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Texas Tech astrophysicists probe the final frontier, finding new answers and more questions

John Davis

Perhaps the understanding that we are made of star dust compels us to better understand the universe in which we live.

Our tiny planet is swallowed by the Milky Way galaxy, which hangs in a firmament we hardly understand. We've seen perhaps five percent of what exists out there.

However, we're learning more and more about our universe all the time from the warmth of our crib on Planet Earth. This last year, particularly, Texas Tech University astrophysicists contributed some excellent findings to better understand how the machinery of the cosmos works.

Since January of 2013, the Department of Physics has added four new astrophysics faculty members to the single professor who was teaching introductory astronomy courses, said Roger Lichti, chairman of the department. Several post-doctoral research fellows also were hired in that area.

The dividends from beefing up astrophysics are paying off nicely, he said. During the last year or so, astrophysics faculty contributed a third of the department's research publications and more than half of new research funding.

The addition of astrophysics has become a major draw for undergraduate students and is expected to increase graduate students numbers as well. Expanding into astrophysics comes in the middle of rapid growth for the department, he said. The number of physics majors and undergraduate degrees has tripled, while graduate student numbers have roughly doubled in five to seven years.

"In 2011 and 2012, there was a push among different segments of the physics faculty to expand into astrophysics," he said. "There was interest in having astronomy and astrophysics as a recruiting draw and retention tool. Astrophysics has proven extremely attractive. Half of incoming freshmen and transfer students said that was the biggest draw initially attracting them to Texas Tech. Obviously, we're very satisfied with the level of research output. The new folks are very active and still developing their graduate program, so their impact on graduate recruiting is just beginning."

Recent technological advances and new discoveries in the universe have reignited interest in the past decade or so, Lichti said. He predicted that the next 20 to 50 years will see even more discovery and entice even more minds to pursue astrophysics. The work of the Texas Tech astrophysics team may be at the forefront of that new understanding.

Newborn Supernova

With the help of a special spectroscopic camera developed by a Texas Tech physicist, researchers at Caltech and Las Cumbres Observatory Global Telescope Network (LCOGTN) captured rare images of a star in another galaxy going supernova within a day of the star's explosion.

This is the first time scientists have pinpointed a star that eventually exploded as a stripped-envelope supernova, called a type Ib, said David Sand, an assistant professor in the Department of Physics who developed the camera.

The global team of astrophysicists, led by Yi Cao of Caltech, found the supernova on June 16. Their research was published in the peer-reviewed journal, The Astrophysical Journal Letters.

"It is very rare to catch a supernova within a day or two of explosion," Sand said. "Up until now, it has happened at most about a dozen times. It is equally rare that we actually have Hubble Space Telescope imaging of the location of the supernova before it happened, and we were able to see the star that eventually exploded."

Sand said it took 73 million years for the illumination from the star's explosion to travel to Earth.

The supernova came from what is known as a Wolf-Rayet star. Their massive size leads to a speedy demise, Sand said. Where our sun is roughly 5 billion years old, this star was only tens of millions of years old. Wolf-Rayet stars tend to burn up all of their fuel quickly in order to support their own weight because the nuclear burning balances out gravity.

Cues from the spectroscopic camera images led researchers to classify their discovery as a type Ib supernova, which are thought to be the explosions of these massive stars that have lost their outer layers right before their death due to a stellar wind.

Exact details of what happens in these supernovae are murky, he said. When they do explode, they burn roughly as bright as five billion of our suns.

Sand's work is part of the Intermediate Palomar Transient Factory project, which is a scientific collaboration with California Institute of Technology, Los Alamos National Laboratory, University of Wisconsin and several others, is an automated survey of the night sky dedicated to finding transient supernova events. The survey finds hundreds of new supernovae annually, and scientists here try to understand what types of stars become which types of supernovae.

Sand led the development and operations of the special camera, the FLOYDS spectrograph, which was used to help identify this specific kind of supernova. Taking a spectroscopic image helps scientists to tell what kind of supernova they're looking at by splitting the supernova's light up into the colors of the rainbow.

A normal photograph isn't enough to tell, he said.

The FLOYDS spectrographs, of which there are only two in the world, are attached to two-meter telescopes located in Hawaii and Australia. The cameras operate completely robotically allowing scientists to confirm supernova earlier than ever before.

In the last two years, Sand and others have confirmed 100 different supernovae with the new camera, leading to well over a dozen publications thus far.

"This is where FLOYDS comes in, and its robotic nature, which lets us study supernovae young," Sand said.

