NSF-FUNDED SCIENTIST LOOKS AT WHY GALAXIES CHANGE

FROM:  NATIONAL SCIENCE FOUNDATION 

Exploring dramatic changes in galaxies

Scientist hopes to uncover physical process behind the changes, including cosmic webs and supermassive black holes

The evolution of galaxies over billions of years offers any number of tantalizing clues about the origins of the universe. Alison Coil is trying to solve some of these mysteries by studying how galaxies have been changing over time, and why.

"Galaxies have changed dramatically," says Coil, an associate professor of physics at the University of California, San Diego. "In the past, for example, they formed more stars, and had smaller supermassive black holes. These black holes were more active, brighter and gobbling up materials faster."

The National Science Foundation (NSF)-funded scientist is conducting three research projects with the goal of uncovering some of the physical processes underlying these dramatic changes.

"We are looking at large statistical samples of what galaxies are doing, including nearby galaxies and distant ones," she says.

These include looking at the stellar mass of both nearby and distant galaxies, comparing the properties of galaxies that still are forming stars with those that are not; studying the "clustering" behavior of distant galaxies, that is, the process by which they form a "cosmic web," a filamentary-like structure that resembles a sponge; and quantifying supermassive black holes among distant galaxies.

"This may help us discover if we live in a typical kind of galaxy, and how we came to be, and why we're here," she says. "It tells us something about where we came from. I'd like to understand the galaxy population as a whole, as it helps to put our own Milky Way into context."

Coil is conducting her research under an NSF Faculty Early Career Development (CAREER) award, which she received in 2011. The award supports junior faculty who exemplify the role of teacher-scholars through outstanding research, excellent education, and the integration of education and research within the context of the mission of their organization.

One project is looking halfway back to the Big Bang to compare galaxies still forming stars with those that have stopped. These include nearby galaxies, which formed later and are brighter, as well as distant galaxies, which developed when the universe was younger, and are faint.

"Within the population that is no longer forming stars, there's a lot of growth happening for the lower mass galaxies, they are still getting bigger and there is more of them," she says. "For the population still forming stars, the massive galaxies disappear, and are turning into the other population, those whose star formation has turned off.

"We don't know why," she adds. "We know that one kind turns into the other kind, but we don't know what is shutting off star formation."

Her team also is examining distant galaxies and how they form "cosmic webs," which are clusters of galaxies grouped together like foam, likely the result of gravitational pull.

"The thing that is interesting here is that if you look at the clustering in galaxies still forming stars, and those not forming stars, the clustering is different," she says.

Those galaxies no longer producing stars are more clustered together, and typically are located in the middle of the web, she says.

"Those that are still forming stars tend to be on the outskirts," she says. "We are trying to understand this, whether this has to do with when the galaxies are formed, how much mass they have and whether being near other galaxies shuts off star formation."

Finally, she and her colleagues also are studying supermassive black holes located in the centers of distant galaxies, trying to quantify them.

"We want to know how many galaxies have these super massive black holes, which seem to be fairly common," she says. "They have to be shining for us to see them, and they shine when they are accreting material, that is, when stars and gas fall into the black hole. When you see this light, you know there is a black hole actively accreting. But there are other black holes that exist that we can't see. We want to know why some are shining--accreting--and some aren't."

Much of her work uses data generated by the PRIsm Multi-object Survey (PRIMUS), the largest faint galaxy spectroscopic redshift (which refers to a shift in the spectra of very distant galaxies toward longer wavelengths, usually considered evidence that the universe is expanding) survey taken to date.

As part of the grant's educational component, Coil created and taught a new course during the fall 2013 semester on stars and black holes, and convinced her department to split a pre-existing course entitled "the universe" into two courses, including "stars and black holes," and "galaxies and cosmology."

"The new course is much more fun to teach, as we can spend more time on concepts and class discussions," she says. "I found that the students learned more, at a deeper level. I am very pleased with the new course and hope to attract more students to the lower division astrophysics survey courses as a result."

She also is developing a proposal to add an astrophysics doctoral track in her department, which currently has a physics and a biophysics track. "The astrophysics PhD track would allow students to take more graduate level courses in astrophysics and begin astrophysics research in their first year, a year earlier than the physics PhD track," she says. "I hope to recruit more astrophysics graduate students to UCSD as a result of the new track."

She also informally mentors several female undergraduate and graduate students in the department through individual meetings, and holds monthly meetings of the "women in physics" group she established for women graduate students and postdocs.

During the last year, among other things, the group sponsored two guest speakers--a female faculty in another department and a colloquium speaker who works in science policy--and discussed such topics as diversity in faculty hiring, maternity leave policies, and unconscious bias.

