Astronomers pin down origins of 'mile markers' for expansion and acceleration of universe

Nov. 19, 2012 — A study using a unique new instrument on the world's largest optical telescope has revealed the likely origins of especially bright supernovae that astronomers use as easy-to-spot "mile markers" to measure the expansion and acceleration of the universe.

In a paper to appear in the Astrophysical Journal, researchers describe observations of recent supernova 2011fe that they captured with the Large Binocular Telescope (LBT) using a tool created at Ohio State University: the Multi-Object Double Spectrograph (MODS).

MODS measures the frequencies and intensities of light shining from a star. Stars shine at different frequencies depending on the chemical elements they are made of; a star like the sun, which is made mostly of hydrogen, shines at different frequencies than a star that is made mostly of helium. So astronomers can use spectra to determine what a particular star is made of.

Based on the frequencies of light emanating from supernova 2011fe, this type of supernova -- known as Type Ia -- is most likely caused by the interaction between a pair of dead stars known as white dwarfs, the astronomers concluded. One white dwarf orbits the other and sheds material onto it, until the other white dwarf becomes unstable and explodes, shining billions of times brighter than the sun.

Astronomers worldwide have tried to confirm the origin of Type Ia supernovae for decades. Groups have proposed several different hypotheses, including exotic scenarios involving white dwarfs paired with still-"living" giant stars, or even stars like the sun.

Kris Stanek, professor of astronomy at Ohio State and a co-author of the study, explained why settling this issue is important.

"We really want to know more about these supernovae, given their importance in our understanding of how the universe is expanding," he said. "Many observations have been done over the years, and I think many astronomers are starting to accept one explanation -- that two white dwarfs are probably responsible."

Still, the alternative theories keep re-emerging, he said: "like zombies that won't die."

"With this study, we were looking for a zombie 'kill shot,' and we think we found it."

Rick Pogge, professor of astronomy and lead designer of MODS, said that the spectrograph is the ideal tool for settling the debate.

"MODS is one of the most sensitive optical spectrometers in operation today, and being used on what is currently the world's largest optical telescope. If we couldn't kill this debate with MODS and the LBT, something would be dreadfully wrong," he added.

Type Ia supernovae make good mile markers for the universe because their extreme brightness -- 5 billion times brighter than the sun -- makes them easy to see, and their distinctive pattern of brightening and dimming in the weeks after they appear makes them easily identifiable.

Astronomers use that information to calculate the distance from Earth to the supernova, and in turn, calculate how fast the universe is expanding. Knowing more about the composition of the stars that create the supernovae could open up new ideas in the understanding of that expansion.

Here's what nearly all astronomers agree on: Type Ia supernovae originate in binary systems, where one star or star-like object is orbiting another. The main object -- the one that initiates the explosion -- is a white dwarf, the massive remnants of a dead star. Over time, the white dwarf's gravity peels off gas and dust from the companion and absorbs that material. Eventually, the white dwarf becomes unstable, and explodes in a supernova.

At issue, explained lead study author and doctoral student Ben Shappee, is the identity of the white dwarf's companion -- is it another white dwarf, or a giant star, or even a star like our sun?

The Ohio State astronomers found their answer in the light spectrum emanating from the supernova. If the companion were a star like ours, or even a giant star, a sizeable portion of the debris blown away from the supernova would contain atoms of the element hydrogen.

Supernova 2011fe provided a good chance for the researchers to test for the presence of hydrogen. Located in the Pinwheel Galaxy some 21 million light-years away, it was the closest near-Earth Type Ia supernova to occur in the last 20 years.

"If the companion were a star such as ours or even a red giant, we would expect to see a lot of hydrogen in the signal -- maybe even half a solar mass' worth, as the companion was blown away. But instead, we saw at most only one tenth of one percent of a solar mass' worth of hydrogen. That suggests that the white dwarf's companion had very little if any hydrogen in it, and is likely another white dwarf," Shappee said.

