Friday, October 20, 2017

A glimpse of the future

Credit: ESA/Hubble & NASA

This image, captured by the NASA/ESA Hubble Space Telescope, shows what happens when two galaxies become one. The twisted cosmic knot seen here is NGC 2623 — or Arp 243 — and is located about 250 million light-years away in the constellation of Cancer (The Crab).

NGC 2623 gained its unusual and distinctive shape as the result of a major collision and subsequent merger between two separate galaxies. This violent encounter caused clouds of gas within the two galaxies to become compressed and stirred up, in turn triggering a sharp spike of star formation. This active star formation is marked by speckled patches of bright blue; these can be seen clustered both in the centre and along the trails of dust and gas forming NGC 2623’s sweeping curves (known as tidal tails). These tails extend for roughly 50 000 light-years from end to end. Many young, hot, newborn stars form in bright stellar clusters — at least 170 such clusters are known to exist within NGC 2623.

NGC 2623 is in a late stage of merging. It is thought that the Milky Way will eventually resemble NGC 2623 when it collides with our neighbouring galaxy, the Andromeda Galaxy, in four billion years time.

In contrast to the image of NGC 2623 released in 2009 (heic0912), this new version contains data from recent narrow-band and infrared observations that make more features of the galaxy visible.

 Source: ESA/Hubble/Potw

Thursday, October 19, 2017

Caught in a Dust Trap

Credit: ALMA (ESO/NAOJ/NRAO)/S. Kraus (University of Exeter, UK)

This image from the Atacama Large Millimeter/submillimeter Array (ALMA) shows V1247 Orionis, a young, hot star surrounded by a dynamic ring of gas and dust, known as a circumstellar disc. This disc can be seen here in two parts: a clearly defined central ring of matter and a more delicate crescent structure located further out.

arving its way through the disc. As the planet orbits around its parent star, its motion creates areas of high pressure on either side of its path, similar to how a ship creates bow waves as it cuts through water. These areas of high pressure could become protective barriers around sites of planet formation; dust particles are trapped within them for millions of years, allowing them the time and space to clump together and grow.

The exquisite resolution of ALMA allows astronomers to study the intricate structure of such a dust trapping vortex for the first time. The image reveals not only the crescent-shaped dust trap at the outer edge of the dark strip, but also regions of excess dust within the ring, possibly indicating a second dust trap that formed inside of the potential planet’s orbit. This confirms the predictions of earlier computer simulations.

Dust trapping is one potential solution to a major stumbling block in current theories of how planets form, which predicts that particles should drift into the central star and be destroyed before they have time to grow to planetesimal sizes (the radial drift problem).


Source: ESO/Potw

Wednesday, October 18, 2017

NASA Missions Catch First Light from a Gravitational-Wave Event

Credit X-ray: NASA/CXC/Northwestern U./W. Fong & R. Margutti et al. & NASA/GSFC/E. Troja et al.; Optical:NASA/STScI


Astronomers have used NASA's Chandra X-ray Observatory to make the first X-ray detection of a gravitational wave source. Chandra was one of multiple observatories to detect the aftermath of this gravitational wave event, the first to produce an electromagnetic signal of any type. This discovery represents the beginning of a new era in astrophysics.

The gravitational wave source, GW170817, was detected with the advanced Laser Interferometer Gravitational-Wave Observatory, or LIGO, at 8:41am EDT on Thursday August 17, 2017. Two seconds later NASA's Fermi Gamma-ray Burst Monitor (GBM) detected a weak pulse of gamma-rays. Later that morning, LIGO scientists announced that GW170817 had the characteristics of a merger of two neutron stars.

During the evening of August 17, multiple teams of astronomers using ground-based telescopes reported a detection of a new source of optical and infrared light in the galaxy NGC 4993, a galaxy located about 130 million light years from Earth. The position of the new optical and infrared source agreed with the position of the Fermi and the gravitational wave sources. The latter was refined by combining information from LIGO and its European counterpart, Virgo.

Over the following two weeks, Chandra observed NGC 4993 and the source GW170817 four separate times. In the first observation on August 19th (Principal Investigator: Wen-fai Fong from Northwestern University in Evanston, Illinois), no X-rays were detected at the location of GW170817. This observation was obtained remarkably quickly, only 2.3 days after the gravitational source was detected.

On August 26, Chandra observed GW170817 again and this time, X-rays were seen for the first time (PI: Eleonora Troja from Goddard Space Flight Center in Greenbelt, MD, and the University of Maryland, College Park). This new X-ray source was located at the exact position of the optical and infrared source.

"This Chandra detection is very important because it is the first evidence that sources of gravitational waves are also sources of X-ray emission," said Troja. "This detection is teaching us a great deal of information about the collision and its remnant. It helps to give us an important confirmation that gamma-ray bursts are beamed into narrow jets."

The accompanying graphic shows both the Chandra non-detection, or upper limit of X-rays from GW170817 on August 19th, and the subsequent detection on August 26th, in the two sides of the inset box. The main panel of the graphic is the Hubble Space Telescope image of NGC 4993, which includes data taken on August 22nd. The variable optical source corresponding to GW170817 is located in the center of the circle in the Hubble image.

Chandra observed GW170817 again on September 1st (PI Eleonora Troja) and September 2nd (PI: Daryl Haggard from McGill University in Montreal, Canada), when the source appeared to have roughly the same level of X-ray brightness as the August 26 observation.

The properties of the source's X-ray brightness with time matches that predicted by theoretical models of a short gamma-ray burst (GRB). During such an event, a burst of X-rays and gamma rays is generated by a narrow jet, or beam, of high-energy particles produced by the merger of two neutron stars. The initial non-detection by Chandra followed by the detections show that the X-ray emission from GW170817 is consistent with the afterglow from a GRB viewed "off-axis," that is, with the jet not pointing directly towards the Earth. This is the first time astronomers have ever detected an off-axis short GRB.

"After some thought, we realized that the initial non-detection by Chandra perfectly matches with what we expect," said Fong. "The fact that we did not see anything at first gives us a very good handle on the orientation and geometry of the system."

