Colleagues and Students Gather for Astrophysics Symposium

by Roopesh Ojha (PhD ’98)

Radio Galaxy NGC 4261. (credit: Teddy Cheung)

From June 28th through 30th, about fifty former and current students, colleagues and friends of Brandeis astrophysics Professors John Wardle and David Roberts gathered in the Physics building for a symposium titled “When Brandeis met Jansky: astrophysics and beyond.” This event was organized to celebrate their achievements in astrophysics and their impact on generations of students. Their work has established Brandeis as a major player in radio astronomy.

The symposium title refers to Karl Jansky who is credited with starting an entirely new means of studying the cosmos using radio waves. Radio astronomy arrived at Brandeis with Professor Wardle in 1972. He was joined in 1980 by Professor Roberts and together they pioneered a very powerful observational technique called Very Long Baseline Polarimetry. This involves the use of telescopes separated by thousands of kilometers to produce the sharpest images available to astronomers. Their methods allow astronomers to map the magnetic fields in and near celestial objects. With their students and colleagues, John and Dave have exploited this technique to study the magnetic fields in quasars and active galaxies, and near super massive black holes far outside our Milky Way galaxy as well as black holes closer to home.

Physics Conference Group

Professors John Wardle and David Roberts (front right) with former students and colleagues on the steps of the Abelson physics building (photo: Mike Lovett)

The reach of John and Dave’s work was reflected in the content of the presentations and the composition of the attendees, some of whom had traveled from as far afield as South Korea, India, and Europe. All major centers of radio astronomy were represented. At the conference dinner, several former students expressed their appreciation for the roles Dave and John have played as their mentors.

In their presentations, Dave and John described their current projects and highlighted the work of their undergraduates, graduate students and postdoctoral fellows, who have all gone on to successful careers in academia and industry.

The nineteen PhD theses produced by the Brandeis Radio Astronomy group

Professor Roberts has decided to retire at the end of August, though his retirement plans include a huge program of continuing research into unusual-shaped radio galaxies. These may represent galaxy mergers and the possible merger of their central black holes, and is being carried out with colleagues in India. Professor Wardle has no intention of retiring and is expanding his horizons so to speak — he is part of the Event Horizon Telescope collaboration, an international team of astronomers that is attempting to make the first image of the ‘event horizon’* of a black hole!

The symposium was organized by Teddy Cheung (PhD ’05, now at the Naval Research Laboratory) and Roopesh Ojha (PhD ’98, now at NASA, Goddard Space Flight Center), with generous help and support from the Physics Department.

* The boundary around a black hole beyond which nothing can escape.

Resolving the magnetic field around the galaxy’s central black hole

Credit: M. Weiss/CfA

Credit: M. Weiss/CfA

On December 4, the journal Science (Vol. 350 no. 6265 p 1242) published a paper titled, “Resolved magnetic-field structure and variability near the event horizon of Sagittarius A*” (abstract). The paper reports that the Event Horizon Telescope has detected strong magnetic fields around the supermassive black hole at the center of the Milky Way galaxy. John Wardle, Professor of Astrophysics at Brandeis, is one of the lead authors. A co-author is Michael Kosowsky ’14, who worked on the project as a summer research project at the MIT-Haystack observatory as a junior physics major, and is now an NSF Graduate Research Fellow at Harvard.

Near a black hole, differential rotation of a magnetized accretion disk is thought to produce an instability that amplifies weak magnetic fields, driving accretion and outflow. These magnetic fields would naturally give rise to the observed synchrotron emission in galaxy cores and to the formation of relativistic jets, but no observations to date have been able to resolve the expected horizon-scale magnetic-field structure. The paper reports interferometric observations (made with antennas in Hawaii, California and Arizona) at 1.3-millimeter wavelength that spatially resolve the linearly polarized emission from the Galactic Center supermassive black hole, Sagittarius A*. We have found evidence for partially ordered magnetic fields near the event horizon, on scales of ~6 Schwarzschild radii, and we have detected and localized the intra-hour variability associated with these fields.

The above image is an artist’s impression. With the planned addition of antennas in Mexico, Chile, Europe and the South Pole, the Event Horizon Telescope will be able to make true images with angular resolution of a few tens of microarcseconds.

Pairs of Supermassive Black Holes May Be Rarer Than Earlier Thought

Image by David Roberts

Image by David Roberts

Recent research by David H. Roberts, William R. Kenan, Jr. Professor of Astrophysics at Brandeis, has shown that pairs of supermassive black holes at the centers of galaxies are less common than previously thought. This suggests that the level of gravitational radiation from such systems is lower than earlier predicted. This work was in collaboration with Lakshmi Saripalli and Ravi Subrahmanyan of the Raman Research Institute in Bangalore, and much of the work was done by Brandeis undergraduate students Jake Cohen and Jing Liu. It has recently been published in a pair of papers in the Astrophysical Journal Supplements and Astrophysical Journal Letters.

