Astronomy & Astrophysics Faculty
My research focuses on using multi-messenger astronomy to study the Universe, coming at the same problem from multiple directions to gain a more complete picture. In particular, I study the coalescence of binary neutron stars with both gravitational waves and electromagnetic data, predominantly using wide field-of-view optical telescopes such as the Zwicky Transient Facility (ZTF) to identify these counterparts. I also use these telescopes to search for future sources from the Laser Interferometer Space Antenna (LISA), a space-based gravitational wave detector that will study white dwarf binaries in our galaxy as well as binary black hole mergers.
The VERITAS (Very Energetic Radiation Imaging Telescope Array System) is a set of four 12 meter telescopes located at the Whipple Observatory near Tucson, AZ. They are sensitive to the ultraviolet Cherenkov radiation emitted as a by-product of the air showers generated from gamma-rays hitting the Earth’s atmosphere. By geometrically combining the camera images of each telescope’s “perspective” on the air shower, the array can reconstruct the energy and direction of the originating gamma ray. VERITAS is sensitive to gamma rays between 85 GeV and 30 TeV and as such detects some of the highest energy photons emitted in the Universe. Thus, very high-energy (VHE) gamma-ray astronomy probes the extreme physics of systems such as pulsars or the remnants of supernova explosions in our galaxy or even searching for indirect detection of dark matter through annihilation into gamma rays. Work at UMN focuses on Active Galactic Nuclei (AGN) which are cores of galaxies believed to contain supermassive black holes accreting matter and powering kiloparsec-scale jets of relativistic particles that generate non-thermal emission which often outshines the host galaxy. To date, all but five of the 72 AGN detected at VHE energies are blazars – AGN where we view the jet nearly along its axis; the other five are Radio Galaxies. Radio galaxies are blazars with jets viewed at systematically larger angles to the line of sight than blazars, these objects are inherently more challenging to detect at VHE due to decreased Doppler boosting. Obtaining a larger sample of VHE-detected radio galaxies will allow further study of a range of important topics that have proven difficult to resolve with pure VHE blazar observations alone including the location(s) of gamma-ray emission in radio-loud AGN as well as acceleration mechanisms and the linkages between classic radio galaxies and blazar populations.
Advisees & Collaborators
- Nico Adams, Graduate Student, Astronomy - Galaxy Zoo
- Suhail Alnahari, Graduate Student, Computer Science - Zooniverse
- Ann Isaacs, Graduate Student, Astronomy - VERITAS
- Abinash Sinha, Graduate Student, Computer Science - Zooniverse
- Patrick Wilcox, Postdoctoral Associate, VERITAS
- Darryl Wright, Postdoctoral Associate, Zooniverse
- Erik Ostlund, Zooniverse Web Developer
Gehrz is an expert in infrared ground and space-based observational astrophysics, instrumentation development, and telescope construction. His primary research is on the physical properties of astrophysical grains in interstellar, circumstellar, and solar system environments, the physics of nova explosions and their chemical contributions to the interstellar medium, the physical characteristics of the circumstellar ejecta of luminous evolved stars, the infrared morphology of regions of star formation, and the infrared activity of comet nuclei.
Occasionally (actually fairly often!) the Sun suddenly releases huge amounts of energy from its magnetic fields. While this energy constitutes a negligible fraction of the total energy in the Sun, each energy release provokes intense heating, bright radiation, and efficient acceleration of particles in the Sun's outermost atmosphere. Sometimes these events eject masses of plasma into interplanetary space that can impact the Earth's magnetic field, causing the aurorae and posing radiation risks to spacecraft and astronauts, and electromagnetic risks to power grids. It is still not known how these "solar flares" convert energy so efficiently and accelerate particles to such high energies as are observed. Moreover, it is not apparent what determines when a large flare will occur, nor how large it may be. The solar astrophysics group at UMN attempts to evaluate these phenomena by studying X-rays emitted from solar flares. We study all types of flares on the Sun, from large eruptive events all the way down to small, frequent ones. This requires using data from cutting-edge solar telescopes (RHESSI, NuSTAR, SDO) as well as developing new telescopes to better investigate the high-energy Sun. Our instruments have flown on sounding rockets and high altitude balloons, and we are currently working on CubeSats and a NASA Small Explorer mission. The capabilities of these new instruments will open a new door by which we can understand some of the most energetic phenomena in the solar system.