Pogge called the study "a beautiful demonstration of the kind of data we are able to get on a routine basis with the LBT and MODS. Our entire instrument team is very proud of how well MODS is working."

In fact, this study was done with only one half of the MODS system -- MODS1 -- which is currently installed on one mirror of the LBT. It's twin, MODS2, is currently under construction in Columbus and scheduled to be installed on the second mirror in early 2013.

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The above story is reprinted from materials provided by Ohio State University. The original article was written by Pam Frost Gorder.

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Journal Reference:

Benjamin J. Shappee, K. Z. Stanek, R. W. Pogge, P. M. Garnavich. No Stripped Hydrogen in the Nebular Spectra of Nearby Type Ia Supernova 2011fe. Astrophysical Journal, 2012 [link]

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Dark energy is real, say astronomers

Sep. 11, 2012 — Dark energy, a mysterious substance thought to be speeding up the expansion of the Universe is really there, according to a team of astronomers at the University of Portsmouth and LMU University Munich.

After a two-year study led by Tommaso Giannantonio and Robert Crittenden, scientists conclude that the likelihood of its existence stands at 99.996 per cent. Their findings are published in the Monthly Notices of the Royal Astronomical Society.

Professor Bob Nichol, a member of the Portsmouth team, said: "Dark energy is one of the great scientific mysteries of our time, so it isn't surprising that so many researchers question its existence.

"But with our new work we're more confident than ever that this exotic component of the Universe is real -- even if we still have no idea what it consists of."

Over a decade ago, astronomers observing the brightness of distant supernovae realised that the expansion of the Universe appeared to be accelerating. The acceleration is attributed to the repulsive force associated with dark energy now thought to make up 73 per cent of the content of the cosmos. The researchers who made this discovery received the Nobel Prize for Physics in 2011, but the existence of dark energy remains a topic of hot debate.

Many other techniques have been used to confirm the reality of dark energy but they are either indirect probes of the accelerating Universe or susceptible to their own uncertainties. Clear evidence for dark energy comes from the Integrated Sachs Wolfe effect named after Rainer Sachs and Arthur Wolfe.

The Cosmic Microwave Background, the radiation of the residual heat of the Big Bang, is seen all over the sky. In 1967 Sachs and Wolfe proposed that light from this radiation would become slightly bluer as it passed through the gravitational fields of lumps of matter, an effect known as gravitational redshift.

In 1996, Robert Crittenden and Neil Turok, now at the Perimeter Institute in Canada, took this idea to the next level, suggesting that astronomers could look for these small changes in the energy of the light, or photons, by comparing the temperature of the radiation with maps of galaxies in the local Universe.

In the absence of dark energy, or a large curvature in the Universe, there would be no correspondence between these two maps (the distant cosmic microwave background and relatively closer distribution of galaxies), but the existence of dark energy would lead to the strange, counter-intuitive effect where the cosmic microwave background photons would gain energy as they travelled through large lumps of mass.

The Integrated Sachs Wolfe effect was first detected in 2003 and was immediately seen as corroborative evidence for dark energy, featuring in the 'Discovery of the year' in Science magazine. But the signal is weak as the expected correlation between maps is small and so some scientists suggested it was caused by other sources such as the dust in our galaxy. Since the first Integrated Sachs Wolfe papers, several astronomers have questioned the original detections of the effect and thus called some of the strongest evidence yet for dark energy into question.

In the new paper, the product of nearly two years of work, the team have re-examined all the arguments against the Integrated Sachs Wolfe detection as well as improving the maps used in the original work. In their painstaking analysis, they conclude that there is a 99.996 per cent chance that dark energy is responsible for the hotter parts of the cosmic microwave background maps (or the same level of significance as the recent discovery of the Higgs boson).

"This work also tells us about possible modifications to Einstein's theory of General Relativity," notes Tommaso Giannantonio, lead author of the present study.

"The next generation of cosmic microwave background and galaxy surveys should provide the definitive measurement, either confirming general relativity, including dark energy, or even more intriguingly, demanding a completely new understanding of how gravity works."

