass but shrinking in size so much that it became a black hole. What would happen to Earth and the other planets? Ask almost anyone, including elementary school kids, and they’ll tell you confidently that the planets “would be sucked in.”
Now imagine that you’re a future interstellar traveler. Suddenly, you discover that a black hole lurks off to your left. What should you do? Again, ask around, and you’ll probably be told to fire up your engines to try to get away, and that you’ll be lucky to avoid being “sucked into oblivion.”
But I’ll let you in on a little secret that’s actually important to understanding relativity: Black holes don’t suck. If the sun suddenly became a black hole, Earth would become very cold and dark. However, since we’ve assumed that the black hole will have the same mass as the sun, Earth’s orbit would hardly be affected at all.
As for your future as an interstellar traveler… First of all, you wouldn’t “suddenly” discover a black hole off to your left. We have ways to detect many black holes even from Earth, and if we are someday able to embark on interstellar trips we’ll surely have maps that would alert you to the locations of any black holes along your route. Even in the unlikely event that one wasn’t on your map, the black hole’s gravitational effect on your spacecraft would build gradually as you approached, so there’d still be nothing sudden about it. Second, unless you happened to be aimed almost directly at the black hole, its gravity would simply cause you to swing around it in much the same way that we’ve sent spacecraft (such as the Voyager and New Horizons spacecraft) swinging past Jupiter on trips to the outer solar system.
I realize that this may be very disappointing to some of you. As my middle-school daughter put it, “but it’s cool to think that black holes suck.” I was able to placate her only somewhat by pointing out that being cool and “it sucks” don’t usually go together. Still, you’re probably wondering, if black holes don’t suck, what do they do?
The answer has two parts, one mundane and one so utterly amazing that you’ll never again miss your visions of a cosmic vacuum cleaner. The mundane part applies to black holes observed from afar, because at a distance the gravity of a black hole is no different than the gravity of any other object. That’s why turning the sun into a black hole would not affect Earth’s orbit, and why a spacecraft can swing by a black hole just like it swings by Jupiter. The amazing part comes when you begin to approach a black hole closely. There, you’d begin to observe the dramatic distortions of space and time that we can understand only through Einstein’s theory of relativity.
That brings us to the crux of the matter. I’ve begun this book on relativity by talking about black holes because, although almost everyone has heard of them, you cannot actually understand what black holes are unless you first understand the basic ideas discovered by Einstein. One goal of this book is to help you gain that understanding. But I have a second, more important goal in mind as well.
In the process of learning about relativity, you’ll find that your everyday notions of time and space do not accurately reflect the reality of the universe. In essence, you’ll realize that you have grown up with a “common sense” that isn’t quite as sensible as it seems. It’s not your fault; rather, it is a result of the fact that we don’t commonly experience the extreme conditions under which the true nature of time and space is most clearly revealed. Therefore, the real goal of this book is to help you distinguish reality from the fiction that we grow up with, and in the process to consider some of the profound implications of this reality that Einstein was the first to understand.
To get started, let’s take an imaginary voyage to a black hole. This journey will give you an opportunity to experience the two conditions under which Einstein’s ideas have their most dramatic effects: at speeds approaching the speed of light and in the extreme gravity that exists near black holes. For now, we’ll focus only on what you actually observe on your trip, saving the why that lies behind your observations for the chapters that follow.
Astronomers are one step closer to solving a longstanding mystery — just what our Milky Way galaxy looks like.
It may seem odd that a comprehensive understanding of the Milky Way’s structure has so far eluded researchers. But it’s tough to get a broad view of the galaxy from within.
“We are fairly confident that the Milky Way is a spiral galaxy, but we don’t know much in detail. At the most basic level, we’d like to be able to make a map that would show in detail what it looks like,” said Mark Reid of the Harvard-Smithsonian Center for Astrophysics, who led the new study. [Stunning Photos of Our Milky Way Galaxy (Gallery)]
Using the Very Long Baseline Array (VLBA), a system of 10 radio telescopes spanning the globe from Hawaii to New England to the Virgin Islands, and operated in Socorro, N.M., Reid’s team studied masers — naturally occurring sources of laser-like radio waves from clouds of gas near luminous stars — to map our galaxy in unprecedented detail.
“Mark Reid’s paper presents the most precise data we have on the dynamics and structure of the Milky Way galaxy,” said Harvard theorist Avi Loeb, who did not take part in the study.
Previous studies of the Milky Way’s structure were limited to nearby stars or relied on inferring distances from measurements of the speed of gas clouds approaching or receding from us. But these techniques are not reliable enough to discern the finer points of the Milky Way’s structure. So Reid’s team decided to go one step further.
The researchers first tried to get precise values of the Milky Way’s most fundamental parameters — the distance to the galactic center and the speed with which our sun rotates around it. These parameters directly relate to the size and total mass of the Milky Way.
To do so, they measured parallax — an effect that reflects the apparent position of an object when viewed from two different vantage points. This is essentially the same technique used for surveying on Earth, only carried to extraordinary accuracy with the VLBA.
“Were the human eye to have this accuracy, one could see individual molecules in one’s hand,” Reid said.