Gravitational waves are ripples in space-time predicted by Einstein’s 1915 General Theory of Relativity. Propagating at the speed of light, they are produced in astrophysical events such as supernovae and close binary stars.

No direct experimental evidence of the existence of gravitational waves has been found to date. We know that they exist because they sap energy from the orbits of binary systems, and using ultra-precise radio astronomy it has been shown that the changes in binary orbits of pairs of pulsars (magnetized neutron stars) are precisely as predicted by General Relativity. Hulse and Taylor were awarded the Nobel Prize in Physics for their contributions to this work.

The largest source of gravitational waves is expected to be the coalescence of pairs of supermassive black holes in the centers of large galaxies. We know today that galaxies grow by mergers, and that every galaxy harbors a massive black hole at its center, with mass roughly proportional to the galaxy’s mass. When two massive galaxies merge to form a larger galaxy, it will contain a pair of black holes instead of a single one. Through a process involving the gravitational scattering of ordinary stars the two black holes migrate toward each other and eventually coalesce into a single even more massive black hole. The process of coalescence involves “strong gravity,” that is, it occurs when the separation of the two merging black holes becomes comparable to their Schwarzschild radii. Recent developments in numerical relativity have made it possible to study the coalescence process in the computer, and predictions may be made about the details of the gravitational waves that emerge. Thus direct detection of gravitational waves will enable tests of General Relativity not achievable any other way.

In order to predict the amount of gravitational radiation present in the Universe it is necessary to estimate by other methods the rate at which massive galaxies are colliding and their black holes coalescing. One way to do this is to examine the small number of radio galaxies that have unusual morphologies that suggest that they were created by the process of a spin-flip of a supermassive black hole due to its interaction with a second supermassive black hole. These are the so-called “X-shaped radio galaxies” (“XRGs”), and a naive counting of their numbers suggests that they are about 6% of all radio galaxies. Using this and knowing the lifetime of such an odd radio structure it is possible to determine the rate at which massive galaxies are merging and their black holes coalescing.

Roberts et al. re-examined this idea, and made a critical assessment of the mechanism of formation of XRGs. It turns out that other mechanisms can easily create such odd structures, and according to their work the large majority of XRGs are not the result of black hole-black hole mergers at all. They suggest as a result that the rate of supermassive black hole mergers may have been overestimated by a factor of three to five, with the consequence that the Universe contains that much less gravitational radiation than previously believed. In fact, recent results from searches for such gravitational waves have set upper limits below previous predictions, as might expect from this work.

For more information:

 

Michael Kosowsky ’14 receives NSF Graduate Research Fellowship

KosowskyMichael Kosowsky ’14, who majored in both physics and mathematics at Brandeis, has been awarded a National Science Foundation Graduate Research Fellowship in astronomy and astrophysics.  The fellowships, which are awarded based on a national competition, provide three full years of support for Ph.D. research and are highly valued by students and institutions. Kosowsky worked with Prof. David Roberts in the Physics Department on analyzing the polarization of the X-ray binary SS 433 with the purpose of figuring out the magnetic field structure of the source.  He will be pursuing a Ph.D. in physics at Harvard University starting this fall.

Other 2014 NSF Fellowship recipients from Brandeis include:

Alex Dainis  (BS ’11, Biology, Film, Television, Interactive Media), Stanford University
Abby Finkelstein (BS ’13, Neuroscience),  Arizona State University
Lamia Harper (BS ’12, Biology), NYU
Ariel Hyre  (BS ’13,  Chemistry), Boston University
Anatoly Rinberg (BS ’11, Physics, Mathematics), Stanford University
Seth Werfel  (BA ’10, Economics), Stanford University

 

Brandeis undergrad research on cover of Astronomical Journal

3C345CoverWithAuthors1kThe editors of the Astronomical Journal chose an image from a Brandeis research paper to adorn the cover of the February issue of the Journal (see right). What is sweet about this is that the image was made by Valerie Marchenko, a senior physics major who has been doing research since her freshman year, initially with Dave Roberts, and presently with John Wardle in the Physics Department. Several of the images in the paper were made by Valerie, and of course she is a co-author. This is actually her second publication in a mainline astronomical journal.

Roberts DH, Wardle JFC, Marchenko VV. The Structure and Linear Polarization of the Kiloparsec-scale Jet of the Quasar 3C 345. The Astronomical Journal. 2013;145(2):49.

Brandeis undergraduates publish upward of 20 papers a year in scientific journals along with their faculty, postdoc and grad student mentors.