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The above story is reprinted from materials provided by Royal Astronomical Society (RAS).

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Journal Reference:

T. Ginnantonio, R. Crittenden, R. Nichol, A. Ross. The significance of the integrated Sachs-Wolfe effect revisited. Monthly Notices of the Royal Astronomical Society, 2012; (in press) [link]

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Even brown dwarfs may grow rocky planets: Sizing up grains of cosmic dust around failed star

Nov. 30, 2012 — Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have for the first time found that the outer region of a dusty disc encircling a brown dwarf contains millimetre-sized solid grains like those found in denser discs around newborn stars. The surprising finding challenges theories of how rocky, Earth-scale planets form, and suggests that rocky planets may be even more common in the Universe than expected.

Rocky planets are thought to form through the random collision and sticking together of what are initially microscopic particles in the disc of material around a star. These tiny grains, known as cosmic dust, are similar to very fine soot or sand. However, in the outer regions around a brown dwarf -- a star-like object, but one too small to shine brightly like a star -- astronomers expected that grains could not grow because the discs were too sparse, and particles would be moving too fast to stick together after colliding. Also, prevailing theories say that any grains that manage to form should move quickly towards the central brown dwarf, disappearing from the outer parts of the disc where they could be detected.

"We were completely surprised to find millimetre-sized grains in this thin little disc," said Luca Ricci of the California Institute of Technology, USA, who led a team of astronomers based in the United States, Europe and Chile. "Solid grains of that size shouldn't be able to form in the cold outer regions of a disc around a brown dwarf, but it appears that they do. We can't be sure if a whole rocky planet could develop there, or already has, but we're seeing the first steps, so we're going to have to change our assumptions about conditions required for solids to grow," he said.

ALMA's increased resolution compared to previous telescopes also allowed the team to pinpoint carbon monoxide gas around the brown dwarf -- the first time that cold molecular gas has been detected in such a disc. This discovery, and that of the millimetre-size grains, suggest that the disc is much more similar to the ones around young stars than previously expected.

Ricci and his colleagues made their finding using the partially completed ALMA telescope in the high-altitude Chilean desert. ALMA is a growing collection of high precision, dish-shaped antennas that work together as one large telescope to observe the Universe with groundbreaking detail and sensitivity. ALMA "sees" the Universe in millimetre-wavelength light, which is invisible to human eyes. Construction of ALMA is scheduled to finish in 2013, but astronomers began observing with a partial array of ALMA dishes in 2011.

The astronomers pointed ALMA at the young brown dwarf ISO-Oph 102, also known as Rho-Oph 102, in the Rho Ophiuchi star-forming region in the constellation of Ophiuchus (The Serpent Bearer). With about 60 times the mass of Jupiter but only 0.06 times that of the Sun, the brown dwarf has too little mass to ignite the thermonuclear reactions by which ordinary stars shine. However, it emits heat released by its slow gravitational contraction and shines with a reddish colour, albeit much less brightly than a star.

ALMA collected light with wavelengths around a millimetre, emitted by disc material warmed by the brown dwarf. The grains in the disc do not emit much radiation at wavelengths longer than their own size, so a characteristic drop-off in the brightness can be measured at longer wavelengths. ALMA is an ideal instrument for measuring this drop-off and thus for sizing up the grains. The astronomers compared the brightness of the disc at wavelengths of 0.89 mm and 3.2 mm. The drop-off in brightness from 0.89 mm to 3.2 mm was not as steep as expected, showing that at least some of the grains are a millimetre or more in size.

"ALMA is a powerful new tool for solving mysteries of planetary system formation," commented Leonardo Testi from ESO, a member of the research team. "Trying this with previous generation telescopes would have needed almost a month of observing -- impossibly long in practice. But, using just a quarter of ALMA's final complement of antennas, we were able to do it in less than one hour!" he said.