GW170817 Schematic
Credit: NASA/CXC/K.DiVona

The researchers think that initially the jet was narrow, with Chandra viewing it from the side. However, as time passed the material in the jet slowed down and widened as it slammed into surrounding material, causing the X-ray emission to rise as the jet came into direct view. The Chandra data allow researchers to estimate the angle between the jet and our line of sight. The three different Chandra observing teams each estimate angles between 20 and 60 degrees. Future observations may help refine these estimates.

The detection of this off-axis short GRB helps explain the weakness of the gamma-ray signal detected with Fermi GBM for a burst that is so close by. Because our telescopes are not looking straight down the barrel of the jet as they have for other short GRBs, the gamma-ray signal is much fainter.

The optical and infrared light is likely caused by the radioactive glow when heavy elements such as gold and platinum are produced in the material ejected by the neutron star merger. This glow had been predicted to occur after neutron stars merged.

By detecting an off-axis short GRB at the location of the radioactive glow, the Chandra observations provide the missing observational link between short GRBs and gravitational waves from neutron star mergers.

This is the first time astronomers have all of the necessary pieces of information of neutron stars merging — from the production of gravitational waves followed by signals in gamma rays, X-rays, optical and infrared light, that all agree with predictions for a short GRB viewed off-axis.

"This is a big deal because it's an entirely new level of knowledge," said Haggard. "This discovery allows us to link this gravitational wave source up to all the rest of astrophysics, stars, galaxies, explosions, growing massive black holes, and of course neutron star mergers."

Papers describing these results have been accepted for publication in Nature (Troja et al.), and The Astrophysical Journal Letters (Haggard et al. and Margutti et al.). Raffaella Margutti is a collaborator of Fong's, also from Northwestern.

NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra's science and flight operations.

Fast Facts for GW170817:

Scale: Full field optical is about 0.9 arcmin across (about 367,000 light years); X-ray inset is about 0.1 arcmin across (about 41,000 light years)
Category: Neutron Stars/X-ray Binaries
Coordinates (J2000): RA 13h 09m 48.1s | Dec -23° 22´ 53.4"
Constellation: Hydra
Observation Date: August 19, 2017, August 26, 2017, September 1, 2017, September 2, 2017 2017
Observation Time: 46 hours 23 minutes
Obs. ID: 18955, 19294, 20728, 18988
Instrument: ACIS
References: Margutti et al.: The Astrophysical Journal Letters, 848:L20 (7pp), 2017 October 20
Haggard et al.: The Astrophysical Journal Letters, 848:L25 (6pp), 2017 October 20
Troja et al. Nature (2017) Published online 16 October 2017
Color Code: X-rays (Purple); Optical (Red, Green, Blue)
Distance Estimate: About 130 million light years

Tuesday, October 17, 2017

NASA Missions Catch First Light From a Gravitational-Wave Event

Gravitational Wave Source in NGC 4993
Credits: NASA and ESA
Acknowledgment: A. Levan (U. Warwick), N. Tanvir (U. Leicester), and A. Fruchter and O. Fox (STScI)

Neutron Star Collision Creates Kilonova
Credits: NASA, ESA, and A. Feild (STScI)
 Release images

For the first time, NASA scientists have detected light tied to a gravitational-wave event, thanks to two merging neutron stars in the galaxy NGC 4993, located about 130 million light-years from Earth in the constellation Hydra. 

Shortly after 8:41 a.m. EDT on Aug. 17, NASA's Fermi Gamma-ray Space Telescope picked up a pulse of high-energy light from a powerful explosion, which was immediately reported to astronomers around the globe as a short gamma-ray burst. The scientists at the National Science Foundation’s Laser Interferometer Gravitational-Wave Observatory (LIGO) detected gravitational waves dubbed GW170817 from a pair of smashing stars tied to the gamma-ray burst, encouraging astronomers to look for the aftermath of the explosion. Shortly thereafter, the burst was detected as part of a follow-up analysis by ESA’s (European Space Agency’s) INTEGRAL satellite.

NASA's Swift, Hubble, Chandra, and Spitzer missions, along with dozens of ground-based observatories, including the NASA-funded PanSTARRS survey, later captured the fading glow of the blast's expanding debris. 

"This is extremely exciting science," said Paul Hertz, director of NASA’s Astrophysics Division at the agency’s headquarters in Washington. "Now, for the first time, we've seen light and gravitational waves produced by the same event. The detection of a gravitational-wave source’s light has revealed details of the event that cannot be determined from gravitational waves alone. The multiplier effect of study with many observatories is incredible."

Neutron stars are the crushed, leftover cores of massive stars that previously exploded as supernovas long ago. The merging stars likely had masses between 10 and 60 percent greater than that of our Sun, but they were no wider than Washington, D.C. The pair whirled around each other hundreds of times a second, producing gravitational waves at the same frequency. As they drew closer and orbited faster, the stars eventually broke apart and merged, producing both a gamma-ray burst and a rarely seen flare-up called a "kilonova."

"This is the one we've all been waiting for," said David Reitze, executive director of the LIGO Laboratory at Caltech in Pasadena, California. "Neutron star mergers produce a wide variety of light because the objects form a maelstrom of hot debris when they collide. Merging black holes — the types of events LIGO and its European counterpart, Virgo, have previously seen — very likely consume any matter around them long before they crash, so we don't expect the same kind of light show."

"The favored explanation for short gamma-ray bursts is that they're caused by a jet of debris moving near the speed of light produced in the merger of neutron stars or a neutron star and a black hole," said Eric Burns, a member of Fermi's Gamma-ray Burst Monitor team at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "LIGO tells us there was a merger of compact objects, and Fermi tells us there was a short gamma-ray burst. Together, we know that what we observed was the merging of two neutron stars, dramatically confirming the relationship."

Within hours of the initial Fermi detection, LIGO and the Virgo detector at the European Gravitational Observatory near Pisa, Italy, greatly refined the event's position in the sky with additional analysis of gravitational wave data. Ground-based observatories then quickly located a new optical and infrared source — the kilonova — in NGC 4993. 

To Fermi, this appeared to be a typical short gamma-ray burst, but it occurred less than one-tenth as far away as any other short burst with a known distance, making it among the faintest known. Astronomers are still trying to figure out why this burst is so odd, and how this event relates to the more luminous gamma-ray bursts seen at much greater distances. 