Astronomers measure parallax by observing how stars appear to move back and forth as the Earth orbits the sun. Using this technique, Reid’s team first measured the position of a bright maser spot coming from a dense cloud surrounding a newly formed and massive star.
Six months later, the astronomers measured the position again, when the Earth had moved halfway around the sun.
“This gives us two different vantage points, and the bright spot will appear to have moved by a small angle on the sky between the two observations,” said Reid.
Then they made a third measurement, when the Earth returned to its original position, to account for the motions of the sun and the target object. “Knowing the Earth-sun distance and the change in angle allows us to calculate the distance to the starby simple trigonometry,” Reid said.
The results have been impressive. As Reid and his colleagues describe in the paper, published this month in The Astrophysical Journal, it has been possible to determine the location of bright young stars that trace spiral structures in our galaxy, and even to measure how tightly wound the Milky Way’s spiral arms are.
“A typical spiral arm starts near the center of the Milky Way and wraps around once before fading away for lack of material to form stars,” Reid said.
But Loeb said that the most important results of the recent study were the much more accurate estimates of the distance to the galactic center and the circular rotation speed at the sun’s location.
“These values are of fundamental importance to many other studies of the Milky Way,” Loeb said.
Together with Gaia
Since the VLBA is in the Northern Hemisphere, it can only “see” about half of the Milky Way. So the next step is to take the same measurements in the Southern Hemisphere.
Once that’s done, Reid is confident that it should be possible to trace the Milky Way’s arms from their origin in the inner regions of the galaxy around to the outer parts.
His team’s ground-based observations will soon be greatly extended by the European Space Agency’s Gaia spacecraft, which launched in December. Gaia aims to measure the distances to one billion stars by about 2020. [Photos: Gaia Spacecraft to Map Milky Way Galaxy]
“Gaia is an optical telescope and cannot peer through the dusty plane of the Milky Way, where spiral structures dominate, whereas the VLBA uses radio waves that are unaffected by dust,” Reid said, “so the two approaches are quite complementary.”
Instead of measuring parallax distances and mapping the Milky Way, an alternative would be to design a space probe that could move at nearly the speed of light, Reid said.
“In about 10,000 years it would get out of the Milky Way and could take a picture and send it back to us and we would know what the Milky Way looks like,” he said. “Of course it would take another 10,000 years to transmit the image back to us. I’d like to know the answer sooner.”
You can read the paper at the online preprint site ArXiv here: http://xxx.lanl.gov/abs/1401.5377
The Atacama Large Millimeter/submillimeter Array (ALMA) is a telescope array in Chile that includes 66 receivers scattered across the Atacama Desert. The array is located at the European Southern Observatory and bills itself as “the largest astronomical project in existence.”
ALMA is able to peer through the dust that obscures planetary systems under construction, or to look back at stars and galaxies that formed in the early days of the universe and emit their radiation in millimeter light — waves that are about 1,000 times longer than visible-light wavelengths.
Although there are other observatories that can do the same thing, what distinguishes ALMA is its sheer size and number of receivers. Working together, they provide more sensitivity in astronomical observations and allow astronomers to look at the universe in high definition.
ALMA can be viewed as an amalgamation of three separate projects under conception: the Millimeter Array (MMA) of the United States, the Large Southern Array (LSA) of Europe and the Large Millimeter Array (LMA) of Japan. Through conversations between researchers, scientists concluded it would be easier to collaborate on one large project rather than creating several smaller ones.
The first large milestone occurred in 1997, when the European Southern Observatory (ESO) and the National Radio Astronomy Observatory (NRAO) agreed to merge MMA and LSA, a move they formalized in 1999. The ALMA agreement between the entities was subsequently signed in February 2003, with Japan joining in 2004.
ESO — which already has facilities in Chile — negotiated an agreement with the country to build ALMA at Llano de Chajnantor, a high-altitude site that would make it easier to observe the cosmos because the atmosphere is thinner there. In exchange, Chile received 10 percent observing time and additional “cultural, educational and production activities,” according to ALMA. The $1.3 billion cost was borne principally by North America and Europe, with Japan close behind.
Contracts for the antennas were awarded in 2005, with the first antenna coming to Chile in 2007. As antennas arrived and were checked for health and interferometry capabilities, ESO put out a call for the first science observations. Antennas began moving to Chajnantor in 2009, and observations began with a partially completed ALMA in 2011. The 66th and final receiver was installed in 2013.
ALMA’s extreme altitude is an aid in performing observations. Its highest receivers are about 16,500 feet (5,000 meters) above sea level, far above much of the atmosphere and water vapor that can make it hard to see what’s in the sky. Astronomers work in a facility at 9,500 feet (2,900 m), where they receive supplemental oxygen if they’re going to stay awhile.
The 66 receivers can be arranged in many different configurations, ranging from very close together to quite spread apart. At its greatest, the receivers can be moved as far as 9.9 miles (16 kilometers) apart. Each telescope receives information individually, then transmits the data to a supercomputer that combines the information to trace the signal direction — a high-tech version of how human ears combine to locate a sound.
This technology allows astronomers to look at three principal questions, according to ALMA’s website: the nature of the universe’s first stars and galaxies, how planets and stars come together, and what the chemistry is of the gas and dust clouds that may eventually collapse to form planets and stars.
