A new cosmic survey offers unprecedented view of galaxies

View of the galaxies
A color composite image in the green, red and infrared bands of a patch of the sky known as the COSMOS field, as imaged by the Subaru Telescope in Hawaii. The galaxies are seen at such large distances that the light from them has taken billions of years to reach Earth. The light from the faintest galaxies in this image was emitted when the universe was less than 10 percent of its present age. Click here to pan around the image. (Credit: Princeton University/HSC Project)

By the Office of the Dean for Research

The universe has come into sharper focus with the release this week of new images from one of the largest telescopes in the world. A multinational collaboration led by the National Astronomical Observatory of Japan that includes Princeton University scientists has published a “cosmic census” of a large swath of the night sky containing roughly 100 million stars and galaxies, including some of the most distant objects in the universe. These high-quality images allow an unprecedented view into the nature and evolution of galaxies and dark matter.

The images and accompanying data were collected using a digital optical-imaging camera on the Subaru Telescope, located at the Mauna Kea Observatory in Hawaii. The camera, known as Hyper Suprime-Cam, is mounted directly in the optical path, at the “prime focus,” of the Subaru Telescope. A single image from the camera captures an amount of sky equal to the area of about nine full moons.

The project, known as the Hyper Suprime-Cam Subaru Strategic Program, is led by the National Astronomical Observatory of Japan (NAOJ) in collaboration with the Kavli Institute for the Physics and Mathematics of the Universe in Japan, the Academia Sinica Institute of Astronomy and Astrophysics in Taiwan, and Princeton University.

The release includes data from the first one-and-a-half years of the project, consisting of 61.5 nights of observations beginning in 2014. The project will take 300 nights over five to six years.

The data will allow researchers to look for previously undiscovered galaxies and to search for dark matter, which is matter that neither emits nor absorbs light but which can be detected via its effects on gravity. A 2015 study using Hyper Suprime-Cam surveyed 2.3 square degrees of sky and found gravitational signatures of nine clumps of dark matter, each weighing as much as a galaxy cluster (Miyazaki et al., 2015). The current data release covers about 50 times more sky than was used in that study, showing the potential of these data to reveal the statistical properties of dark matter.

The survey consists of three layers: a Wide survey that will eventually cover an area equal to 7000 full moons, or 1400 square degrees; a Deep survey that will look farther into the universe and encompass 26 square degrees; and an UltraDeep survey that will cover 3.5 square degrees and penetrate deep into space, allowing observations of some of the most distant galaxies in the universe. The surveys use optical and near infrared wavelengths in five broad wavelength bands (green, red, infrared, z, and y) and four narrow-band filters. In the multi-band images, the images are extremely sharp, with star images only 0.6 to 0.8 arcseconds across. (One arcsecond equals 3600th part of a degree.)

Figure 2: Cluster of galaxies
An image of a massive cluster of galaxies in the Virgo constellation showing numerous strong gravitational lenses. The distance to the central galaxy is 5.3 billion light years, while the lensed galaxies, apparent as the arcs around the cluster, are much more distant. This is a composite image in the green, red, and infrared band, and has a spatial resolution of about 0.6 arcsecond. (Credit: NAOJ/HSC Project)

The ability to capture images from deep in space is made possible by the light-collection power of the Subaru Telescope’s mirror, which has an aperture of 8.2 meters, as well as the image exposure time. The depth into space that one can look is measured in terms of the magnitude, or brightness of objects that can be seen from Earth in a given wavelength band. The depths of the three surveys are characterized by magnitudes in the red band of 26.4, 26.6 and 27.3 in the Wide, Deep and Ultradeep data, respectively. As the survey continues, the Deep and Ultradeep surveys will be able to image fainter objects.

The Hyper Suprime-Cam contains 104 scientific charge-coupled devices (CCDs) for a total of 870 million pixels. The total amount of data taken so far comprises 80 terabytes, which is comparable to the size of about 10 million images by a typical digital camera, and covers 108 square degrees. Because it is difficult to search such a huge dataset with standard tools, NAOJ has developed a dedicated database and interface for ease of access and use of the data.