NASA’s Swift, Hubble and Spitzer missions followed the evolution of the kilonova to better understand the composition of this slower-moving material, while Chandra searched for X-rays associated with the remains of the ultra-fast jet.

When Swift turned to the galaxy shortly after Fermi’s gamma-ray burst detection, it found a bright and quickly fading ultraviolet (UV) source. 

"We did not expect a kilonova to produce bright UV emission," said Goddard’s S. Bradley Cenko, principal investigator for Swift. "We think this was produced by the short-lived disk of debris that powered the gamma-ray burst."

Over time, material hurled out by the jet slows and widens as it sweeps up and heats interstellar material, producing so-called afterglow emission that includes X-rays. But the spacecraft saw no X-rays — a surprise for an event that produced higher-energy gamma rays. 

NASA’s Chandra X-ray Observatory clearly detected X-rays nine days after the source was discovered. Scientists think the delay was a result of our viewing angle, and it took time for the jet directed toward Earth to expand into our line of sight.

"The detection of X-rays demonstrates that neutron star mergers can form powerful jets streaming out at near light speed," said Goddard's Eleonora Troja, who led one of the Chandra teams and found the X-ray emission. "We had to wait for nine days to detect it because we viewed it from the side, unlike anything we had seen before."

On Aug. 22, NASA’s Hubble Space Telescope began imaging the kilonova and capturing its near-infrared spectrum, which revealed the motion and chemical composition of the expanding debris.

"The spectrum looked exactly like how theoretical physicists had predicted the outcome of the merger of two neutron stars would appear," said Andrew Levan at the University of Warwick in Coventry, England, who led one of the proposals for Hubble spectral observations. "It tied this object to the gravitational wave source beyond all reasonable doubt." 

Astronomers think a kilonova's visible and infrared light primarily arises through heating from the decay of radioactive elements formed in the neutron-rich debris. Crashing neutron stars may be the universe's dominant source for many of the heaviest elements, including platinum and gold.
Because of its Earth-trailing orbit, Spitzer was uniquely situated to observe the kilonova long after the Sun moved too close to the galaxy on the sky for other telescopes to see it. Spitzer's Sept. 30 observation captured the longest-wavelength infrared light from the kilonova, which unveils the quantity of heavy elements forged. 

"Spitzer was the last to join the party, but it will have the final word on how much gold was forged," says Mansi Kasliwal, Caltech assistant professor and principal investigator of the Spitzer observing program. 

Numerous scientific papers describing and interpreting these observations have been published in Science, Nature, Physical Review Letters and The Astrophysical Journal.

Gravitational waves were directly detected for the first time in 2015 by LIGO, whose architects were awarded the 2017 Nobel Prize in physics for the discovery.

NASA's Hubble Studies Source of Gravitational Waves

On August 17, 2017, weak ripples in the fabric of space-time known as gravitational waves washed over Earth. Unlike previously detected gravitational waves, these were accompanied by light, allowing astronomers to pinpoint the source. NASA’s Hubble Space Telescope turned its powerful gaze onto the new beacon, obtaining both images and spectra. The resulting data will help reveal details of the titanic collision that created the gravitational waves, and its aftermath.

The Laser Interferometer Gravitational-Wave Observatory (LIGO) detected gravitational waves at 8:41 a.m. EDT on August 17. Two seconds later, NASA’s Fermi Gamma-ray Space Telescope measured a short pulse of gamma rays known as a gamma-ray burst. Many observatories, including space telescopes, probed the suspected location of the source, and within about 12 hours several spotted their quarry.

In a distant galaxy called NGC 4993, about 130 million light-years from Earth, a point of light shone where nothing had been before. It was about a thousand times brighter than a variety of stellar flare called a nova, putting it in a class of objects astronomers call “kilonovae.” It also faded noticeably over 6 days of Hubble observations.

“This appears to be the trifecta for which the astronomical community has been waiting: Gravitational waves, a gamma-ray burst, and a kilonova all happening together,” said Ori Fox of the Space Telescope Science Institute, Baltimore, Maryland.

The source of all three was the collision of two neutron stars, the aged remains of a binary star system. A neutron star forms when the core of a dying massive star collapses, a process so violent that it crushes protons and electrons together to form subatomic particles called neutrons. The result is like a giant atomic nucleus, cramming several Suns’ worth of material into a ball just a few miles across.

In NGC 4993, two neutron stars once spiraled around each other at blinding speed. As they drew closer together, they whirled even faster, spinning as fast as a blender near the end. Powerful tidal forces ripped off huge chunks while the remainder collided and merged, forming a larger neutron star or perhaps a black hole. Leftovers spewed out into space. Freed from the crushing pressure, neutrons turned back into protons and electrons, forming a variety of chemical elements heavier than iron.

“We think neutron star collisions are a source of all kinds of heavy elements, from the gold in our jewelry to the plutonium that powers spacecraft, power plants, and bombs,” said Andy Fruchter of the Space Telescope Science Institute.

Several teams of scientists are using Hubble’s suite of cameras and spectrographs to study the gravitational wave source. Fruchter, Fox, and their colleagues used Hubble to obtain a spectrum of the object in infrared light. By splitting the light of the source into a rainbow spectrum, astronomers can probe the chemical elements that are present. The spectrum showed several broad bumps and wiggles that signal the formation of some of the heaviest elements in nature. 

“The spectrum looked exactly like how theoretical physicists had predicted the outcome of the merger of two neutron stars would appear. It tied this object to the gravitational wave source beyond all reasonable doubt,” said Andrew Levan at the University of Warwick in Coventry, England, who led one of the proposals for Hubble spectral observations. Additional spectral observations were led by Nial Tanvir of the University of Leicester, England.

Spectral lines can be used as fingerprints to identify individual elements. However, this spectrum is proving a challenge to interpret.

“Beyond the fact that two neutron stars flung a lot of matter out into space, we’re not yet sure what else the spectrum is telling us,” explained Fruchter. “Because the material is moving so fast, the spectral lines are smeared out. Also, there are all kinds of unusual isotopes, many of which are short-lived and undergo radioactive decay. The good news is that it’s an exquisite spectrum, so we have a lot of data to work with and analyze.”