“Many other astronomical specialties also will benefit from the new capabilities of ALMA,” the website added, such as the ability to “map gas and dust in the Milky Way and other galaxies, investigate ordinary stars, analyze gas from an erupting volcano on Jupiter’s moon, Io [and] study the origin of the solar wind.”
The first image from ALMA was a combined view of the Antennae Galaxies, which are about 75 million light-years from Earth. Another early image pierced dust surrounding the Centaurus A galaxy to show its bright center. The telescope is also capable of producing three-dimensional visualizations of gas, such as the image above of NGC 253 (the Sculptor Galaxy) released in 2013.
One of ALMA’s most prominent finds was announced in 2014 from examining a famed supernova remnant — the leftovers of supernova 1987A — and uncovering dust spewing in the area.
“We have found a remarkably large dust mass concentrated in the central part of the ejecta from a relatively young and nearby supernova,” astronomer Remy Indebetouw, of NRAO and the University of Virginia, said in a statement. “This is the first time we’ve been able to really image where the dust has formed, which is important in understanding the evolution of galaxies.”
A baby star was captured on camera in 2013 showing the youngster blasting out starstuff at 84,477 mph (144,000 kph). Once the material crashed into the surrounding gas, it produced a glow. This could provide some clues as to how the sun came together, astronomers said at the time. “The sun is a star, so if we want to understand how our solar system was created, we need to understand how stars are formed,” Héctor Arce, the lead author of the Astrophysical Journal study, said in a statement.
The formation of stars determines where new planetary systems can arise as well as the structure and evolution of galaxies. Learning more about star formation could help scientists understand the remaining mysteries about the birth of galaxies, including how the supermassive black holes at their hearts apparently originated so early in the universe’s history.
A dearth of data on the way matter is distributed within star-forming clouds has limited the accuracy of star formation models. Now scientists have developed a way to determine these details by using so-called dust-extinction maps, or observations of how dust scatters and absorbs light.
The birth of stars
Stars are born when pockets of gas and dust within interstellar molecular clouds exceed critical density and collapse under their own gravity. Once the pressure and the temperature inside get high enough for nuclear fusion to ignite, it creates a star. The rate at which stars form depends mainly on the number and density of these clumps within stellar nurseries.
Scientists have now used one feature of these clouds to better understand star formation. As the light of distant stars penetrates through a stellar nursery, the molecular cloud’s dust dims the light. By measuring how this process dims thousands of different stars, scientists can reconstruct the cloud’s 3D structure, helping pinpoint how matter is distributed within the cloud.
Using data from 16 nearby molecular clouds, each within about 850 light-years of Earth, “we could devise a very concrete recipe for how new stars are born in the interstellar gas clouds,” study lead author Jouni Kainulainen, an astrophysicist at the Max Planck Institute for Astronomy in Heidelberg, Germany, told Space.com. “And the ingredients of this recipe are simple to understand — only those regions in the clouds in which density is higher than about 5,000 molecules per cubic centimeter produce stars, and about a tenth of the gas in these regions collapses into the new stars.”
A clue for cosmic clouds
One key consequence of these findings “is that we have given astronomers a new tool to estimate the rates at which molecular clouds form stars, without even seeing the new stars in the clouds,” Kainulainen said. “I quote my collaborator Thomas Henning — ‘Show us your image of a cloud, and we can tell you how much it forms stars right now.’”
Such a capability “gives us a great possibility to use new, powerful telescopes such as the Atacama Large Millimeter/submillimeter Array (ALMA) to observe gas clouds in the galaxy and better map its star-formation capacity,” Kainulainen said. ALMA consists of 66 high-precision microwave antennas spread over distances of up to 10 miles (16 kilometers) in the Chilean desert, which can act like a single, high-resolution telescope. The array has just commenced operations.
Intriguingly, the researchers found it was simpler for stars to form in these clouds than once predicted by theory. But the findings have yet to address the question of how galaxies and supermassive black holes formed in the early universe, Kainulainen said. “I think it is currently too early to speculate about the possible effects of our findings to the early universe,” he said.
“We have to remember that physical conditions during the early galaxy formation were quite different from those of the galaxies today,” Kainulainen said. “It would be an unreasonable leap in conclusions to assume that the process of star formation would proceed exactly in the same way as in the current, local universe. Our results are best applicable to spiral galaxies such as the Milky Way in the local universe.”
Future studies should address the assumptions made in creating the team’s 3D models for the molecular clouds, Kainulainen said. “It is a good topic for future discussions — how this 3D modeling could potentially be improved,” he said. “I hope we will have comments and ideas from other scientists regarding this.”
Future research could determine what processes are most responsible for shaping molecular clouds.
“We know today very well that processes such as gravitational forces and magneto-hydrodynamic turbulence have a key role in shaping the clouds,” Kainulainen said. “With the parameters we have derived, we can answer questions such as, ‘In which regions is the cloud structure most crucially affected by turbulence, and in which by gravity?’ Such questions are of great importance in understanding the origin of interstellar gas clouds in galaxies and their evolution.”