In the near future, the completed ALMA telescope will be powerful enough to make detailed images of the discs around Rho-Oph 102 and other objects. Ricci explained, "We will soon be able to not only detect the presence of small particles in discs, but to map how they are spread across the circumstellar disc and how they interact with the gas that we've also detected in the disc. This will help us better understand how planets come to be."

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Journal Reference:

L. Ricci, L. Testi, A. Natta, A. Scholz and I. De Gregorio-Monsalvo. ALMA OBSERVATIONS OF ?-OPH 102: GRAIN GROWTH AND MOLECULAR GAS IN THE DISK AROUND A YOUNG BROWN DWARF. Astrophysical Journal Letters, 2012 (in press)

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When the first stars blinked on: Very first stars may have turned on when the universe was 750 million years old

Dec. 5, 2012 — As far back in time as astronomers have been able to see, the universe has had some trace of heavy elements, such as carbon and oxygen. These elements, originally churned from the explosion of massive stars, formed the building blocks for planetary bodies, and eventually for life on Earth.

Now researchers at MIT, the California Institute of Technology, and the University of California at San Diego have peered far back in time, to the era of the first stars and galaxies, and found matter with no discernible trace of heavy elements. To make this measurement, the team analyzed light from the most distant known quasar, a galactic nucleus more than 13 billion light-years from Earth.

These quasar observations provide a snapshot of our universe during its infancy, a mere 750 million years after the initial explosion that created the universe. Analysis of the quasar's light spectrum provided no evidence of heavy elements in the surrounding gaseous cloud -- a finding that suggests the quasar dates to an era nearing that of the universe's first stars.

"The first stars will form in different spots in the universe … it's not like they flashed on at the same time," says Robert Simcoe, an associate professor of physics at MIT. "But this is the time that it starts getting interesting."

Simcoe and his colleagues have published the results from their study this week in the journal Nature.

Hitting the universal wall

Based on numerous theoretical models, most scientists agree on a general sequence of events during the universe's early development: Nearly 14 billion years ago, an immense explosion, now known as the Big Bang, threw off massive amounts of matter and energy, creating a rapidly expanding universe. In the minutes following the explosion, protons and neutrons collided in nuclear fusion reactions to form hydrogen and helium.

Eventually, the universe cooled to a point where fusion stopped generating these basic elements, leaving hydrogen as the dominant constituent of the universe. Heavier elements, such as carbon and oxygen, would not form until the first stars appeared.

Astronomers have attempted to identify the point at which the first stars were born by analyzing light from more distant bodies. (The farther away an object is in space, the older it is.) Until now, scientists have only been able to observe objects that are less than about 11 billion years old. These objects all exhibit heavy elements, suggesting stars were already plentiful, or at least well established, at that point in the universe's history.

"[The astrophysics community] sort of hit this wall," says Simcoe, an astrophysicist at MIT's Kavli Institute for Astrophysics and Space Research. "When this [quasar] was discovered, we could sort of leapfrog further back in time and make a measurement that was substantially earlier."

Looking for nothing

The quasar in question, discovered in August 2011, is the most distant of its kind. To study such distant objects, Simcoe and his colleagues built an infrared spectrometer, which they fitted onto the Magellan Telescope, a massive ground-based telescope in Chile. This past January, the team trained the telescope on the newly discovered quasar, and collected data from its light.

The spectrometer split the incoming light into different wavelengths, which the team plotted on a graph. Simcoe then looked for telltale dips in the data, correlating various wavelengths with the light given off by different chemicals.

"Each chemical has its own fingerprint," Simcoe says. "Based on the pattern of what light is absorbed, it tells you the chemical composition."

Simcoe and his colleagues determined the quasar's "intrinsic spectrum" -- the amount of light naturally given off by such a body -- and compared this with the observed data to search for the presence of heavy elements. The group found evidence of hydrogen, but no oxygen, silicon, iron or magnesium in the light data. But confirming the absence of evidence for heavy elements was a challenging task.