Hubble also picked up visible light from the event that gradually faded over the course of several days. Astronomers believe that this light came from a powerful “wind” of material speeding outward. These observations hint that astronomers viewed the collision from above the orbital plane of the neutron stars. If seen from the side (along the orbital plane), matter ejected during the merger would have obscured the visible light and only infrared light would be visible.

“What we see from a kilonova might depend on our viewing angle. The same type of event would appear different depending on whether we’re looking at it face-on or edge-on, which came as a total surprise to us,” said Eleonora Troja of the University of Maryland, College Park, Maryland, and NASA’s Goddard Space Flight Center, Greenbelt, Maryland. Troja is also a principal investigator of a team using Hubble observations to study the object.

The gravitational wave source now is too close to the Sun on the sky for Hubble and other observatories to study. It will come back into view in November. Until then, astronomers will be working diligently to learn all they can about this unique event.

The launch of NASA’s James Webb Space Telescope also will offer an opportunity to examine the infrared light from the source, should that glow remain detectable in the months and years to come.


Christine Pulliam / Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4366 / 410-338-4514 /

Felicia Chou
NASA Headquarters, Washington, D.C.

Dewayne Washington
Goddard Space Flight Center, Greenbelt, Maryland


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Monday, October 16, 2017

The Far Side of the Milky Way

Artist’s view of the Milky Way with the location of the Sun and the star forming region (maser source G007.47+00.05) at the opposite side in the Scutum-Centaurus spiral arm. © Bill Saxton. NRAO/AUI/NSF; Robert Hurt, NASA.

Mapping Spiral Structure for an Improved Picture of our Home Galaxy

Astronomers from the Max Planck Institute for Radio Astronomy in Bonn, Germany, and the Harvard-Smithsonian Center for Astrophysics have directly measured the distance to a star-forming region on the opposite side of our Milky Way Galaxy from the Sun, using the Very Long Baseline Array. Their achievement reaches deep into the Milky Way’s terra incognita and nearly doubles the previous record for distance measurement within our Galaxy.

Their results are published in the 13 October issue of the journal Science.

Distance measurements are crucial for an understanding of the structure of the Milky Way. Most of our Galaxy's material, consisting principally of stars, gas, and dust, lies within a flattened disk, in which our Solar System is embedded. Because we cannot see our Galaxy face-on, its structure, including the shape of its spiral arms, can only be mapped by measuring distances to objects elsewhere in the Galaxy.

The astronomers used a technique called trigonometric parallax, first applied by Friedrich Wilhelm Bessel in 1838 to measure the distance to the star 61 Cygni in the constellation of the Swan. This technique measures the apparent shift in the sky position of a celestial object as seen from opposite sides of the Earth's orbit around the Sun. This effect can be demonstrated by holding a finger in front of one's nose and alternately closing each eye -- the finger appears to jump from side to side.

Measuring the angle of an object's apparent shift in position this way allows astronomers to use simple trigonometry to directly calculate the distance to that object. The smaller the measured angle, the greater the distance is. In the framework of the Bar and Spiral Structure Legacy (BeSSeL) Survey, it is now possible to measure parallaxes a thousand times more accurate than Friedrich Bessel. The Very Long Baseline Array (VLBA), a continent-wide radio telescope system, with ten dish antennas distributed across North America, Hawaii, and the Caribbean, can measure the minuscule angles associated with great distances. In this case, the measurement was roughly equal to the angular size of a baseball on the Moon.

"Using the VLBA, we now can accurately map the whole extent of our Galaxy," says Alberto Sanna, of the Max Planck Institute for Radio Astronomy in Germany (MPIfR).

The new VLBA observations, made in 2014 and 2015, measured a distance of more than 66,000 light-years to the star-forming region G007.47+00.05 on the opposite side of the Milky Way from the Sun, well past the Galaxy's center in a distance of 27,000 light-years. The previous record for a parallax measurement was about 36,000 light-years.

"Most of the stars and gas in our Galaxy are within this newly-measured distance from the Sun. With the VLBA, we now have the capability to measure enough distances to accurately trace the Galaxy's spiral arms and learn their true shapes," Sanna explains.

The VLBA observations measured the distance to a region where new stars are being formed. Such regions include areas where molecules of water and methanol act as natural amplifiers of radio signals -- masers, the radio-wave equivalent of lasers for light waves. This effect makes the radio signals bright and readily observable with radio telescopes.

The Milky Way has hundreds of such star-forming regions that include masers. "So we have plenty of 'mileposts' to use for our mapping project. But this one is special: Looking all the way through the Milky Way, past its center, way out into the other side", says the MPIfR's Karl Menten.

The astronomers' goal is to finally reveal what our own Galaxy looks like if we could leave it, travel outward perhaps a million light-years, and view it face-on, rather than along the plane of its disk. This task will require many more observations and much painstaking work, but, the scientists say, the tools for the job now are in hand. How long will it take?

"Within the next 10 years, we should have a fairly complete picture," predicts Mark Reid of the Harvard-Smithsonian Center for Astrophysics.

Distance determination by measuring the angle of apparent shift in an object's position, as seen from opposite sides of Earth's orbit around the Sun (trigonometric parallax technique).© Bill Saxton, NRAO/AUI/NSF; Robert Hurt, NASA.

The research team consists of Alberto Sanna of the Max Planck Institute for Radio Astronomy (MPIfR), the first author, along with colleagues Mark Reid and Thomas Dame of the Harvard-Smithsonian Center for Astrophysics and Karl Menten and Andreas Brunthaler, also of the MPIfR. They report their findings in the 13 October issue of the journal Science.

The Long Baseline Observatory (LBO) runs the “Very Long Baseline Array” (VLBA) as a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

The BeSSeL Survey (Bar and Spiral Structure Legacy Survey) is a VLBA Key Science project. The survey is named in honor of Friedrich Wilhelm Bessel (1784-1846) who measured the first stellar parallax in 1838. The goal of the survey is to study the spiral structure and kinematics of the Milky Way.