Galaxies — those vast collections of stars that populate our universe — are all over the place. Perhaps the most resonant example of this fact is the Hubble eXtreme Deep Field, a collection of photographs from the Hubble Space Telescope revealing thousands of galaxies in a single composite picture.
Estimating how many galaxies are throughout the universe is a tougher job, however. Sheer numbers is one problem — once the count gets into the billions, it takes a while to do the addition. Another problem is the limitation of our instruments. To get the best view, a telescope needs to have a large aperture (the diameter of the main mirror or lens) and be located above the atmosphere to avoid distortion from Earth’s air.
While estimates among different experts vary, an acceptable range is between 100 billion and 200 billion galaxies, Mario Livio, an astrophysicist at the Space Telescope Science Institute in Baltimore, told Space.com.
To the best of Livio’s knowledge, Hubble is the best instrument available for galaxy counting and estimation. The telescope, launched in 1990, initially had a distortion on its main mirror that was corrected during a shuttle visit in 1993. Hubble also went underwent several upgrades and service visits until the final shuttle mission there in May 2009.
In 1995, astronomers pointed the telescope at what appeared to be an empty region of Ursa Major, and collected 10 days’ worth of observations. The result was an estimated 3,000 faint galaxies in a single frame, going as dim as 30th magnitude. (For comparison, the North Star or Polaris is at about 2nd magnitude.) This image composite was called the Hubble Deep Field and was the furthest anyone had seen into the universe at the time. [Related: Brightest Stars: Luminosity & Magnitude]
As the Hubble telescope received upgrades to its instruments, astronomers repeated the experiment twice. In 2003 and 2004, scientists created the Hubble Ultra Deep Field, which in a million-second exposure revealed about 10,000 galaxies in a small spot in the constellation Fornax.
In 2012, again using upgraded instruments, scientists used the telescope to look at a portion of the Ultra Deep Field. Even in this narrower field of view, astronomers were able to detect about 5,500 galaxies. Researchers dubbed this the eXtreme Deep Field.
All in all, Hubble reveals an estimated 100 billion galaxies in the universe or so, but this number is likely to increase to about 200 billion as telescope technology in space improves, Livio said.
Whatever instrument is used, the method of estimating the number of galaxies is the same. You take the portion of sky imaged by the telescope (in this case, Hubble). Then — using the ratio of the sliver of sky to the entire universe — you can determine the number of galaxies in the universe.
“This is assuming that there is no large cosmic variance, that the universe is homogenous,” Livio said. “We have good reasons to suspect that is the case. That is the cosmological principle.”
The principle dates back to Albert Einstein’s theory of general relativity at the turn of the last century. One of general relativity’s findings is that gravity is a distortion of space and time. With that understanding in hand, several scientists (including Einstein) tried to understand how gravity affected the entire universe.
“The simplest assumption to make is that if you viewed the contents of the universe with sufficiently poor vision, it would appear roughly the same everywhere and in every direction,” NASA stated. “That is, the matter in the universe is homogeneous and isotropic when averaged over very large scales. This is called the cosmological principle.”
One example of the cosmological principle at work is the cosmic microwave background, radiation that is a remnant of the early stages of the universe after the Big Bang. Using instruments such as NASA’s Wilkinson Microwave Anisotropy Probe, astronomers have found the CMB is virtually identical wherever one looks.
Would the number of galaxies change with time?
Measurements of the universe’s expansion — through watching galaxies race away from us — show that it is about 13.82 billion years old. As the universe gets older and bigger, however, galaxies will recede farther and farther from Earth. This will make them more difficult to see in telescopes.
The universe is expanding faster than the speed of light (which does not violate Einstein’s speed limit because the expansion is of the universe itself, rather than of objects traveling through the universe). Also, the universe is accelerating in its expansion.
This is where the concept of the “observable universe” — the universe that we can see — comes into play. In 1 trillion to 2 trillion years, Livio said, this means that there will be galaxies that are beyond what we can see from Earth.
“We can only see light from galaxies whose light had enough time to reach us,” Livio said. “It doesn’t mean that that’s all there is in the universe. Hence, the definition of the observable universe.”
Galaxies also change over time. The Milky Way is on a collision course with the nearby Andromeda Galaxy, and both will merge in about 4 billion years. Later on, other galaxies in our Local Group — the galaxies closest to us — will eventually combine. Residents of that future galaxy would have a much darker universe to observe, Livio said.
“Civilizations started then, they would have no evidence that there was a universe with 100 billion galaxies,” he said. “They would not see the expansion. They would probably not be able to tell there was a Big Bang.”
What about other universes?
As the early universe inflated, there are some theories that say that different “pockets” broke away and formed different universes. These different places could be expanding at different rates, include other types of matter, and have different physical laws than our own universe.
Livio pointed out there could be galaxies in these other universes — if they exist — but we have no way right now of knowing for sure. So the number of galaxies could even be greater than 200 billion, when considering other universes.
In our own cosmos, Livio said, astronomers will be better able to refine the number upon the launch of the James Webb Space Telescope (for which his institute will manage the mission operations and science). Hubble is able to peer back at galaxies that formed about 450 million years after the Big Bang. After James Webb launches in 2018, astronomers anticipate they can look as far back as 200 million years after the Big Bang.