Figure 3: Interation between galaxies
A color composite image in the green, red and infrared bands of UGC 10214, known as the Tadpole Galaxy in the ELAIS-N1 region. The distance to this galaxy is about 400 million light years. The long tail of stars is due to gravitational interaction between two galaxies. (Credit: NAOJ/HSC Project)

“Since 2014, we have been observing the sky with HSC, which can capture a wide-field image with high resolution,” said Satoshi Miyazaki, the leader of the project and a scientist at NAOJ. “We believe the data release will lead to many exciting astronomical results, from exploring the nature of dark matter and dark energy, as well as asteroids in our own solar system and galaxies in the early universe. The team members are now preparing a number of scientific papers based on these data. We plan to publish them in a special issue of the Publication of Astronomical Society of Japan. Moreover, we hope that interested members of the public will also access the data and enjoy the real universe imaged by the Subaru telescope, one of the largest the world.”

At Princeton, the project is co-led by Michael Strauss and Robert Lupton of the Department of Astrophysical Sciences. “The HSC data are really beautiful,” Strauss said. “Princeton scientists are using these data to explore the nature of merging galaxies, to search for the most distant quasars in the universe, to map the outer reaches of the Milky Way Galaxy, and for many other projects. We are delighted to make these wonderful images available to the world-wide astronomical community.”

Funding for the HSC Project was provided in part by the following grants: Grant-in-Aid for Scientific Research (B) JP15340065; Grant-in-Aid for Scientific Research on Priority Areas JP18072003; and the Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST) entitled, “Uncovering the origin and future of the Universe-ultra-wide-field imaging and spectroscopy reveal the nature of dark matter and dark energy.” Funding was also provided by Princeton University.

This article was adapted from a press release from the National Astronomical Observatory of Japan.

An explanation for the mysterious onset of a universal process (Physics of Plasmas)

Solar flares
Magnetic reconnection happens in solar flares on the surface in the sun, as well as in experimental fusion energy reactors here on Earth. Image credit: NASA.

By John Greenwald, Princeton Plasma Physics Laboratory Communications

Scientists have proposed a groundbreaking solution to a mystery that has puzzled physicists for decades. At issue is how magnetic reconnection, a universal process that sets off solar flares, northern lights and cosmic gamma-ray bursts, occurs so much faster than theory says should be possible. The answer, proposed by researchers at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) and Princeton University, could aid forecasts of space storms, explain several high-energy astrophysical phenomena, and improve plasma confinement in doughnut-shaped magnetic devices called tokamaks designed to obtain energy from nuclear fusion.

Magnetic reconnection takes place when the magnetic field lines embedded in a plasma — the hot, charged gas that makes up 99 percent of the visible universe — converge, break apart and explosively reconnect. This process takes place in thin sheets in which electric current is strongly concentrated.

According to conventional theory, these sheets can be highly elongated and severely constrain the velocity of the magnetic field lines that join and split apart, making fast reconnection impossible. However, observation shows that rapid reconnection does exist, directly contradicting theoretical predictions.

Detailed theory for rapid reconnection

Now, physicists at PPPL and Princeton University have presented a detailed theory for the mechanism that leads to fast reconnection. Their paper, published in the journal Physics of Plasmas in October, focuses on a phenomenon called “plasmoid instability” to explain the onset of the rapid reconnection process. Support for this research comes from the National Science Foundation and the DOE Office of Science.

Plasmoid instability, which breaks up plasma current sheets into small magnetic islands called plasmoids, has generated considerable interest in recent years as a possible mechanism for fast reconnection. However, correct identification of the properties of the instability has been elusive.

Luca Comisson, PPPL
Luca Comisso, lead author of the study. Photo courtesy of PPPL.

The Physics of Plasmas paper addresses this crucial issue. It presents “a quantitative theory for the development of the plasmoid instability in plasma current sheets that can evolve in time” said Luca Comisso, lead author of the study. Co-authors are Manasvi Lingam and Yi-Min Huang of PPPL and Princeton, and Amitava Bhattacharjee, head of the Theory Department at PPPL and Princeton professor of astrophysical sciences.

Pierre de Fermat’s principle

The paper describes how the plasmoid instability begins in a slow linear phase that goes through a period of quiescence before accelerating into an explosive phase that triggers a dramatic increase in the speed of magnetic reconnection. To determine the most important features of this instability, the researchers adapted a variant of the 17th century “principle of least time” originated by the mathematician Pierre de Fermat.

Use of this principle enabled the researchers to derive equations for the duration of the linear phase, and for computing the growth rate and number of plasmoids created. Hence, this least-time approach led to a quantitative formula for the onset time of fast magnetic reconnection and the physics behind it.