Dr. Alberto Sanna
Phone:+49 228 525-304
Max-Planck-Institut für Radioastronomie, Bonn

Prof. Dr. Karl M. Menten
Direktor und Leiter der Forschungsabteilung "Millimeter- und Submillimeter-Astronomie"
Phone:+49 228 525-297
Max-Planck-Institut für Radioastronomie, Bonn

Dr. Norbert Junkes
Press and Public Outreach
Phone:+49 228 525-399
Max-Planck-Institut für Radioastronomie, Bonn

Original Paper

Mapping Spiral Structure on the far side of the Milky Way

Alberto Sanna, Mark J. Reid, Thomas M. Dame, Karl M. Menten, Andreas Brunthaler, 2017, Science (October 13 issue)


Millimeter- und Submillimeter-Astronomie
Research Department "Millimeter and Submillimeter Astronomy" at MPIfR, Bonn, Germany

Harvard-Smithsonian Center for Astrophysics (CfA)

National Radio Astronomy Observatory (NRAO)

Very Long Baseline Array (VLBA)

Bar and Spiral Structure Legacy Survey (BeSSeL)

Sunday, October 15, 2017

The Remarkable Jet of the Quasar 4C+19.44

A Chandra X-ray image of the quasar 3C19.44. The overlayed contours show the radio emission (the dimension 100kpc corresponds to 329,000 light-years; the extremely bright core produces a line of bright pixels as an artifact). NASA/Chandra VLA and Harris et al.

Quasars are galaxies with massive black holes at their cores. So much energy is being radiated from near the nucleus of a quasar that it is much brighter than the rest of the entire galaxy. Much of that radiation is at radio wavelengths, produced by electrons ejected from the core at speeds very close to that of light, often in narrow, bipolar jets that are hundreds of thousands of light-years long. The fast-moving charged particles can also scatter photons of light, kicking them up in energy into the X-ray range. Even after more than two decades of study, however, there is still no clear conclusion as to the physical mechanism actually responsible for the X-ray emission. In more powerful quasars, it does appear that this scattering process dominates. In lower power jets, however, the emission characteristics suggest that the X-ray emission is dominated by magnetic field effects, not scattering.

The lead author of a new paper on the remarkable jet in the quasar 4C+19.44 is CfA astronomer Dan Harris, who very sadly passed away in December, 2015, after a long and productive career. His CfA teammates on this project, Dan Schwartz with Nicholas Lee and Aneta Siemiginowska, worked to finish the research together with an international team of colleagues. The scientists undertook a detailed, high spatial resolution study of the straight, three hundred thousand light-year long jet in this quasar using multiwavelength data from the Chandra (X-ray), Spitzer (infrared), and Hubble (optical) space observatories as well as from the Very Large Array (radio).

The combination of multiwavelength observations with high spatial resolution enabled the team to measure the characteristics of the emission systematically in ten distinct knots along the jets. They find that both the magnetic field strength and the particle velocities are (remarkably) quite constant all along the length of this jet, at least when presuming the scattering process dominates. But the scientists are not able to exclude magnetic effects as producing some of the X-ray emission. They do conclude, however, that for the magnetic process to be active, any electrons contributing to it must belong to a separate population that is distinct from the electrons that dominate the scattering.


"A Multi-band Study of the Remarkable Jet in Quasar 4C+19.44," D. E. Harris, N. P. Lee, D. A. Schwartz, A. Siemiginowska, F. Massaro, M. Birkinshaw, D. M. Worrall, C. C. Cheung, J. M. Gelbord, Svetlana G. Jorstad, Alan P. Marscher, H. Landt, H. Marshall, E. S. Perlman, L. Stawarz, Y. Uchiyama, and C. M. Urry, ApJ 846, 119, 2017.

Saturday, October 14, 2017

Astronomical airplane trails do not evade but lighten up

The image shows a galaxy in orange that moves to the left and leaves a gas trail. The trail seems to extinguish slowly, but lightens up again near the second, white-yellow galaxy. Most white dots in the image are complete galaxies. (c) Francesco de Gasperin (Leiden University).  Source

An international team of astronomers led by Francesco de Gasperin (Leiden University, the Netherlands) has witnessed an unexpected phenomenon in a merger of a two clusters of galaxies. The astronomers discovered a gas trail that slowly extinguished, but then lit up again. It is unclear where the energy for the rejuvenation of this trail comes from. The researchers publish their findings in Science Advances.

The astronomers investigated Abell 1033. This is a cluster of galaxies consisting of two smaller clusters that are in the process of merging. Abell 1033 is located in the northern constellation of Leo Minor (near Ursa Major). Clusters of galaxies are the largest structures in the universe. They can contain hundreds to thousands of galaxies similar to our Milky Way. Smaller clusters can merge together to form a larger cluster.

The astronomers observed that an individual galaxy in Abell 1033 leaves a trail of gas as it traveled through the cluster. On astronomical scale, such a trail resembles the trail of colored smoke behind a stunt plane.

The astronomers had expected that the gas trail, like the ones behind a stunt plane, would slowly fade and eventually disappear. To their astonishment they saw that the end of the gas trail was brighter than the middle.

"This was totally unexpected," says Francesco de Gasperin, the first author of the research paper that is published in Science Advances. "As these clouds of electrons radiate away their energy over time, they should become fainter and disappear. Instead, in this case, after more than a hundred million years, the trail of electrons is glowing brightly."

There is no precise explanation for this phenomenon, yet. It seems that the trail brightens near the center of the cluster of galaxies. De Gasperin: "Part of the energy released in the merger event must have been transferred to rejuvenate the cloud of electrons." The research on merging clusters of galaxies is complicated because astronomers only see a snapshot of a process that takes billions of years to complete. In addition to that, the telescopes that are needed for the investigation must receive signals at extremely low frequencies. The astronomers combined data from the Indian Giant Metrewave Radio Telescope and LOFAR, the Low Frequency Array. LOFAR was designed and built by the Dutch research institute ASTRON. The telescope consists of thousands of antennas spread across eight countries. The heart of LOFAR is in Drenthe in the north-east of the Netherlands.