She also runs a one-day physics outreach program for the Reach for Tomorrow foundation, which targets underprivileged middle school youth. About 50 students are participating this year.

Finally, every June she leads a similar program with the Tech Trek program, which is aimed at middle school girls interested in math and science. About 25 girls participate in the physics portion of the program, which involves hands-on demonstrations, and a soldering lab, all run by graduate students and postdocs in the department.

"I specifically recruit mostly women students and postdocs to help run the event, so that the girls can meet and interact with many women who were actively pursuing physics careers," she says.

-- Marlene Cimons, National Science Foundation
Investigators
Alison Coil
Related Institutions/Organizations
University of California-San Diego

SUPERCOMPUTERS PREDICT SIGNS OF BLACK HOLES CONSUMING STARS

Right:  Black Hole Caught in a Stellar Homicide.  This computer-simulated image shows gas from a star that is ripped apart by tidal forces as it falls into a black hole. Some of the gas also is being ejected at high speeds into space.  Image Credit: NASA, S. Gezari (The Johns Hopkins University), and J. Guillochon (University of California, Santa Cruz).

FROM:  NATIONAL SCIENCE FOUNDATION 

Cosmic slurp

Georgia Tech researchers use supercomputers to understand and predict signs of black holes swallowing stars

April 14, 2014

Somewhere out in the cosmos an ordinary galaxy spins, seemingly at slumber. Then all of a sudden, WHAM! A flash of light explodes from the galaxy's center. A star orbiting too close to the event horizon of the galaxy's central supermassive black hole has been torn apart by the force of gravity, heating up its gas and sending out a beacon to the far reaches of the universe.

In a universe with tens of billions of galaxies, how would we see it? What would such a beacon look like? And how would we distinguish it from other bright, monumental intergalactic events, such as supernovas?

"Black holes by themselves do not emit light," said Tamara Bogdanovic, an assistant professor of physics at the Georgia Institute of Technology. "Our best chance to discover them in distant galaxies is if they interact with the stars and gas that are around them."

In recent decades, with improved telescopes and observational techniques designed to repeatedly survey the vast numbers of galaxies in the sky, scientists noticed that some galaxies that previously looked inactive would suddenly light up at their very center.

"This flare of light was found to have a characteristic behavior as a function of time. It starts very bright and its luminosity then decreases in time in a particular way," she explained. "Astronomers have identified those as galaxies where a central black hole just disrupted and 'ate' a star. It's like a black hole putting up a sign that says 'Here I am.'"

Using a mix of theoretical and computer-based approaches, Bogdanovic tries to predict the dynamics of events such as the black-hole-devouring-star scenario described above, also known as a "tidal disruption." Such events would have a distinct signature to someone analyzing data from a ground-based or space-based observatory.

Using National Science Foundation-funded supercomputers at the Texas Advanced Computing Center (Stampede) and the National Institute for Computational Sciences (Kraken), Bogdanovic and her collaborators recently simulated the dynamics of these super powerful forces and charted their behavior using numerical models.

Tidal disruptions are relatively rare cosmic occurrences. Astrophysicists have calculated that a Milky Way-like galaxy stages the disruption of a star only once in about 10,000 years. The luminous flare of light, on the other hand, can fade away in only a few years. Because it is such a challenge to pinpoint tidal disruptions in the sky, astronomical surveys that monitor vast numbers of galaxies simultaneously are crucial.

Huge difference

So far, only a few dozen of these characteristic flare signatures have been observed and deemed "candidates" for tidal disruptions. But with data from PanSTARRS, Galex, the Palomar Transient Factory and other upcoming astronomical surveys becoming available to scientists, Bogdanovic believes this situation will change dramatically.

"As opposed to a few dozen that have been found over the past 10 years, now imagine hundreds per year--that's a huge difference!" she said. "It means that we will be able to build a varied sample of stars of different types being disrupted by supermassive black holes."

With hundreds of such events to explore, astrophysicists' understanding of black holes and the stars around them would advance by leaps and bounds, helping determine some key aspects of galactic physics.

"A diversity in the type of disrupted stars tells us something about the makeup of the star clusters in the centers of galaxies," Bodganovic said. "It may give us an idea about how many main sequence stars, how many red giants, or white dwarf stars are there on average."

Tidal disruptions also tell us something about the population and properties of supermassive black holes that are doing the disrupting.

"We use these observations as a window of opportunity to learn important things about the black holes and their host galaxies," she continued. "Once the tidal disruption flare dims below some threshold luminosity that can be seen in observations, the window closes for that particular galaxy."