We are building instruments with which we observe the cosmic microwave background radiation (CMB). The CMB is a relic remnant from the big bang. Detailed characterization of the properties of the CMB have given and will continue to give a tremendous amount of information about the evolution of the universe. By 'evolution of the universe' we mean: throughout most of the age of the universe, from immediately after the bang until galaxies and clusters of galaxies formed.
I am a theoretical astrophysicist working primarily on problems that involve study of fluid dynamics (including magnetohydrodynamics -- MHD), turbulence, plasma processes and the resulting acceleration, transport, and emissions of high energy particles (cosmic rays). Our efforts center on a broad range of astrophysical contexts ranging from supernova remnants in the galaxy on parsec scales, to high powered plasma jets associated with radio galaxies on kiloparsec scales, to the formation and evolution of galaxy clusters, especially the plasma media filling them on megaparsec scales. We often focus on the complex interactions among these diverse phenomena and their consequences. Much of our work involves numerical simulations carried out largely using codes developed in our group.
Right: The example image shows the simulated radio emission from a radio galaxy in a cluster of galaxies just after it has been overrun by a shock wave propagating through the cluster. Such interactions may be useful as lamp posts to reveal the physics of radio galaxies and of galaxy clusters.
My research focuses on supernova (SN) explosions and gravitational lensing by galaxies and galaxy clusters. Supernovae (SNe) offer the opportunity to study the deaths of individual stars far beyond the local universe, and thermonuclear SNe Ia are excellent tools for measuring the expansion history of the universe. Recent work includes the discovery of an individual, luminous blue supergiant star at redshift z=1.5 (a look-back time of 9.3 billion years) which is highly magnified (~2000 at peak) by a foreground galaxy cluster. The star's microlensing flux variations offer a new window into the nature of galaxy-cluster dark matter, the initial mass function, and the evolution of massive stars. The blue supergiant star was found in the same lensed galaxy where the first-known multiply imaged, strongly lensed SN (dubbed SN Refsdal) appeared in late 2014 in an Einstein cross configuration. The timing of a subsequent reappearance of the SN, at an offset of ~8 arcseconds from its original location, in 2015 disagrees with most but not all predictions, and illustrates a promising approach for identifying the most accurate cluster-modeling techniques and magnification maps.
My research involves the theory and modeling of Ultra-Low-Frequency (ULF) waves in the magnetospheres of Earth and other planets and their relationship to auroral particle acceleration. ULF waves (frequencies less than 5 Hz) are described by the theory of magnetohydrodynamics (MHD), which in the ideal case predicts no electric field parallel to the background magnetic field. However, in the auroral zone, the ideal MHD approximation is violated and parallel electric fields can develop and accelerate auroral particles. The propagation of ULF waves is complicated by the inhomogeneous plasma in the magnetosphere. In particular, a strong gradient in the phase velocity of these waves (known as the Alfven speed) can cause reflections in the topside ionosphere that can help explain the particle acceleration in the aurora. This region is called the ionospheric Alfven resonator.
Dr. Kazue Takahashi (Applied Physics Lab, Johns Hopkins University)
Professor Colin Waters (University of Newcastle, New South Wales, Australia)
Dr. Murray Sciffer (University of Newcastle, New South Wales, Australia)
Professor Ian Mann (University of Alberta, Edmonton, Canada)
My research aims to answer the following questions: (1) How were the chemical elements produced in the universe? (2) How did the universe get enriched in these elements? (3) How did galaxies form? (4) How did fundamental particles, especially neutrinos, influence the above processes?
Dr. Scarlata's research focuses on the poorly-understood balance between gas inflow (accretion of gas from the circumgalactic medium), outflow (energetic feedback into the interstellar medium due to stellar winds, supernova explosions, and black-hole activity) and gas consumption rate (star-formation). This energy exchange determines the physical properties of the gas (e.g., temperature, density, kinematics) that shape the mass-metallicity relation and the galaxy luminosity function, enrich and pollute the intergalactic medium, and regulate the transport of ionizing photons. Dr. Scarlata's work is mostly observational in nature, targeting galaxies at different cosmic epochs to directly trace their evolution while using local galaxies as laboratories to understand their distant analogs.
I have been using the Hubble Space Telescope to study the resolved stellar populations of nearby galaxies. These observations allow the reconstruction of the star formation histories of these galaxies. For the dwarf galaxies, we can look for the imprint of the epoch of reionization. I have also been using the Large Binocular Telescope (LBT) to derive chemical abundances from star-forming regions. Using the Multi-Object Double Spectrograph (MODS) on the allows the simultaneous measurements of dozens of star-forming regions in spiral galaxies so that we can measure the gradients and dispersions in their chemical abundances.