The paper also produced a surprise. The authors found that such relationships do not reflect traditional power laws, in which one quantity varies as a power of another. “It is common in all realms of science to seek the existence of power laws,” the researchers wrote. “In contrast, we find that the scaling relations of the plasmoid instability are not true power laws – a result that has never been derived or predicted before.”

PPPL, on Princeton University’s Forrestal Campus in Plainsboro, N.J., is devoted to creating new knowledge about the physics of plasmas — ultra-hot, charged gases — and to developing practical solutions for the creation of fusion energy. The Laboratory is managed by Princeton University for the U.S. Department of Energy’s Office of Science, which is the largest single supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

Read the abstract here: Comisso, L.; Lingam, M.; Huang, Y.-M.; Bhattacharjee, A. General theory of the plasmoid instability. Physics of Plasmas 23, 2016. DOI: 10.1063/1.4964481





European satellite could discover thousands of planets in Earth’s galaxy (arXiv)

By Morgan Kelly, Office of Communications

Celestial Sphere
Princeton University and Lund University researchers project that the recently launched European satellite Gaia could discover tens of thousands of planets during its five-year mission. In this image, the colored portions indicate the number of observations Gaia would make of a particular part of the sky during its mission; the scale at the bottom indicates the number of observations from zero (purple) to 200 (red). The total number of observations of any part of the sky ranges from about 60 at low ecliptic latitudes to about 80 at high ecliptic latitudes, with a maximum of about 150-200 at intermediate latitudes. From these many different observations of each star, the highly accurate Gaia measurements will reveal the tiny star motion, or “wobble,” that results from any orbiting planet. (Image by Lennart Lindegren, Lund University)

A recently launched European satellite could reveal tens of thousands of new planets within the next few years, and provide scientists with a far better understanding of the number, variety and distribution of planets in our galaxy, according to research published today.

Researchers from Princeton University and Lund University in Sweden calculated that the observational satellite Gaia could detect as many as 21,000 exoplanets, or planets outside of Earth’s solar system, during its five-year mission. If extended to 10 years, Gaia could detect as many as 70,000 exoplanets, the researchers report. The researchers’ assessment is accepted in the Astrophysical Journal and was published Nov. 6 in advance-of-print on arXiv, a preprint database run by Cornell University.

Exoplanets will be an important “by-product” of Gaia’s mission, explained first author Michael Perryman, who made the assessment while serving as Princeton’s Bohdan Paczyński Visiting Fellow in the Department of Astrophysical Sciences. Built and operated by the European Space Agency (ESA) and launched in December 2013, Gaia will capture the motion, physical characteristics and distance from Earth — and one another — of roughly 1 billion objects, mostly stars, in the Milky Way galaxy with unprecedented precision. The presence of an exoplanet will be determined by how its star “wobbles” as a result of the planet’s orbit around it.

More important than the numbers of predicted discoveries are the kinds of planets that the researchers expect Gaia to detect, many of which — such as planets with multi-year orbits that pass directly, or transit, in front of their star as seen from Earth — are currently difficult to find, Perryman said. The satellite’s instruments could reveal objects that are considered rare in the Milky Way, such as an estimated 25 to 50 Jupiter-sized planets that orbit faint, low-mass stars known as red dwarfs. Unique planets and systems — such as planets that orbit in the opposite direction of their companions — can inspire years of research, he said.

Transit Distances
One of the main objectives of the Gaia mission is to establish the currently uncertain distance from Earth to various stars using high-precision triangulation, which would allow a much better understanding of the properties of the stars and the planets orbiting them. Of the 1,163 confirmed transiting planets, which pass directly in front of their stars as seen from Earth, there are 644 distinct host stars; less than 200 have accurately known distances from Earth. This image shows the distances from Earth (center) to the stars (black dots) of transiting exoplanets. The inner dashed circle has a radius of 100 parsecs (about 326 light years) with the middle and outer circles corresponding to 500 parsecs (1,630 light years) and 1,000 parsecs (3,260 light years), respectively. The cluster of points to the lower right represents the transiting planets discovered by NASA’s Kepler satellite. For each star, the straight lines extending from the circle indicate the current uncertainty of its distance from Earth. (Image by Michael Perryman)

“It’s not just about the numbers. Each of these planets will be conveying some very specific details, and many will be highly interesting in their own way,” Perryman said. “If you look at the planets that have been discovered until now, they occupy very specific regions of discovery space. Gaia will not only discover a whole list of planets, but in an area that has not been thoroughly explored so far.”