Role of supercomputer

In a recent paper submitted to the Astrophysical Journal, Bogdanovic, working with Roseanne Cheng (Center for Relativistic Astrophysics at Georgia Tech) and Pau Amaro-Seoane (Albert Einstein Institute in Potsdam, Germany), considered the tidal disruption of a red giant star by a supermassive black hole using computer modeling.

The paper comes on the heels of the discovery of a tidal disruption event in which a black hole disrupted a helium-rich stellar core, thought to be a remnant of a red giant star, named PS1-10jh, 2.7 billion light years from Earth.

The sequence of events they described aims to explain some unusual aspects of the observational signatures associated with this event, such as the absence of the hydrogen emission lines from the spectrum of PS1-10jh.

As a follow-up to this theoretical study, the team has been running simulations on Kraken and Stampede, as well as Georgia Tech's Keeneland supercomputer. The simulations reconstruct the chain of events by which a stellar core, similar to the remnant of a tidally disrupted red giant star, might evolve under the gravitational tides of a massive black hole.

"Calculating the messy interplay between hydrodynamics and gravity is feasible on a human timescale only with a supercomputer," Cheng said. "Because we have control over this virtual experiment and can repeat it, fast forward, or rewind as needed, we can examine the tidal disruption process from many perspectives. This in turn allows us to determine and quantify the most important physical processes at play."

The research shows how supercomputer simulations complement and constrain theory and observation.

"There are many situations in astrophysics where we cannot get insight into a sequence of events that played out without simulations. We cannot stand next to the black hole and look at how it accretes gas. So we use simulations to learn about these distant and extreme environments," Bogdanovic said.

One of Bogdanovic's goals is to use the knowledge gained from simulations to decode the signatures of observed tidal disruption events.

"The most recent data on tidal disruption events is already outpacing theoretical understanding and calling for the development of a new generation of models," she explained. "The new, better quality data indicates that there is a great diversity among the tidal disruption candidates. This is contrary to our perception, based on earlier epochs of observation, that they are a relatively uniform class of events. We have yet to understand what causes these differences in observational appearance, and computer simulations are guaranteed to be an important part of this journey."

Investigators
Roseanne Cheng
Pau Amaro-Seoane
Tamara Bogdanovic

NEW GAMMA-RAY OBSERVATORY HAS BEGUN THE STUDY OF THE ENERGETIC UNIVERSE

FROM:  LOS ALAMOS NATIONAL LABORATORY 
New Gamma-Ray Observatory Begins Operations at Sierra Negra Volcano In The State Of Puebla, Mexico

New Site to Observe Supernovas and Supermassive Black Holes

LOS ALAMOS, N.M., August 21, 2013—The High-Altitude Water Cherenkov (HAWC) Gamma Ray Observatory has begun formal operations at its site in Mexico. HAWC is designed to study the origin of very high-energy cosmic rays and observe the most energetic objects in the known universe. This extraordinary observatory, using a unique detection technique that differs from the classical astronomical design of mirrors, lenses, and antennae, is a significant boost to international scientific and technical knowledge.

“The HAWC observatory will search for signals from dark matter and to study some of the most extreme objects in the universe, such as supermassive black holes and exploding stars,” said Brenda Dingus, principal investigator and a research fellow at Los Alamos National Laboratory. Dingus is a Fellow of the American Physical Society, and in 2000 was a recipient of the Presidential Early Career Award for Scientists and Engineers.

HAWC is located at an altitude of 4100 meters on the slope of the volcanoes Sierra Negra and Pico de Orizaba at the border between the states of Puebla and Veracruz. The observatory, which is still under construction, uses an array of Cherenkov detectors to observe high-energy cosmic rays and gamma rays. Currently 100 out of 300 Cherenkov detectors are deployed and taking data. Each Cherenkov detector consists of 180,000 liters of extra-pure water stored inside an enormous tank (5 meters high and 7.3 meters in diameter) with four highly sensitive light sensors fixed to the bottom of the tank.

“Los Alamos has a long history of working in this field and built the predecessor to the HAWC observatory, called Milagro, located at the Los Alamos site in New Mexico,” Dingus said.

HAWC 15 Times More Sensitive Than Predecessor

“HAWC will be more than 15 times more sensitive than Milagro was, and it will detect many new sources of high-energy photons. Los Alamos also studies these high-energy phenomena through complex computer simulations to understand the physical mechanisms that accelerate particles to energies millions of times greater than man-made accelerators,” Dingus said.