"That's the first story. The second story is this lucky Hubble imaging from 2005. Someone took an image with Hubble of the galaxy where this supernova happened. Just sheer luck – nothing to do with the supernova or seeing into the future or anything. Zoom to 2013, and we discover the supernova within a day of its explosion. We look in the Hubble data archive and notice the image from eight years prior, and we just match it up with our most recent data to see if there is a star in the old image at the exact same position as the supernova today.

"The second story really is luck, but it is happening more and more these days as the Hubble telescope collects more images of nearby galaxies."

Sand said scientists can take another Hubble image at the location of the supernova after it has faded away. If the star that he and others identified as the progenitor to the supernova has disappeared, then they will know which star died. Otherwise, if the star is still there, then the supernova came from some other object too faint for researchers to see, and the mystery continues.

Galactic Milemarker

With the help of the same special spectroscopic camera, Sand and others from the Intermediate Palomar Transient Factory observed a different supernova that was a mere 12 million light years away from Earth. Finding one so close is important, he said, because astrophysicists use these stars to map distances in the universe.

The supernova, called a type Ia, is the closest discovered to Earth in a generation. The proximity to Earth could yield better understanding of this particular type of milemarker supernova and its expansion history.

The research on this supernova alone has already led to over a dozen publications.

"These type Ia supernovae are very important to astronomers in general," he said. "They let us measure distances to within about 10 percent or so. These supernovae are very uniform, and the explosions emit the same intrinsic amount of light. So, if you think about the supernova being like a 60-watt light bulb, the light bulb in a room gives a brighter light intensity compared to a light bulb a mile away. They still give off the same amount of light though, so we're able to figure out about how far away it is."

Sand said a type Ia supernova may begin as a carbon/oxygen white dwarf star that feeds off a neighboring normal star. Once the white dwarf star accretes enough material to reach a mass that's 1.4 times the size of our sun compressed into a ball about the size of Earth, it becomes unstable and explodes into a supernova in a process that still isn't fully understood.

Because they tend to explode at nearly the same mass and luminosity, they make excellent guides for judging distances.

However, Ariel Goobar from the Oskar Klein Center at Stockholm University, said evidence shows this particular supernova may have formed differently. Goobar and collaborators used pre-explosion images of the region of the M82 galaxy where the supernova went off, both from the Hubble Space Telescope and from the Palomar Oschin Telescope near San Diego, to search for a star in the location of the explosion, or possible earlier nova eruptions.

Novae eruptions happen when temporary nuclear fusion occurs on the surface of a white dwarf after it has eaten some gas from a companion star. Scientists believe they could be precursors to type Ia supernovas.

The lack of a pre-explosion detection suggests the supernova may have originated in the merging of two white dwarf stars.

"Until very recently, the leading model for standard candle supernovae was thought to include a companion star from which material was stripped by the white dwarf until the accumulated mass could no longer be sustained by the outwards pressure, leading to a runaway thermonuclear explosion," Goobar said. "The observations of this supernova are challenging for this theoretical picture."

Three scientists won the Nobel Prize in Physics in 2011 after they used type Ia supernovae to document the universe not only is expanding but also the expansion process is accelerating.

The acceleration phenomenon is called dark energy, and Sand said he believes studying closer type Ia supernovae will lead to better distance-judging in the universe as well as a better understanding of the dark energy phenomena.

"If we can judge distances better than plus-or-minus 10 percent or so, I think this will lead to uncovering the details of dark energy we still need to understand," he said. "We didn't even know about dark energy until a few years ago. It's almost like the universe is going to fly apart. Right now, there's no good physical model or explanation for dark energy."

Theory Buster

A Texas Tech astrophysicist was part of a team of researchers that discovered the first examples of black holes in globular star clusters in our own galaxy, upsetting 40 years of theories against their possible existence.

Tom Maccarone, an associate professor of physics, said finding black holes in globular clusters might provide a way for them to merge into bigger black holes.

"These mergers may produce the 'ripples in spacetime' we call gravitational waves," he said. "Trying to detect gravitational waves is one of the biggest problems in physics right now, because it would be the strongest test of whether Einstein's theory of relativity is correct."

The research team detected the existence of the black holes by using an array of radio telescopes to pick up a certain type of radio frequency released by these black holes as they eat a star next to them.

The results were published in The Astrophysical Journal and featured in the National Radio Astronomy Observatory's ENews news bulletin.