Record-Setting X-ray Jet Discovered

X-ray jet

X-ray jet from quasar GB 1428, located 12.4 billion light years from Earth. (X-ray: NASA/CXC/NRC/C.Cheung et al; Optical: NASA/STScI; Radio: NSF/NRAO/VLA)

On November 28, NASA posted a press release announcing the record breaking discovery of an x-ray emitting jet in a quasar at a distance of 12.4 billion light years from Earth. The discovery is published in the Astrophysical Journal Letters, and the lead author is C. (Teddy) Cheung (Brandeis PhD 2004). Co-authors include Doug Gobeille (Brandeis PhD 2011), Brandeis professor of astrophysics John Wardle, and colleagues from the Harvard-Smithsonian Center for Astrophysics. Teddy Cheung made the x-ray image, using the orbiting Chandra X-ray observatory, and Doug Gobeille made the radio image as part of his PhD research at Brandeis using the 27 antennas of the Very Large Array in New Mexico.

A jet of X-ray emitting plasma from a supermassive black hole 12.4 billion light years from Earth has been detected by NASA’s Chandra X-ray Observatory. This is the most distant X-ray jet ever observed and gives astronomers a glimpse into the explosive activity associated with the growth of supermassive black holes in the early universe. The jet was produced by a quasar named GB 1428+4217, or GB 1428 for short. Giant black holes at the centers of galaxies can pull in matter at a rapid rate producing the quasar phenomenon. The energy released as particles fall toward the black hole generates intense radiation and powerful beams of high-energy particles that blast away from the black hole at nearly the speed of light. These particle beams can interact with magnetic fields or ambient photons to produce jets of radiation.

“We’re excited about this result not just because it’s a record holder, but because very few X-ray jets are known in the early universe,” said Teddy Cheung of the National Academy of Sciences, resident at the Naval Research Laboratory in Washington DC, and lead author of the paper describing these results.

As the electrons in the jet fly away from the quasar, they move through a sea of background photons left behind after the Big Bang. When a fast-moving electron collides with one of these so-called cosmic microwave background photons, it can boost the photon’s energy into the X-ray band.

“Since the brightness of the jet in X-rays depends, among other things, on how fast the electrons are moving away from the black hole, discoveries like the jet in GB 1428 tell us something about the environment around supermassive black holes and their host galaxies not that long after the Big Bang,” said co-author Lukasz Stawarz from the Japan Aerospace Exploration Agency, in Kanagawa, Japan.

Because the quasar is seen when the universe is at an age of about 1.3 billion years, less than 10% of its current value, the cosmic background radiation is a thousand times more intense than it is now. This makes the jet much brighter, and compensates in part for the dimming due to distance.

“We’re lucky that the universe gives us this natural amplifier and lets us detect this object with relatively short exposures,” said co-author Aneta Siemiginowska, of the Harvard-Smithsonian Center for Astrophysics in Cambridge, MA, “Otherwise we might miss important physical processes happening at very large distances from Earth and as far away as GB 1428.”

While there is another possible source of X-rays for the jet — radiation from electrons spiraling around magnetic field lines in the jet — the authors favor the idea that the cosmic background radiation is being boosted because the jet is so bright.

Prior to the discovery of the jet in GB 1428, the most distant X-ray jet known was 12.2 billion light years away, and another is located at about 12 billion light years, both discovered by authors of the GB 1428 paper. A very similar shaped jet in GB 1428 was also detected in radio waves with the NSF’s Very Large Array (VLA).

The particle beams that produce these three extremely distant X-ray jets appear to be moving slightly more slowly than jets from galaxies that are not as far away. This may be because the jets were less energetic when launched from the black hole or because they are slowed down more by their environment.

The researchers think the length of the jet in GB 1428 is at least 230,000 light years, or about twice the diameter of the entire Milky Way galaxy. This jet is only seen on one side of the quasar in the Chandra and VLA data. When combined with previously obtained evidence, this suggests the jet is pointed almost directly toward us. This configuration would boost the X-ray and radio signals for the observed jet and diminish those for a jet presumably pointed in the opposite direction.

Observations were also taken of GB 1428 with a set of radio telescopes at different locations around the Earth that allows details to be resolved on exceptionally small scales. They revealed the presence of a much smaller jet, about 1,900 light years long, which points in a similar direction to the X-ray jet.

This result appeared in the September 1st, 2012 issue of The Astrophysical Journal Letters. Other co-authors of the paper are Doug Gobeille from University of South Florida in Tampa, FL; John Wardle from Brandeis University in Waltham, MA; and Dan Harris and Dan Schwartz from the Harvard-Smithsonian Center for Astrophysics.

NASA’s Marshall Space Flight Center in Huntsville, Ala., manages the Chandra Program for NASA’s Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory controls Chandra’s science and flight operations from Cambridge, Mass.

More information, including images and other multimedia, can be found at:

http://www.nasa.gov/chandra
and
http://chandra.si.edu

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