The construction and operation of HAWC has been made possible by the financial support of several Mexican institutions such as the Consejo Nacional de Ciencia y Tecnología (CONACYT), the Universidad Nacional Autónoma de México (UNAM), and the Instituto Nacional de Astrofísica, Óptica y Electrónica (INAOE). Funding has also been provided by the United States through the National Science Foundation (NSF), the Department of Energy (DOE) Office of Science, the Los Alamos National Laboratory (LANL), and the University of Maryland. The University of Maryland is the managing institute of the project overall.

The HAWC array, operating with 100 Cherenkov detectors since August 1 and growing each week, will be sensitive to high-energy particles and radiation between 100 GeV and 100 TeV, energy equivalent to a billion times the energy of visible light. For more information online see http://www.hawc-observatory.org/.

In 2009, HAWC was identified as the Mexican astronomical project with the highest expected impact on high-energy astrophysics. Shortly thereafter a test array with three Cherenkov detectors was installed at the volcano Sierra Negra and successfully observed cosmic rays and gamma rays. Following these early tests, a prototype array of seven Cherenkov detectors was built in 2009 to test the tank design, simulate real data-taking, and study the logistics of deploying a large-scale observatory in this remote location. In 2012, the first 30 of 300 HAWC detectors were deployed, and since that time have been operated nearly continuously. The 30-detector stage of HAWC permitted calibration of the observatory via the observation of the shadow of the moon as it blocked cosmic rays. (http://1.usa.gov/14jjT8w)

Today, the scientific team of HAWC will formally begin observations of the most violent phenomena in the known universe, such as supernovae explosions and the evolution of supermassive black holes.

Image captions:

Figure 1: Artist’s conception of a black hole in the center of a distant galaxy emitting gamma rays, one of which reaches the Earth. Upon entering the terrestrial atmosphere, the gamma ray will produce a cascade of energetic particles that travels to detectors on the ground. Credit: Aurore Simonnet, Sonoma State University.

Figure 2. Diagram of a HAWC Cherenkov detector, with a person shown for scale. Inside the Cherenkov detector, a high-energy charged particle (red line) produces Cherenkov light (green lines) as it moves from top to bottom through the tank. The Cherenkov light is recorded by four highly sensitive photo-sensors placed at the bottom of the Cherenkov detector. By combining measurements from many tanks the properties of the original gamma ray or cosmic ray can be inferred.

Figure 3. Image of an event produced by particle cascade in the HAWC observatory. The larger circles represent each Cherenkov detector in HAWC, each contains 4 photo-sensors represented in the figure as smaller circles. The color of each small circle or photo-sensor represents the arrival time of the particle cascade to each sensor. This is one of the first images recorded by HAWC since the beginning of operations. In particular, this cascade arrived from the upper left to the bottom right and its center hit HAWC at the “X” mark. The time scale is given in the lower scale in nanometers.

Figure 4. The HAWC Observatory taken in August 2013 from the summit of Sierra Negra. The image has been digitally altered to show HAWC as it will appear when construction is complete in 2014. The 111 Cherenkov detectors currently installed (100 Cherenkov detectors in operation) are colored white and located in the upper right quadrant of the array.

Background: The Most Energetic Particles in the Known Universe

Gamma rays (electromagnetic radiation of very high frequency) and cosmic rays (subatomic particles of very high energy) are products of the most energetic and cataclysmic events in the known universe. These phenomena include the collisions of two neutron stars, the explosions of supernovae, binary systems of stars with stellar accretion, and active gal actic nuclei which host black holes millions of times more massive than the sun.

When high-energy cosmic rays and gamma rays reach the Earth, they interact with air molecules in the upper atmosphere. Gamma rays, for example, are converted into pairs of charged matter and anti-matter particles (mainly electrons and positrons). These particles rapidly interact with other air molecules, producing additional gamma rays of reduced energy, which then create further charged particle pairs. This chain reaction proceeds until a large cascade of particles and radiation reaches ground level, where it can be recorded in the HAWC detectors.

When the charged particle cascade from an extra-terrestrial gamma ray passes through a Cherenkov detector, its particles are traveling faster than the speed of light in water. The resulting effect is similar to the shock wave produced in the atmosphere by a supersonic airplane (a "sonic boom"), but instead of producing sound the particles produce a visible cone of light. The flash of light, called Cherenkov radiation, is measured by the light sensor fixed to the bottom of each detector in HAWC. By combining the light signal observed in many tanks with fast electronics and high precision computing equipment, it is possible for scientists to determine the time of arrival, energy, and direction of the original extraterrestrial gamma ray or cosmic ray.