"It's always hard to establish the absence of something," Simcoe says.

To do so, the researchers considered every other scenario that might explain the light patterns they observed, including newborn galaxies and other matter situated in front of the quasar. Their efforts ultimately confirmed that the quasar's light spectrum indicated an absence of heavy elements 750 million years after the Big Bang.

"[The birth of the first stars] is one of these important moments in the history of the universe," Simcoe says. "It went from looking like the early universe, which was just gas and dark matter, to looking like it does today, where there are stars and galaxies … it's the point when the universe started to resemble what it looks like today. And it's sort of amazing how early that happens. It didn't take long."

John O'Meara, an associate professor of physics at St. Michael's College in Vermont, says while scientists will have to analyze many more distant quasars to confirm the absence of heavy elements, the MIT group's discovery is an "impressive and important step in advancing our knowledge of the universe when it was very young."

"Prior to this result, we have not seen regions of the universe this old and devoid of heavy elements, so there was a missing link in our understanding of how the elemental content of the universe has evolved with time," O'Meara adds. "[This] discovery possibly provides such a rare environment where the universe had yet to form stars."

Going forward, Simcoe hopes to analyze other quasars from this early era to further confirm the absence of heavy elements.

"If we can find things in this epoch, we can start to characterize them," Simcoe says. "There's always something interesting at the edge."

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The above story is reprinted from materials provided by Massachusetts Institute of Technology. The original article was written by Jennifer Chu.

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Journal Reference:

Robert A. Simcoe, Peter W. Sullivan, Kathy L. Cooksey, Melodie M. Kao, Michael S. Matejek, Adam J. Burgasser. Extremely metal-poor gas at a redshift of 7. Nature, 2012; 492 (7427): 79 DOI: 10.1038/nature11612

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Biggest black hole blast discovered: Most powerful quasar outflow ever found

Nov. 26, 2012 — Astronomers using ESO's Very Large Telescope (VLT) have discovered a quasar with the most energetic outflow ever seen, at least five times more powerful than any that have been observed to date. Quasars are extremely bright galactic centres powered by supermassive black holes. Many blast huge amounts of material out into their host galaxies, and these outflows play a key role in the evolution of galaxies. But, until now, observed quasar outflows weren't as powerful as predicted by theorists.

Quasars are the intensely luminous centres of distant galaxies that are powered by huge black holes. This new study has looked at one of these energetic objects -- known as SDSS J1106+1939 -- in great detail, using the X-shooter instrument on ESO's VLT at the Paranal Observatory in Chile [1]. Although black holes are noted for pulling material in, most quasars also accelerate some of the material around them and eject it at high speed.

"We have discovered the most energetic quasar outflow known to date. The rate that energy is carried away by this huge mass of material ejected at high speed from SDSS J1106+1939 is at least equivalent to two million million times the power output of the Sun. This is about 100 times higher than the total power output of the Milky Way galaxy -- it's a real monster of an outflow," says team leader Nahum Arav (Virginia Tech, USA). "This is the first time that a quasar outflow has been measured to have the sort of very high energies that are predicted by theory."

Many theoretical simulations suggest that the impact of these outflows on the galaxies around them may resolve several enigmas in modern cosmology, including how the mass of a galaxy is linked to its central black hole mass, and why there are so few large galaxies in the Universe. However, whether or not quasars were capable of producing outflows powerful enough to produce these phenomena has remained unclear until now [2].

The newly discovered outflow lies about a thousand light-years away from the supermassive black hole at the heart of the quasar SDSS J1106+1939. This outflow is at least five times more powerful than the previous record holder [3]. The team's analysis shows that a mass of approximately 400 times that of the Sun is streaming away from this quasar per year, moving at a speed of 8000 kilometres per second.

"We couldn't have got the high-quality data to make this discovery without the VLT's X-shooter spectrograph," says Benoit Borguet (Virginia Tech, USA), lead author of the new paper. "We were able to explore the region around the quasar in great detail for the first time."