"It’s like being among the last explorers. As soon as we move in uncharted territories, or in this case at unexplored frequencies, our universe is still full of surprises," says De Gasperin. "And this is just a first step. Much is still to be done to understand the complexity of galaxy clusters, and find what is lurking at low radio frequencies"


Gentle re-energisation of electrons in merging galaxy clusters. By: F. de Gasperin, H.T. Intema, T.W. Shimwell, G. Brunetti, M. Brüggen, T.A. Enßlin, R.J. van Weeren, A. Bonafede, H.J.A. Röttgering. Accepted for publication in Science Advances. (open access)

Friday, October 13, 2017

Size can be deceptive

Credit: ESA/Hubble & NASA

As far as galaxies are concerned, size can be deceptive. Some of the largest galaxies in the Universe are dormant, while some dwarf galaxies, such as ESO 553-46 imaged here by the NASA/ESA Hubble Space Telescope, can produce stars at a hair-raising rate. In fact, ESO 553-46 has one of the highest rates of star formation of the 1000 or so galaxies nearest to the Milky Way. No mean feat for such a diminutive galaxy!

Clusters of young, hot stars are speckling the galaxy, burning with a fierce blue glow. The intense radiation they produce also causes surrounding gas to light up, which is bright red in this image. The small mass and distinctive colouring of galaxies of this type prompted astronomers to classify them, appropriately, as blue compact dwarfs (BCD).

Lacking the clear core and structure that many larger galaxies — such as the Milky Way — have, BCDs such as ESO 553-46 are composed of many large clusters of stars bound together by gravity. Their chemical makeup is interesting to astronomers, since they contain relatively little dust and few elements heavier than helium, which are produced in stars and distributed via supernova explosions. Such conditions are strikingly similar to those that existed in the early Universe, when the first galaxies were beginning to form.

Thursday, October 12, 2017

Astronomers discover unusual spindle-like galaxies

An elliptical galaxy in prolate rotation. The galaxy resembles the shape of a cigar, with its stars rotating around the galaxy's long axis, similar to a spindle. the background image is a snapshot of a simulation by A. Tsatsi and colleagues. [less] Image: J. Chang, PMO / T. Müller, HdA

Galaxies are majestic, rotating wheels of stars? Not in the case of the spindle-like galaxies studied by Athanasia Tsatsi (Max Planck Institute for Astronomy) and her colleagues. Using the CALIFA survey, the astronomers found that these slender galaxies, which rotate along their longest axis, are much more common than previously thought. The new data allowed the astronomers to create a model for how these unusual galaxies probably formed, namely out of a special kind of merger of two spiral galaxies. The results have been published in the journal Astronomy & Astrophysics.

When most people think of galaxies, they think of majestic spiral galaxies like that of our home galaxy, the Milky Way: billions of stars, rotating in a flat disk similar to the way that a wheel rotates around its central axis. But there is another kind of galaxy, which used to be thought very rare: so-called prolate rotators, each shaped like a cigar, which rotates along its long axis, like a spindle.

Now, a group of astronomers led by Athanasia Tsatsi of the Max Planck Institute for Astronomy has completed a thorough study of these cosmic spindles. Using data from the CALIFA survey, a systematic study that examined the velocity structure of more than 600 galaxies, the astronomers discovered eight new prolate rotating galaxies, almost doubling the total known number of such galaxies (from 12 to 20). Cosmic spindles are considerably less rare than astronomers had thought!

Given the high quality of their data, the astronomers were able to propose a plausible explanation for how these cosmic spindles come into existence. In general, galaxies grow when they merge with other galaxies. Several mergers with smaller galaxies have made our own Milky Way the stately disk it is today. To make a cosmic spindle, two large disk galaxies need to collide at right angles, as shown in this animation:

The formation of an elliptical galaxy in prolate rotation. The mechanism shown here was proposed by Athanasia Tsatsi and her colleagues in order to explain the recent discoveries of galaxies of this kind with the CALIFA survey. The formation involves a polar merger of two spiral galaxies. One of the spiral galaxies develops a marked elongated structure (a "bar," to use the technical term) before the merger, which gives the resulting elliptical galaxy its cigar-like (prolate) shape. The stars of the second spiral galaxy end up orbiting around the bar of the first companion. Together they form a cigar-shaped elliptical galaxy that rotates like a spindle around its long axis.

As the galaxies begin to interact via gravitational attraction, one of them forms a bar: an elongated structure near the center. That bar becomes the cigar-like shape of the merged galaxy, while the orbiting stars of the other galaxy imbue the merged galaxy with its overall sense of rotation.

The results are an interesting piece of the puzzle, explaining a likely formation scenario for an unusual, but not all that uncommon type of galaxy. Tsatsi's team of researchers having put to good use all the information contained in the CALIFA data, the ball is now in the court of the observing astronomers again: the merger simulations make some additional predictions for the detailed properties of prolate rotators. These cannot be distinguished with the current observations, but could be tested with instruments like MUSE, the Multi Unit Spectral Explorer at ESO's Very Large Telescope, an 8-meter-telescope at Paranal Observatory in Chile.

Background information

The results here will be published in the journal Astronomy & Astrophysics as Tsatsi et al., "CALIFA reveals prolate rotation in massive early-type galaxies: A polar galaxy merger origin?"

The team members are Athanasia Tsatsi, Glenn van de Ven, and Andrea V. Macciò (also New York University Abu Dhabi) in collaboration with Mariya Lyubenova (University of Groningen, Netherland, now at ESO), J. Chang (Purple Mountain Observatory, Nanjing, China), J. A. L. Aguerri and J. Falcón-Barroso (both Instituto de Astrofísica de Canarias and Universidad de La Laguna, Tenerife, Spain).

Calar Alto Observatory was founded in 1979 and is located in Andalusia, Spain. It is operated jointly by the Max Planck Institute for Astronomy (MPIA) and the Astrophysical Institute of Andalusia (IAA-CSIC, Granada, Spain). The Observatory has granted 250 observing nights over the course of three years, using the 3.5 metre telescope for the CALIFA survey. This project is a joint effort of more than 80 scientists from 25 different research institutes in 13 different countries world wide.

The integral field spectrograph used for the CALIFA survey at Calar Alto Observatory, PMAS (in a special configuration called PPAK), uses more than 350 optical fibres to cover a field of view of one square arcminute (equivalent to the apparent size of a 1 euro coin placed at a distance of approximately 80 metres). This allows a complete extended object, such as a galaxy, to be fully mapped in detail in just one exposure.