“The numbers are not going to change much,” Livio added, pointing out the first galaxies probably formed not too long before that. “So a number like 200 billion [galaxies] is probably it for our observable universe.”
The central black holes in dwarf galaxies — the “seeds” that grow into the monsters at the core of the Milky Way and other large galaxies — are probably surprisingly weighty, containing 1,000 to 10,000 times the mass of our sun, a new study reports.
The finding goes against one popular theory of supermassive black hole evolution, suggesting that galaxy mergers aren’t necessary to create these behemoths, which can harbor billions of times more mass than the sun.
“We still don’t know how the monstrous black holes that reside in galaxy centers formed,” lead author Shobita Satyapal, of George Mason University in Virginia, said in a statement. “But finding big black holes in tiny galaxies shows us that big black holes must somehow have been created in the early universe, before galaxies collided with other galaxies.”
It’s also possible that supermassive black holes grow primarily by gobbling up gas and dust, getting bigger relatively sedately along with their host galaxies, researchers said.
Satyapal and her colleagues analyzed observations of dwarf galaxies made by NASA’s Wide-field Infrared Survey Explorer spacecraft, or WISE.
Dwarf galaxies have changed relatively little over time, and they resemble the types of galaxies that existed when the universe was young. So they’re a good place to look for nascent supermassive black holes, researchers said.
WISE’s all-sky survey picked out hundreds of dwarf galaxies, which appear to sport strikingly large black holes.
“Our findings suggest the original seeds of supermassive black holes are quite massive themselves,” Satyapal said.
While the results are intriguing, follow-up study will be necessary to fully flesh them out, outside researchers said.
“Though it will take more research to confirm whether the dwarf galaxies are indeed dominated by actively feeding black holes, this is exactly what WISE was designed to do: find interesting objects that stand out from the pack,” astronomer Daniel Stern, of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., said in a statement. Stern was not part of the study team.
WISE launched to Earth orbit in December 2009 on a 10-month mission to scan the entire sky in infrared light. It was shut down in February 2011, then reactivated in September 2013 with a new mission and a new name. Now called NEOWISE, the spacecraft is hunting for potentially dangerous asteroids, some of which could be promising targets for human exploration.
The new study was published in the March issue of The Astrophysical Journal.
The first direct evidence of cosmic inflation — a period of rapid expansion that occurred a fraction of a second after the Big Bang — also supports the idea that our universe is just one of many out there, some researchers say.
On Monday (March 17), scientists announced new findings that mark the first-ever direct evidence of primordial gravitational waves — ripples in space-time created just after the universe began. If the results are confirmed, they would provide smoking-gun evidence that space-time expanded at many times the speed of light just after the Big Bang 13.8 billion years ago.
The new research also lends credence to the idea of a multiverse. This theory posits that, when the universe grew exponentially in the first tiny fraction of a second after the Big Bang, some parts of space-time expanded more quickly than others. This could have created “bubbles” of space-time that then developed into other universes. The known universe has its own laws of physics, while other universes could have different laws, according to the multiverse concept.
“It’s hard to build models of inflation that don’t lead to a multiverse,” Alan Guth, an MIT theoretical physicist unaffiliated with the new study, said during a news conference Monday. “It’s not impossible, so I think there’s still certainly research that needs to be done. But most models of inflation do lead to a multiverse, and evidence for inflation will be pushing us in the direction of taking [the idea of a] multiverse seriously.”
Other researchers agreed on the link between inflation and the multiverse.
“In most of the models of inflation, if inflation is there, then the multiverse is there,” Stanford University theoretical physicist Andrei Linde, who wasn’t involved in the new study, said at the same news conference. “It’s possible to invent models of inflation that do not allow [a] multiverse, but it’s difficult. Every experiment that brings better credence to inflationary theory brings us much closer to hints that the multiverse is real.”
When Guth and his colleagues thought up cosmic inflation more than 30 years ago, scientists thought it was untestable. Today, however, researchers are able to study light left over from the Big Bang called cosmic microwave background radiation (CMB).
In the new study, a team led by John Kovac of the Harvard-Smithsonian Center for Astrophysics found telltale signs of inflation in the microwave background. The researchers discovered a distinct curl in the polarization pattern of the CMB, a sign of gravitational waves created by the rapid expansion of space-time just after the Big Bang.
Linde, one of the main contributers to inflation theory, says that if the known universe is just one bubble, there must be many other bubbles in the cosmic fabric.
“Think about some unstable state,” Linde explained. “You are standing on a hill, and you can fall in this direction, you can fall in that direction, and if you’re drunk, eventually you must fall. Inflation is instability of our space with respect to its expansion.
“You have something growing exponentially,” he added. “If you just let it go … it will continue exponentially growing, so this [the known universe] is one possibility of something going wrong with this instability, which is very, very right for us because it has created all of our space. Now, we know that if anything can go wrong, it will go wrong once and a second time and a third time and into infinity as long as it can go.”
Hundreds of wandering “rogue” black holes may dwell in the Milky Way — and now researchers say they know how to detect them. Discovering these strange objects could shed light on the formation of the Milky Way and other galaxies.