As well as SDSS J1106+1939, the team also observed one other quasar and found that both of these objects have powerful outflows. As these are typical examples of a common, but previously little studied, type of quasars [4], these results should be widely applicable to luminous quasars across the Universe. Borguet and colleagues are currently exploring a dozen more similar quasars to see if this is the case.

"I've been looking for something like this for a decade," says Nahum Arav, "so it's thrilling to finally find one of the monster outflows that have been predicted!"

[1] The team observed SDSS J1106+1939 and J1512+1119 in April 2011 and March 2012 using the X-shooter spectrograph instrument attached to ESO's VLT. By splitting the light up into its component colours and studying in detail the resultant spectrum the astronomers could deduce the velocity and other properties of the material close to the quasar.

[2] The powerful out?ow observed in SDSS J1106+1939 carries enough kinetic energy to play a major role in active galaxy feedback processes, which typically require a mechanical power input of roughly 5% of the luminosity of the quasar. The rate at which kinetic energy is being transferred by the outflow is described as its kinetic luminosity.

[3] SDSS J1106+1939 has an outflow with a kinetic luminosity of at least 1046 ergs s-1. The distances of the outflows from the central quasar (300-8000 light-years) was greater than expected suggesting that we observe the outflows far from the region in which we assume them to initially accelerated (0.03-0.4 light-years).

[4] A class known as Broad Absorption Line (BAL) quasars.

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Journal Reference:

Benoit C.J. Borguet, Nahum Arav, Doug Edmonds, Carter Chamberlain, Chris Benn. Major contributor to AGN feedback: VLT X-shooter observations of S iv BAL QSO outflows. The Astrophysical Journal, (accepted)

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Hubble sees a galaxy hit a bullseye

Dec. 4, 2012 — In Hubble's image, NGC 922 clearly reveals itself not to be a normal spiral galaxy. The spiral arms are disrupted, a stream of stars extends out towards the top of the image, and a bright ring of nebulae encircles the core. Observing with NASA's Chandra X-ray Observatory reveals more chaos in the form of ultraluminous X-ray sources dotted around the galaxy.

NGC 922's current unusual form is a result of a cosmic bullseye millions of years ago. A smaller galaxy, catalogued as 2MASXI J0224301-244443, plunged right through the heart of NGC 922 and shot out the other side. In wide-field views of the NGC 922, the small interloper can be still be seen shooting away from the scene of the crash.

As the small galaxy passed through the middle of NGC 922, it set up ripples that disrupted the clouds of gas, and triggered the formation of new stars whose radiation then lit up the remaining gas. The bright pink colour of the resulting nebulae is a characteristic sign of this process, and it is caused by excited hydrogen gas (the dominant element in interstellar gas clouds). This process of excitation and emission of light by gases is similar to that in neon signs.

In theory, if two galaxies are aligned just right, with the small one passing through the centre of the larger one, the ring of nebulae should form a perfect circle, but more often the two galaxies are slightly off kilter, leading to a circle that, like this one, is noticeably brighter on one side than the other.

These objects, called collisional ring galaxies, are relatively rare in our cosmic neighbourhood. Although galaxy collisions and mergers are commonplace, the precise alignment and ratio of sizes necessary to form a ring like this is not, and the ring-like phenomenon is also thought to be relatively short-lived.

The chances of seeing one of these galaxies nearby is therefore quite low. Despite the immense number of galaxies in the Universe, this is one of only a handful known in our cosmic neighbourhood. Observations of the more distant Universe (where we see further into the past) show that these rings were more common in the past, however.

Hubble's image of NGC 922 consists of a series of exposures taken in visible light with Hubble's Wide Field Camera 3, and in visible and near-infrared light with the Wide Field and Planetary Camera 2.

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Astronomers uncover a surprising trend in galaxy evolution

Oct. 19, 2012 — A comprehensive study of hundreds of galaxies observed by the Keck telescopes in Hawaii and NASA's Hubble Space Telescope has revealed an unexpected pattern of change that extends back 8 billion years, or more than half the age of the universe.