For the CALIFA survey, care has been taken to select the possible observation targets at random from the overall population of galaxies. In that way, the galaxies under study should be representative of the whole: Statistical conclusions from the analysis of their data should thus allow astronomers to draw conclusions about local galaxies in general. 

The CALIFA member institutions are: Astrophysical Institute, Academy of Sciences of the Czech Republic, Prague; Australian Astronomical Observatory, Australia; Centro Astronómico Hispano Alemán, Spain; Centro de Astrofísica da Universidade do Porto, Portugal; Institut d'Astrophysique de Paris, France; Instituto de Astrofisica de Andalucia, Spain; Instituto de Astrofisica de Canarias, Spain; Instituto de Física de Cantabria, Spain; Laboratoire d'Astrophysique de Marseille, France; Leibniz Institut für Astrophysik, Potsdam, Germany; Max Planck Institute for Astronomy, Heidelberg, Germany; Observatoire de Paris, France; Peking University – Kavli Institute for Astronomy and Astrophysics, China; Royal Military College of Canada, Canada; Tianjin Normal University, China; Universidad Autónoma de Madrid, Spain; Universidad de Complutense de Madrid, Spain; Universidad de Granada, Spain; Universidad de Zaragoza, Spain; University of Bochum, Germany; University of Cambridge, UK; University of Copenhagen – Dark Cosmology Centre, Denmark; University of Edingurgh, UK; University of Groningen – Kapteyn Astronomical Institute, The Netherlands; University of Heidelberg – Landessternwarte Königstuhl, Germany; University of Lisbon, Portugal; University of Missouri-Kansas City, USA; University of Porto, Portugal; University of Sidney, Australia; University of Vienna, Austria

Science Contact:

Athanasia Tsatsi
Max Planck Institute for Astronomy

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Markus Pössel
Public Information Officer
Phone:(+49|0) 6221 528-261

Wednesday, October 11, 2017

Giant Exoplanet Hunters: Look for Debris Disks

This artist's rendering shows a giant exoplanet causing small bodies to collide in a disk of dust. A study in The Astronomical Journal finds that giant exoplanets with long-period orbits are more likely to be found around young stars that have a disk of dust and debris than those without disks. The study focused on planets more than five times the mass of Jupiter. The astronomers are conducting the largest survey to date of stars with dusty debris disks, and finding the best evidence yet that giant planets are responsible for keeping that material in check.  Credit NASA/JPL-Caltech/T. Pyle (IPAC)

There's no map showing all the billions of exoplanets hiding in our galaxy -- they're so distant and faint compared to their stars, it's hard to find them. Now, astronomers hunting for new worlds have established a possible signpost for giant exoplanets.

A new study finds that giant exoplanets that orbit far from their stars are more likely to be found around young stars that have a disk of dust and debris than those without disks. The study, published in The Astronomical Journal, focused on planets more than five times the mass of Jupiter. This study is the largest to date of stars with dusty debris disks, and has found the best evidence yet that giant planets are responsible for keeping that material in check.

"Our research is important for how future missions will plan which stars to observe," said Tiffany Meshkat, lead author and assistant research scientist at IPAC/Caltech in Pasadena, California. Meshkat worked on this study as a postdoctoral researcher at NASA's Jet Propulsion Laboratory in Pasadena. "Many planets that have been found through direct imaging have been in systems that had debris disks, and now we know the dust could be indicators of undiscovered worlds."

Astronomers found the likelihood of finding long-period giant planets is nine times greater for stars with debris disks than stars without disks. Caltech graduate student Marta Bryan performed the statistical analysis that determined this result.

Researchers combined data from 130 single-star systems with debris disks detected by NASA's Spitzer Space Telescope, and compared them with 277 stars that do not appear to host disks. The two star groups were between a few million and 1 billion years old. Of the 130 stars, 100 were previously scanned for exoplanets. As part of this study, researchers followed up on the other 30 using the W. M. Keck Observatory in Hawaii and the European Southern Observatory's Very Large Telescope in Chile. They did not detect any new planets in those 30 systems, but the additional data helped characterize the abundance of planets in systems with disks.

The research does not directly resolve why the giant exoplanets would cause debris disks to form. Study authors suggest the massive gravity of giant planets causes small bodies called planetesimals to collide violently, rather than form proper planets, and remain in orbit as part of a disk.

"It's possible we don't find small planets in these systems because, early on, these massive bodies destroyed the building blocks of rocky planets, sending them smashing into each other at high speeds instead of gently combining," said co-author Dimitri Mawet, a Caltech associate professor of astronomy and a JPL senior research scientist.

On the other hand, giant exoplanets are easier to detect than rocky planets, and it is possible that there are some in these systems that have not yet been found.

Our own solar system is home to gas giants responsible for making "debris belts" -- the asteroid belt between Mars and Jupiter, shaped by Jupiter, and the Kuiper Belt, shaped by Neptune. Many of the systems Meshkat and Mawet studied also have two belts, but they are also much younger than ours -- up to 1 billion years old, compared to our system's present age of 4.5 billion years. The youth of these systems partly explains why they contain much more dust -- resulting from the collisions of small bodies -- than ours does.

One system discussed in the study is Beta Pictoris, which has been directly imaged from ground-based telescopes. This system has a debris disk, comets and one confirmed exoplanet. In fact, scientists predicted this planet's existence well before it was confirmed, based on the presence and structure of the prominent disk.

In a different scenario, the presence of two dust belts in a single debris disk suggests there are likely more planets in the system whose gravity maintains these belts, as is the case in the HR8799 system of four giant planets. The gravitational forces of giant planets nudge passing comets inward toward the star, which could mimic the period of our solar system's history about 4 billion years ago known as the Late Heavy Bombardment. Scientists think that during that period, the migration of Jupiter, Saturn, Uranus and Neptune deflected dust and small bodies into the Kuiper and asteroid belts we see today. When the Sun was young, there would have been a lot more dust in our solar system as well.

"By showing astronomers where future missions such as NASA's James Webb Space Telescope have their best chance to find giant exoplanets, this research paves the way to future discoveries," said Karl Stapelfeldt of JPL, chief scientist of NASA's Exoplanet Exploration Program Office and study co-author.