No one knows exactly how the Milky Way came to exist. But according to one popular model of galaxy formation, the building blocks of the Milky Way were dwarf galaxies that collided and merged shortly after the Big Bang.
This idea assumes that floating black holes, each containing 1,000 to 100,000 more mass than the sun, could be left over from those early cosmic times — fossil evidence for the growth and mergers of black holes in the infant universe. [The Strangest Black Holes in the Universe]
Each of the Milky Way’s building-block galaxies had its own central black hole. During mergers between dwarf galaxies, these black holes also came together. In the process, the new single black hole received a rocket-like kick from the emission of excess gravitational waves in the opposite direction, said astrophysicist Avi Loeb of Harvard University, who wrote the paper together with his graduate student Xiawei Wang.
In most cases, this kick would make the black hole speed up enough to move it away from its newly enlarged dwarf galaxy — but not far enough to leave the region that eventually would become the Milky Way. (A new central black hole could then form in the dwarf galaxy via gas accretion.)
Once the host galaxy became massive enough, the black holes near it would have been unable to escape. One of them grew and became the supermassive black hole that is believed to exist at the center of the Milky Way, weighing four million suns. But there should be hundreds of rogue black holes floating in the distant “halo” of the Milky Way, left over from the pre-Milky Way time when only dwarf galaxies existed, Loeb said.
“The Milky Way halo serves as a kind of a ‘reservoir’ of wandering black holes that originally lived in the cores of the small galaxies that merged to make it,” he said.
Black Hole Quiz: How Well Do You Know Nature’s Weir…
Bow shock detection
But how does one detect them, if it is impossible to observe black holes directly, and they are”rogues” floating somewhere in space? Loeb and Wang say they have founda way.
“When such black holes pass through the gas disk of the Milky Way galaxy, they produce a bow shock — similar to the sonic boom produced intheair by supersonic jets,” said Loeb. “The shock accelerates electrons to high energies and these emit radio waves that we can detect.”
“And the radio emission from these bow shocks should be detectable with existing radio observatories,” he added. “Of course, if such a bow shock is discovered, one would be able to also observe the cluster of stars attached to the floating black hole and possibly the X-ray emission from the black hole itself as it accretes gas.”
This method would be”a nice new way to identify the theoretically predicted [wandering black holes],” said astrophysicist Jeremy Darling of the University of Colorado, who did not take part in the study. “The Wang and Loeb paper shows how these black holes can create a bow shock as they pass through the disk of our galaxy, effectively lighting up and making themselves available for observation.”
And he agrees that in principle, these bow shocks should be “easily detectable with current facilities,” using radio and infrared waves.
But it won’t be easy, cautions Darling, as the difficulty is “the ‘needle-in-a-haystack” problem common in astronomy: There are many objects emitting in the radio and infrared range in the disk of our galaxy, and Wang and Loeb predict that only a few black holes (in some scenarios maybe not even a single one) should be in the disk at any given time.
“Moreover, we view our galaxy edge-on, so there is tremendous confusion as objects overlap one another and pile up along the line of sight. Extant radio surveys of the galaxy lack the angular resolution to distinguish the black hole bow shock from other phenomena, which is a pity.”
During earlier research, Loeb and his former student Ryan O’Leary proposed another way to detect these floating black holes. They suggested that such black holes are likely surrounded by a cluster of stars that were originally tightly bound to them.
These clusters would bevery different from globular star clusters, as they would be held together by the gravity of the black hole. As a result, they would be very compact, just a few light-years in size.
Loeb and O’Leary have identified candidate star clusters and are currently collecting spectroscopic data on some,to test if any of them has a central black hole.
“There may be a treasure trove in the backyard of the Milky Way that could inform us about the first generation of black holes in the universe,” Loeb said.
The oceans soured into a deadly sulfuric-acid stew after the huge asteroid impact that wiped out the dinosaurs, a new study suggests.
Eighty percent of the planet’s species died off at the end of the Cretaceous Period 65.5 million years ago, including most marine life in the upper ocean, as well as swimmers and drifters in lakes and rivers. Scientists blame this mass extinction on the asteroid or comet impact that created the Chicxulub crater in the Gulf of Mexico.
A new model of the disaster finds that the impact would have inundated Earth’s atmosphere with sulfur trioxide, from sulfate-rich marine rocks called anhydrite vaporized by the blast. Once in the air, the sulfur would have rapidly transformed into sulfuric acid, generating massive amounts of acid rain within a few days of the impact, according to the study, published today (March 9) in the journal Nature Geoscience.
The model helps explain why most deep-sea marine life survived the mass extinction while surface dwellers disappeared from the fossil record, the researchers said. The intense acid rainfall only spiked the upper surface of the ocean with sulfuric acid, leaving the deeper waters as a refuge. The model could also account for another extinction mystery: the so-called fern spike, revealed by a massive increase in fossil fern pollen just after the impact. Ferns are one of the few plants that tolerate ground saturated in acidic water, the researchers said.
The Chicxulub impact devastated the Earth with more than just acid rain. Other killer effects included tsunamis, a global firestorm and soot from burning plants. [The 10 Best Ways to Destroy Earth]
The ocean-acidification theory has been put forth before, but some scientists questioned whether the impact would have produced enough global acid rain to account for the worldwide extinction of marine life. For example, the ejected sulfur could have been sulfur dioxide, which tends to hang out in the atmosphere instead of forming aerosols that become acid rain.