Ultimately, a comprehensive census allows scientists to more accurately determine how many planets and planetary systems exist, the detailed properties of those planets, and how they are positioned throughout the galaxy, Perryman said.

Perryman worked with Joel Hartman, an associate research scholar in Princeton’s astrophysical sciences department, Gáspár Bakos, an associate professor of astrophysical sciences, and Lennart Lindegren, a professor of astronomy at Lund University. Gaia is based on a satellite proposal led by Lindegren and Perryman that was submitted to the ESA in 1993.

Research on exoplanets has increased dramatically in the 15 years since Gaia was accepted by the ESA in 2000. The new estimate is based on a highly detailed model of how stars and planets are positioned in the Milky Way; more accurate details of Gaia’s measurement and data-analysis capabilities; and current estimates of exoplanet distributions, particularly those derived from NASA’s Kepler satellite, which has identified nearly 1,000 confirmed planets and more than 3,000 candidates. Crucial to conducting the assessment is the much-improved knowledge that now exists about distant planets, Perryman said, such as the types of stars that exoplanets orbit.

The first exoplanet was detected in 1995. Nearly 1,900 have since been discovered. Bakos, who focuses much of his research on exoplanets, launched and oversees HATNet (Hungarian-made Automated Telescope Network) and HATSouth, planet-hunting networks of fully automated, small-scale telescopes installed on four continents that scan the sky every night for planets as they transit in front of their parent star. The projects have discovered more than 50 planets since 1999.

“Our assessment will help prepare exoplanet researchers for what to expect from Gaia,” Perryman said. “We’re going to be adding potentially 20,000 new planets in a completely new area of discovery space. It’s anyone’s guess how the field will develop as a result.”

This work was partly supported by the National Science Foundation (grant no. 1108686) and NASA (grant no. NNX13AJ15G).

Read the article.

Michael Perryman, Joel Hartman, Gáspár Bakos and Lennart Lindegren. 2014. Astrometric exoplanet detection with Gaia. Astrophysical Journal. Arti­cle first pub­lished to the arXiv preprint database: Nov. 6, 2014.


Rife with hype, exoplanet study needs patience and refinement (PNAS)

By Morgan Kelly, Office of Communications

Exoplanet transiting in front of its star. Princeton’s Adam Burrows argues against drawing too many conclusions about such distant objects with today’s technologies. Photo credit: ESA/C. Carreau

Imagine someone spent months researching new cities to call home using low-resolution images of unidentified skylines. The pictures were taken from several miles away with a camera intended for portraits, and at sunset. From these fuzzy snapshots, that person claims to know the city’s air quality, the appearance of its buildings, and how often it rains.

This technique is similar to how scientists often characterize the atmosphere — including the presence of water and oxygen — of planets outside of Earth’s solar system, known as exoplanets, according to a review of exoplanet research published in the Proceedings of the National Academy of Sciences.

A planet’s atmosphere is the gateway to its identity, including how it was formed, how it developed and whether it can sustain life, stated Adam Burrows, author of the review and a Princeton University professor of astrophysical sciences.

But the dominant methods for studying exoplanet atmospheres are not intended for objects as distant, dim and complex as planets trillions of miles from Earth, Burrows said. They were instead designed to study much closer or brighter objects, such as planets in Earth’s solar system and stars.

Nonetheless, scientific reports and the popular media brim with excited depictions of Earth-like planets ripe for hosting life and other conclusions that are based on vague and incomplete data, Burrows wrote in the first in a planned series of essays that examine the current and future study of exoplanets. Despite many trumpeted results, few “hard facts” about exoplanet atmospheres have been collected since the first planet was detected in 1992, and most of these data are of “marginal utility.”

The good news is that the past 20 years of study have brought a new generation of exoplanet researchers to the fore that is establishing new techniques, technologies and theories. As with any relatively new field of study, fully understanding exoplanets will require a lot of time, resources and patience, Burrows said.

“Exoplanet research is in a period of productive fermentation that implies we’re doing something new that will indeed mature,” Burrows said. “Our observations just aren’t yet of a quality that is good enough to draw the conclusions we want to draw.