"Astronomers thought disk galaxies in the nearby universe had settled into their present form by about 8 billion years ago, with little additional development since," said Susan Kassin, an astronomer at NASA's Goddard Space Flight Center in Greenbelt, Md., and the study's lead researcher. "The trend we've observed instead shows the opposite, that galaxies were steadily changing over this time period."

Today, star-forming galaxies take the form of orderly disk-shaped systems, such as the Andromeda Galaxy or the Milky Way, where rotation dominates over other internal motions. The most distant blue galaxies in the study tend to be very different, exhibiting disorganized motions in multiple directions. There is a steady shift toward greater organization to the present time as the disorganized motions dissipate and rotation speeds increase. These galaxies are gradually settling into well-behaved disks.

Blue galaxies -- their color indicates stars are forming within them -- show less disorganized motions and ever-faster rotation speeds the closer they are observed to the present. This trend holds true for galaxies of all masses, but the most massive systems always show the highest level of organization.

Researchers say the distant blue galaxies they studied are gradually transforming into rotating disk galaxies like our own Milky Way.

"Previous studies removed galaxies that did not look like the well-ordered rotating disks now common in the universe today," said co-author Benjamin Weiner, an astronomer at the University of Arizona in Tucson. "By neglecting them, these studies examined only those rare galaxies in the distant universe that are well-behaved and concluded that galaxies didn't change."

Rather than limit their sample to certain galaxy types, the researchers instead looked at all galaxies with emission lines bright enough to be used for determining internal motions. Emission lines are the discrete wavelengths of radiation characteristically emitted by the gas within a galaxy. They are revealed when a galaxy's light is separated into its component colors. These emission lines also carry information about the galaxy's internal motions and distance.

The team studied a sample of 544 blue galaxies from the Deep Extragalactic Evolutionary Probe 2 (DEEP2) Redshift Survey, a project that employs Hubble and the twin 10-meter telescopes at the W. M. Keck Observatory in Hawaii. Located between 2 billion and 8 billion light-years away, the galaxies have stellar masses ranging from about 0.3 percent to 100 percent of the mass of our home galaxy.

A paper describing these findings will be published Oct. 20 in The Astrophysical Journal.

The Milky Way galaxy must have gone through the same rough-and-tumble evolution as the galaxies in the DEEP2 sample, and gradually settled into its present state as the sun and solar system were being formed.

In the past 8 billion years, the number of mergers between galaxies large and small has decreased sharply. So has the overall rate of star formation and disruptions of supernova explosions associated with star formation. Scientists speculate these factors may play a role in creating the evolutionary trend they observe.

Now that astronomers see this pattern, they can adjust computer simulations of galaxy evolution until these models are able to replicate the observed trend. This will guide scientists to the physical processes most responsible for it.

The DEEP2 survey is led by Lick Observatory at the University of California at Santa Cruz in collaboration with the University of California at Berkeley, the University of Hawaii at Manoa, Johns Hopkins University in Baltimore, Md., the University of Chicago and the California Institute of Technology in Pasadena.

The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. NASA's Goddard Space Flight Center in Greenbelt, Md., manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Md., conducts Hubble science operations. STScI is operated by the Association of Universities for Research in Astronomy, Inc. in Washington.

For more information about NASA's Hubble Space Telescope, visit: http://www.nasa.gov/hubble

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Journal Reference:

Susan A. Kassin, Benjamin J. Weiner, S. M. Faber, Jonathan P. Gardner, C. N. A. Willmer, Alison L. Coil, Michael C. Cooper, Julien Devriendt, Aaron A. Dutton, Puragra Guhathakurta, David C. Koo, A. J. Metevier, Kai G. Noeske, Joel R. Primack. The Epoch of Disk Settling: z ~ 1 to Now. The Astrophysical Journal, 2012; 758 (2): 106 DOI: 10.1088/0004-637X/758/2/106

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