Most of my work has to do with dark matter: measuring the amount, and, more importantly, its spatial distribution on a wide range of astrophysical scales, from sub-galactic to super-cluster. Detailed knowledge of dark matter distribution will enable us to place constraints on the physical properties of dark matter particles. Since dark matter is invisible, mapping out its distribution must rely on indirect methods. While a few mass reconstruction methods exist, gravitational lensing is the optimal one, as it does not rely on any assumptions about the physical state of the dark matter or the light emitting matter. Another interesting aspect of dark matter is the study of its dynamics in galaxies and clusters of galaxies. In the last decade we have learned much about the properties of relaxed dark matter halos from numerical computer simulations, however, understanding the physics behind these results is still an open issue. This is the other major direction of my research.
Dr. Woodward is an international expert in XUVOIR ground, and space-based observational astrophysics, instrumentation development, and telescope construction, management, and operations. He also has significant experience in national space policy. Dr. Woodward’s primary research is on the physical properties of astrophysical grains in interstellar, circumstellar, and solar system environments, the physics of nova explosions and their chemical contributions to the interstellar medium, the physical characteristics of the circumstellar ejecta of evolved stars, evolved stellar population demographics, and the infrared activity of comet nuclei and small solar system bodies. He played a significant role in the programmatic development of NASA's Space Infrared Telescope Facility (Spitzer) legacy sciences opportunities, and has participated in mentoring programs to enhance diversity in the field of astrophysics. His research is supported by NSF and NASA.
Fellow, American Astronomical Society (AAS)
Board of Directors, American Institute of Physics (AIP)
NASA Astrophysics Advisory Committee (APAC)
Fellow of the African Science Institute (ASI)
Coordinator, Large Binocular Telescope Observatory (LBTO)
NASA Advisory Council (NAC)
Chair, SOFIA Users Group
Below: The AGOS adaptive optics system on the Large Binocular Telescope (Arizona) is used by Woodward to pursue a variety of ground based planetary and stellar astrophysics research.
Over the past several years, we have been simulating brief but important events in the evolution of stars that stretch our ability to harness the power of present large computing systems and that will continue to put demanding requirements on future systems. These events are brief, in that they last only a few days, or even less, while typical time scales for stars are very much longer. Nevertheless, they are not as brief as explosive events, and for this reason, we must simulate the behavior of the star for a great many dynamical times. These events also depend critically upon processes that are inherently 3D. The events we simulate occur in deep convection zones that develop above nuclear burning shells. We find that the most important modes of convection in these shells are comparable in size to the entire shells themselves, forcing us to include in our simulation domain the entire interior region of the star, not just a small section of it. Our simulations therefore present the challenge that we must describe the whole stellar interior over a long time interval, which requires millions of time steps. Because we are unwilling to wait the necessary time for these simulations to complete on a small cluster of machines—which would be years—we must find a way to get all this done in a reasonable time by using large numbers of tightly coupled machines in a large, single computing system. We must find ways to have enormous numbers of computational engines simultaneously involved productively in the work, despite the fact that the greatest challenge is the number of time steps needed rather than the number of spatial grid cells. A focus of recent work is the hydrogen ingestion flash. This can occur in a late stage of evolution for an intermediate mass star. In the phase of their evolution after core helium burning, asymptotic giant branch (AGB) stars have periodic outbursts called thermal pulses. Each such thermal pulse begins with helium that is accumulating between the hydrogen-burning shell and the degenerate carbon-oxygen core suddenly igniting in what is called the helium shell flash. The energy generation in the just-ignited helium burning shell rises so rapidly at this point that the energy cannot be carried outward effectively by radiative heat transport, and therefore a convection zone develops above the helium burning shell. This convection zone is called the pulse-driven convection zone (PDCZ). The PDCZ grows in mass by incorporating more and more of the gas above it, and it also grows in radius by lifting the overlying layers upward. This upward lifting of the hydrogen burning shell reduces its temperature by an expansion of the gas, and the hydrogen burning effectively ceases. What is special about this sequence of events in AGB stars of the early universe (as well as in some post-AGB stars of the present universe) is that the PDCZ can grow by incorporating gas from above it right up to the point where it encounters gas where the hydrogen has not yet burned. Once this fresh hydrogen fuel is entrained into the convection flow, even in small concentrations, it can burn violently in a hydrogen ingestion flash.