“There’s a lot of hype in this subject, a lot of irrational exuberance. Popular media have characterized our understanding as better than it actually is,” he said. “They’ve been able to generate excitement that creates a positive connection between the astrophysics community and the public at large, but it’s important not to hype conclusions too much at this point.”

The majority of data on exoplanet atmospheres come from low-resolution photometry, which captures the variation in light and radiation an object emits, Burrows reported. That information is used to determine a planet’s orbit and radius, but its clouds, surface, and rotation, among other factors, can easily skew the results. Even newer techniques such as capturing planetary transits — which is when a planet passes in front of its star, and was lauded by Burrows as an unforeseen “game changer” when it comes to discovering new planets — can be thrown off by a thick atmosphere and rocky planet core.

All this means that reliable information about a planet can be scarce, so scientists attempt to wring ambitious details out of a few data points. “We have a few hard-won numbers and not the hundreds of numbers that we need,” Burrows said. “We have in our minds that exoplanets are very complex because this is what we know about the planets in our solar system, but the data are not enough to constrain even a fraction of these conceptions.”

Burrows emphasizes that astronomers need to acknowledge that they will never achieve a comprehensive understanding of exoplanets through the direct-observation, stationary methods inherited from the exploration of Earth’s neighbors. He suggests that exoplanet researchers should acknowledge photometric interpretations as inherently flawed and ambiguous. Instead, the future of exoplanet study should focus on the more difficult but comprehensive method of spectrometry, wherein the physical properties of objects are gauged by the interaction of its surface and elemental features with light wavelengths, or spectra. Spectrometry has been used to determine the age and expansion of the universe.

Existing telescopes and satellites are likewise vestiges of pre-exoplanet observation. Burrows calls for a mix of small, medium and large initiatives that will allow the time and flexibility scientists need to develop tools to detect and analyze exoplanet spectra. He sees this as a challenge in a research environment that often puts quick-payback results over deliberate research and observation. Once scientists obtain high-quality spectral data, however, Burrows predicted, “Many conclusions reached recently about exoplanet atmospheres will be overturned.”

“The way we study planets out of the solar system has to be radically different because we can’t ‘go’ to those planets with satellites or probes,” Burrows said. “It’s much more an observational science. We have to be detectives. We’re trying to find clues and the best clues since the mid-19th century have been in spectra. It’s the only means of understanding the atmosphere of these planets.”

A longtime exoplanet researcher, Burrows predicted the existence of “hot-Jupiter” planets — gas planets similar to Jupiter but orbiting very close to the parent star — in a paper in the journal Nature months before the first such planet, 51 Pegasi b, was discovered in 1995.

Read the abstract.

Citation: Burrows, Adam S. 2014. Spectra as windows into exoplanet atmospheres. Proceedings of the National Academy of Sciences. Article first published online: Jan. 13, 2014. DOI: 10.1073/pnas.1304208111

Shape from sound — new methods to probe the universe (Physical Review Letters)

By Morgan Kelly, Office of Communications

As the universe expands, it is continually subjected to energy shifts, or “quantum fluctuations,” that send out little pulses of “sound” into the fabric of spacetime. In fact, the universe is thought to have sprung from just such an energy shift.

A recent paper in the journal Physical Review Letters reports a new mathematical tool that should allow one to use these sounds to help reveal the shape of the universe. The authors reconsider an old question in spectral geometry that asks, roughly, to what extent can the shape of a thing be known from the sound of its acoustic vibrations? The researchers approached this problem by breaking it down into small workable pieces, according to author Tejal Bhamre, a Princeton University graduate student in the Department of Physics.

To understand the authors’ method, consider a vase. If one taps a vase with a spoon, it will make a sound that is characteristic of its shape. Similarly, the technique Bhamre and her coauthors developed could, in principle, determine the shape of spacetime from the perpetual ringing caused by quantum fluctuations.

The researchers’ technique also provides a unique connection between the two pillars of modern physics — quantum theory and general relativity — by using vibrational wavelengths to define the geometric property that is spacetime.

Bhamre worked with coauthors David Aasen, a physics graduate student at Caltech, and Achim Kempf, a Waterloo University professor of physics of information.

Read the abstract.

David Aasen, Tejal Bhamre and Achim Kempf. 2013. Shape from Sound: Toward New Tools for Quantum Gravity. Physical Review Letters. Article first published online: March 18, 2013. DOI: 10.1103/PhysRevLett.110.121301.

This research was supported by the Natural Sciences and Engineering Research Council of Canada.