Lead author Sohsuke Ohno, of the Chiba Institute of Technology in Japan, and his co-authors simulated the Chicxulub impact conditions in a lab, zapping sulfur-rich anhydrite rocks with a laser to mimic the forces of an asteroid colliding with Earth. The resulting vapor was mostly sulfur trioxide, rather than sulfur dioxide, the researchers found. In Earth’s atmosphere, the sulfur trioxide would have quickly combined with water to form sulfuric acid aerosols. These aerosols played a key role in quickly getting sulfur out of the sky and into the ocean, the researchers said. The tiny droplets likely stuck to pulverized silicate rock debris raining down on the planet, thus removing sulfuric acid from the atmosphere in just a matter of days.
“Our experimental results indicate that sulfur trioxide is expected to be the major sulfide component in the sulfur oxide gas released during the impact,” Ohno told Live Science in an email interview. “In addition to that, by the scavenging or sweeping out of acid aerosols by coexisting silicate particles, sulfuric acid would have settled to the ground surface within a very short time,” Ohno said.
For the first time, astronomers have directly measured how fast a black hole spins, clocking its rotation at nearly half the speed of light.
The distant supermassive black hole would ordinarily be too faint to measure, but a rare lineup with a massive elliptical galaxy created a natural telescope known as a gravitational lens that allowed scientists to study the faraway object.
“The gravitational lens is crucial,” study co-author Mark Reynolds of the University of Michigan told Space.com via email..”Without this, we would not be able to collect X-ray photons to measure the spin of a black hole that is so distant.”
Nature’s free telescope
Just more than 6 billion light-years from Earth, a supermassive black hole powers the quasar . Quasars, the most luminous objects in the universe, shine brightly across vast distances, fed by material that falls into their black holes.
Black holes are massive objects whose gravitational pull is so powerful that even light cannot escape their grasp. Most form when a star at the end of its lifetime explodes, its outer core collapsing into a tiny dense ball.
Supermassive black holes have masses millions of times that of the sun and are found at the center of most galaxies, including the Milky Way. Their origins are still unknown.
The only features scientists are able to measure about the voracious objects are their mass and spin. Astronomers can determine the mass of a black hole by measuring its interactions with gas and other objects, but characterizing its rotation has remained a challenge, especially for more distant supermassive black holes.
In the new study, a team led by Rubens Reis of the University of Michigan used NASA’s Chandra X-ray Observatory and the European Space Agency’s XMM-Newton — the largest X-ray space telescopes currently available — to observe the X-rays generated in the innermost regions of the disk of material circling and feeding the supermassive black hole that powers the quasar J1131.
Measuring the radius of the disk allowed the astronomers to calculate the black hole’s spin speed, which was almost half the speed of light.
The team would have been unable to measure the spin without a rare lineup in space. A giant elliptical galaxy lies between Earth and the quasar J1131. The huge galaxy acts as a gravitational lens to bend and magnify objects that lie behind it — in this case, the supermassive black hole.
“It acts like a telescope, but a free one provided by nature,” Reynolds said.
“Such a quadruple lens of a quasar is a very rare object,” Guido Risaliti, of the Harvard-Smithsonian Center for Astrophysics, told Space.com in an email. “Until a few years ago, none of them was known.”
Risaliti, who was not involved in the research, also studies supermassive black holes. Last year, he made the first reliable measurement of the spin of a nearby supermassive black hole. He authored a News & Views article that appeared along with the research in the journal Nature today (March 5). [No Escape: Dive Into a Black Hole (Infographic)]
The spin of a supermassive black hole can reveal information about how it accretes the material it consumes. To achieve a rapid spin, material must fall into the black hole in a direction similar to its rotation, ultimately revving it up like a child spinning a merry-go-round.
A slower spin indicates that the gas and dust supplying the black hole fall into it from multiple directions, spinning the black hole up or down depending on whether it comes in with or against the rotation. In this case, the random influx of material acts like a child alternating pushing and pulling the merry-go-round.
The quick spin of J1131 indicates that the black hole is being fed by a bountiful supply of gas and dust. These large volumes could be provided by collisions and mergers between galaxies, among other sources, Reynolds said.
A slower spin and more haphazard feeding process would be caused by material arriving in spurts, from interstellar gas clouds and stars wandering too close from a variety of directions.
“Observational studies over the past 20 years have shown a clear link between the mass of the supermassive black hole at the center of a galaxy and the properties of the galaxy in which it resides,” Reynolds said. “These relations suggest a symbiotic relationship between the central black hole and its host galaxy.”
By studying the black hole, astronomers can learn more about the origin and evolution of galaxies — and spin plays a very important role.
“The growth history of a supermassive black hole is encoded in its spin,” Reynolds said.
High spin values throughout most black holes would suggest that galaxy mergers have played a significant role in galactic evolution throughout the life of the universe. Determining how common rapid spin rates are will require the study of multiple distant supermassive black holes that lie in the active galactic nuclei (AGN) of nearby galaxies.