Below: Two volume-rendered views of the far hemisphere of the simulated interior of a 2 MꙨ AGB star of the early universe, with metallicity Z = 10-5, at time 2,699 min. A violent wave of combustion of entrained, H-rich gas is now seen at the lower right, about to cross the star, pulling down large concentrations of H-rich fuel. At this stage, the presence of our outer bounding sphere becomes strongly felt, and our simulation can no longer be trusted. All the hydrogen within the bounding sphere is quickly consumed after the combustion wave reaches the opposite side of the star. We are now testing a new code that will be able to move the bounding sphere outward a factor of 2 in radius.
My research focuses on in-situ measurements of electric fields associated with convection and waves in the collision-less, magnetized, plasmas of the Earth's magnetosphere, the solar wind, and the outer corona of the sun. We are interested in the structure of interplanetary and planetary shock waves, magnetic field line reconnection, and auroral particle acceleration-- and the role of the electric fields in these structures in driving dynamics and particle acceleration. We are currently the lead (P-I) institution for the Electric Fields and Waves Instrument on NASA's Van Allen Probe Spacecraft and Co-Is on the Fields Instrument on the NASA's Solar Probe Mission to the Sun. Solar Probe will pass to distances of 10-30 solar radii of the sun-- the closest approach to the Sun of any spacecraft and study the processes that power the acceleration of the solar wind.
Kris Davidson (Emeritus)
In astrophysics, one superb example is usually worth hundreds of objects that have "ordinary" data -- and often far more than that. Meanwhile, the most extreme or most exotic objects can give clues that we won't find elsewhere. That's why I've concentrated on non-routine, exceptionally significant objects, special sets of objects, and puzzles. Decades ago this meant quasars, pulsar dispersion measures, compact X-ray sources, cocoon stars, and non-thermal photoionization models -- all new topics at the time, and I originated some of the concepts that later became standard. In recent years, though, I've focused on giant stellar eruptions ("supernova impostors") because frankly, they're more mysterious than ordinary supernova explosions. No one knows exactly what causes a giant eruption, but the instability almost certainly involves radiation pressure. Within this topic, I have obtained most of the successful HST spectra of Eta Carinae, the extremely massive star which is our only well-observable supernova impostor. (An interesting aspect is that we had to push the instrument's capability envelope, far more than most HST programs.) Eta Car is not a specialized topic; it embraces stellar structure and instabilities, super-Eddington wind acceleration, exotic emission processes seen almost nowhere else, binary-perturbed gas flows with a broad range of velocities, special data reduction tricks, and other matters both theoretical and observational.
Roberta Humphreys (Emerita)
My research focuses on the most massive stars and the final stages in their evolution before the terminal explosion or collapse to a black hole. With their very short lifetimes and high mass loss events, we observe changes in their properties during the course of a human lifetime revealed by spectroscopy, multi-wavelength photometry, and imaging with the Hubble Space Telescope, the Large Binocular Telescope, and other ground and space-based observatories.
Terry Jay Jones (Emeritus)
I build instruments for deployment on ground-based telescopes such as the Large Binocular Telescope and the MMT. My research covers a broad range of topics, but I specialize in Infrared imaging and astronomical polarimetry. Recent instrument projects include LMIRCam, the 2-5 micron imager on the LBT and MMTPol, the 1-4 micron imaging polarimeter on the MMT. With these instruments, I study dusty hypergiants, very young stars, and extragalactic sources.
Lawrence Rudnick (Emeritus)
I am interested in clusters of galaxies, their origins out of large-scale structure and their continuing evolution. My focus is on the intracluster medium, which I study through the accompanying radio and X-ray emission. I am also interested in radio galaxies, both in and out of clusters, how they evolve and interact with their surrounding medium. I am involved with several next-generation radio surveys in New Mexico, Australia and South Africa, using total and polarized intensity emission to study these questions.
Associations & Collaborations:
Australia Square Kilometer Array Pathfinder project (ASKAP)
Science Board for the Canadian Initiative for Radio Astronomy Data Analysis (CIRADA)
Builders Group for the Evolutionary Map of the Universe (EMU) Survey
Coordinator, Technical Working Group for the Galaxy Cluster Legacy Survey on South Africa's MeerKAT telescope
Executive Committee, Polarization Sky Survey of the Universe's Magnetism (POSSUM)
Science Survey Group for the Very Large Array Sky Survey (VLASS)