For more information about exoplanets, visit:

Elizabeth Landau
Jet Propulsion Laboratory, Pasadena, CA

Monday, October 09, 2017

Mysterious Dimming of Tabby's Star May Be Caused by Dust

This illustration depicts a hypothetical uneven ring of dust orbiting KIC 8462852, also known as Boyajian's Star or Tabby's Star. Credit: NASA/JPL-Caltech.  › Full image and caption

One of the most mysterious stellar objects may be revealing some of its secrets at last.

Called KIC 8462852, also known as Boyajian's Star, or Tabby's Star, the object has experienced unusual dips in brightness -- NASA's Kepler space telescope even observed dimming of up to 20 percent over a matter of days. In addition, the star has had much subtler but longer-term enigmatic dimming trends, with one continuing today. None of this behavior is expected for normal stars slightly more massive than the Sun. Speculations have included the idea that the star swallowed a planet that it is unstable, and a more imaginative theory involves a giant contraption or "megastructure" built by an advanced civilization, which could be harvesting energy from the star and causing its brightness to decrease.

A new study using NASA's Spitzer and Swift missions, as well as the Belgian AstroLAB IRIS observatory, suggests that the cause of the dimming over long periods is likely an uneven dust cloud moving around the star. This flies in the face of the "alien megastructure" idea and the other more exotic speculations.

The smoking gun: Researchers found less dimming in the infrared light from the star than in its ultraviolet light. Any object larger than dust particles would dim all wavelengths of light equally when passing in front of Tabby's Star.

"This pretty much rules out the alien megastructure theory, as that could not explain the wavelength-dependent dimming," said Huan Meng, at the University of Arizona, Tucson, who is lead author of the new study published in The Astrophysical Journal. "We suspect, instead, there is a cloud of dust orbiting the star with a roughly 700-day orbital period."

Why Dust is Likely

We experience the uniform dimming of light often in everyday life: If you go to the beach on a bright, sunny day and sit under an umbrella, the umbrella reduces the amount of sunlight hitting your eyes in all wavelengths. But if you wait for the sunset, the sun looks red because the blue and ultraviolet light is scattered away by tiny particles.The new study suggests the objects causing the long-period dimming of Tabby's Star can be no more than a few micrometers in diameter (about one ten-thousandth of an inch).

From January to December 2016, the researchers observed Tabby's Star in ultraviolet using Swift, and in infrared using Spitzer. Supplementing the space telescopes, researchers also observed the star in visible light during the same period using AstroLAB IRIS, a public observatory with a 27-inch-wide (68 centimeter) reflecting telescope located near the Belgian village of Zillebeke.

Based on the strong ultraviolet dip, the researchers determined the blocking particles must be bigger than interstellar dust, small grains that could be located anywhere between Earth and the star. Such small particles could not remain in orbit around the star because pressure from its starlight would drive them farther into space. Dust that orbits a star, called circumstellar dust, is not so small it would fly away, but also not big enough to uniformly block light in all wavelengths. This is currently considered the best explanation, although others are possible.

Collaboration with Amateur Astronomers

Citizen scientists have had an integral part in exploring Tabby's Star since its discovery. Light from this object was first identified as "bizarre" and "interesting" by participants in the Planet Hunters project, which allows anyone to search for planets in the Kepler data. That led to a 2016 study formally introducing the object, which is nicknamed for Tabetha Boyajian, now at Louisiana State University, Baton Rouge, who was the lead author of the original paper and is a co-author of the new study. The recent work on long-period dimming involves amateur astronomers who provide technical and software support to AstroLAB.

Several AstroLAB team members who volunteer at the observatory have no formal astronomy education. Franky Dubois, who operated the telescope during the Tabby's Star observations, was the foreman at a seat belt factory until his retirement. Ludwig Logie, who helps with technical issues on the telescope, is a security coordinator in the construction industry. Steve Rau, who processes observations of star brightness, is a trainer at a Belgian railway company.

Siegfried Vanaverbeke, an AstroLAB volunteer who holds a Ph.D. in physics, became interested in Tabby's Star after reading the 2016 study, and persuaded Dubois, Logie and Rau to use Astrolab to observe it.

"I said to my colleagues: 'This would be an interesting object to follow,'" Vanaverbeke recalled. "We decided to join in."

University of Arizona astronomer George Rieke, a co-author on the new study, contacted the AstroLAB group when he saw their data on Tabby's Star posted in a public astronomy archive. The U.S. and Belgium groups teamed up to combine and analyze their results.

Future Exploration

While study authors have a good idea why Tabby's Star dims on a long-term basis, they did not address the shorter-term dimming events that happened in three-day spurts in 2017. They also did not confront the mystery of the major 20-percent dips in brightness that Kepler observed while studying the Cygnus field of its primary mission. Previous research with Spitzer and NASA's Wide-field Infrared Survey Explorer suggested a swarm of comets may be to blame for the short-period dimming. Comets are also one of the most common sources of dust that orbits stars, and so could also be related to the long-period dimming studied by Meng and colleagues.

Now that Kepler is exploring other patches of sky in its current mission, called K2, it can no longer follow up on Tabby's Star, but future telescopes may help unveil more secrets of this mysterious object.

"Tabby's Star could have something like a solar activity cycle. This is something that needs further investigation and will continue to interest scientists for many years to come," Vanaverbeke said.

NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the Swift mission in collaboration with Pennsylvania State University in University Park, the Los Alamos National Laboratory in New Mexico, and Orbital Sciences Corp. in Dulles, Virginia. Other partners include the University of Leicester and Mullard Space Science Laboratory in the United Kingdom, Brera Observatory and the Italian Space Agency in Italy, with additional collaborators in Germany and Japan.

NASA's Jet Propulsion Laboratory, Pasadena, California, manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at Caltech in Pasadena, California. Spacecraft operations are based at Lockheed Martin Space Systems Company, Littleton, Colorado. Data are archived at the Infrared Science Archive housed at the Infrared Processing and Analysis Center at Caltech. Caltech manages JPL for NASA.

For more information about Spitzer, visit: -

News Media Contact

Elizabeth Landau
Jet Propulsion Laboratory, Pasadena, Calif.