“The next immediate step is to obtain a few more black hole spins in the nearby AGN, but it will be difficult to repeat observations like the one of Reis’ team due to the rarity of these sources,” Risaliti said. “The big step forward will be the measurements of the black hole spins with the next generation of high sensitivity X-ray telescopes, such as the ESA’s Athena.”
In-pouring rivers of hydrogen gas could explain how spiral galaxies maintain the constant star formation that dominates their hearts, a new study reports.
Using the Green Bank Telescope (GBT) in West Virginia, scientists observed a tenuous filament of gas streaming into the galaxy NGC 6946, known as the “Fireworks Galaxy” because of the large number of supernovae observed within it. The find may provide insight into the source of fuel that powers the ongoing birth of young stars, researchers said.
“We knew that the fuel for star formation had to come from somewhere,” study lead author D.J. Pisano, of West Virginia University, said in a statement. “So far, however, we’ve detected only about 10 percent of what would be necessary to explain what we observe in many galaxies.”
Located 22 million light-years from Earth on the border of the constellations Cepheus and Cygnus, NGC 6946 is a medium-sized spiral galaxy pointed face-on toward the Milky Way.
Previous studies revealed a halo of hydrogen gas around NGC 6946 common to spiral galaxies. Such halos are formed by hydrogen ejected from the galaxies by star formation and violent supernova explosions. These interactions heat the gas in the halo to extreme temperatures.
When Pisano turned the GBT toward the spiral galaxy for further examination, however, he discovered a ribbon of gastoo cool to have suffered the heating processes undergone by halo gas.
On average, the Milky Way churns out between 1 to 5 new stars per year. Rich in gas, NGC 6946 is far more active. For example, it has hosted at least 9 explosive supernovae in the past century.
“Starburst” galaxies are even more prolific. These productive galaxiesshould have burned through the gas they were born with over the course of their lifetimes, bringing star formation to a sudden halt. Instead, the process continues today, suggesting that something is continuing to supply them with sufficient gas to keep creating more stars.
“A leading theory is that rivers of hydrogen — known as cold flows — may be ferrying hydrogen through intergalactic space, clandestinely fueling star formation,” Pisano said. “But this tenuous hydrogen has simply been too diffuse to detect, until now.”
The immense, unblocked dish of the Green Bank Telescope, combined with its location in the National Radio Quiet Zone, where radio transmissions are limited, allow the large disk to detect the faint hydrogen signal that would be present in a cold flow.
Another possibility is that the hydrogen detected originated from a close encounter with another galaxy in the past. The gravitational interaction between the two could have stretched out a ribbon of neutral atomic hydrogen, researchers said. Such a ribbon would contain stars that astronomers should be able to easily observe, though none have yet been spotted. Further studies of the streamer hydrogen gas will help clarify its role.
After decades of wondering why young massive stars don’t blow away the gas surrounding them, astronomers have finally found a process that explains how these stellar youngsters hang on to their gassy envelopes.
This star type — more than 10 times the mass of the sun and most active in ultraviolet light — begins shining as a gigantic gas cloud collapses, fusing hydrogen into helium and igniting the star’s nuclear engine. The new research shows that this gas accretion continues even as the star shines, counteracting the stellar radiation that “pushes” against the gas.
A new model reveals that the gas falls unevenly onto the star and also clumps into spiral “filamentary concentrations” because there is so much gas in a small area. When the star moves through the spirals, these filaments absorb the ultraviolet radiation the star emits, protecting the surrounding gas. Once the absorption stops, the gas nebulas shrink. [Top 10 Star Mysteries]
“These transitions from rarefied to dense gas and back again occur quickly compared to most astronomical events,” Mac Low, a curator in the American Museum of Natural History’s Department of Astrophysics and co-author of the paper, said in a statement. “We predicted that measurable changes could occur over times as short as a few decades.”
Massive stars only influential not only when they are alive but also when they die. When a star of this size finishes burning the elements inside it, this triggers a massive collapse and explosion known as a supernova. These explosions created all elements in the universe that are heavier than iron, making Earth and other rocky planets possible.
Young massive stars have been closely studied for decades. Nobody could figure out why the gas around them didn’t blow away, however, as simpler models used previously implied that the gas would expand and dissipate.
The new models, based on observations from the Karl G. Jansky Very Large Array (VLA) in New Mexico, suggest that there are many small ionized hydrogen regions around these stars. The accretion process on the star kept going even after the hydrogen hotspots had formed, which was the opposite of what astronomers expected. Using models, astronomers then supposed that the gas falls unevenly on the star, creating the filaments.
Researchers came to this conclusion after using VLA observations of Sagittarius B2, a huge gas and dust cloud almost 400 light-years away from the center of the Milky Way galaxy. Between observations made in 1989 and 2012, researchers spotted four ionized hydrogen or HII regions getting brighter.
“The long-term trend is still the same, that HII regions expand with time,” said study leader Christopher De Pree, an astronomer at Agnes Scott College. “But in detail, they get brighter or get fainter and then recover. Careful measurements over time can observe this more detailed process.”
The research was recently published in Astrophysical Journal Letters and is also available in preprint form on Arxiv.