Globular star clusters are large groupings of stars thought to contain some of the oldest stars in the universe. In the same distance from our sun to the nearest neighbor, Proxima Centauri, these globular star clusters could have a million to tens of millions of stars, Maccarone said.

"The stars can collide with one another in that environment," Maccarone said. "The old theory believed that the interaction of stars was thought to kick out any black holes that formed. They would interact with each other and slingshot black holes out of the cluster until they were all gone."

He compared it to water vapor coming off a hot cup of coffee. As some water molecules get hot enough to turn to steam, they are let go from their environment to float off into the atmosphere even though the coffee may be below the boiling temperature of water.

The old theory stated that the stars would kick the black holes out in the same fashion – occasionally, some black holes would have enough energy to escape the cluster, and gradually, they all would leave.

While the theory may still be displaced, Maccarone said it might still be somewhat true. Black holes might still get kicked out of globular star clusters, but at a much slower rate than initially believed.

Maccarone made the first discovery several years ago of a black hole in a globular star cluster in the neighboring NGC4472 galaxy. But rather than finding it by using radio waves, Maccarone found it by seeing an X-ray emission from the gas falling into the black hole and heating up to a few million degrees.

"In 2007, I had made the first discoveries in other galaxies," he said. "It's surprisingly easier to find them in other galaxies than in our own, even though they're a thousand times as far away as these in our own galaxy are."

This year, he and his team discovered two examples of globular star clusters in our own galaxy which host black holes by finding radio emission by using the Very Large Array of radio telescopes in New Mexico.

"As the black hole eats a star, these jets of material are coming out," he said. "Most of the material falls into the black hole, but some is thrown outwards in a jet. To see that jet of material, we look for a radio emission. We found some radio emissions coming from this globular star cluster that we couldn't explain any other way."

Star Power

One of the questions that has intrigued astronomers for decades is how galaxies go from forming hundreds of stars each year to none so quickly (if tens of millions of years can be considered quickly).

Potentially powerful, monstrous black holes at the centers of galaxies are commonly considered responsible for heating up and expelling gas needed to form stars. But now, Paul Sell, a Texas Tech postdoctoral research fellow, along with a group of other researchers, has discovered evidence that black holes have less importance than the stars themselves in extreme cases where formation of new stars is quickly halted.

Studying a small set of 12 rare galaxies using the unique capabilities of the Hubble Space Telescope and Chandra X-ray Observatory has provided new insights. The team's work could help determine the evolution of galaxies and why some evolve from ones full of life to passive, star graveyards.

"What we've concluded is the stars are the ones that are blowing out the gas, heating it up and stopping the formation of the stars," said Sell, who began this project during graduate work at the University of Wisconsin-Madison. "We're not ruling out the black hole because it could be doing other things we haven't noticed. Rather, it's just not needed. There is enough energy from all the stars to do all this without needing a black hole."

The team's findings were published in the Monthly Notices of the Royal Astronomical Society.

Two types of galaxies exist. Spiral galaxies, such as the Milky Way, are loaded with the cold gas needed to form stars. Elliptical galaxies do not have much cold gas. In order for stars to form, the cold gas condenses in a star-forming region. Once the density is high enough, stars form.

The most important stars are the heaviest ones with violent, short lives. They burn through their fuel so fast they have a hard time remaining stable and release increasingly powerful winds as they age. Eventually, they collapse when the fuel is used up, resulting in the catastrophic explosion of the star, called a supernova. The combined power from the winds and supernovae of these stars heats up the gas and expels it from the galaxy.

The biggest stars are the brightest and hottest, which is why they appear blue. But they also die off very quickly, and when they do, all that is left are the smaller, dead, red stars. Sell's team's research attempted to understand how galaxies evolve from the active, blue, star-forming spiral galaxies to the red, dead elliptical galaxies. Is it the black hole or is it the stars?

This graphic illustrates how a vibrant, star-forming galaxy quickly transforms into a sedate galaxy composed of old stars. (1) The scenario begins when two galaxies merge, funneling a large amount of gas into the central region. (2) The gas compresses, sparking a firestorm of star birth, which blows out most of the remaining star-forming gas. (3) Devoid of its fuel, the galaxy settles into a quiet existence, composed of aging stars. Credit: A. Feild (STScI)

The intense star-forming episode in these galaxies is triggered by the collision of spiral galaxies. The merging galaxies are producing a galaxy much more compact than the Milky Way, but with the same mass that resembles neither spiral nor elliptical galaxies.

In each collision, the cold gas churns together and falls to the center very rapidly to produce a massive, compact core. In the core, the gas condenses to form a single, large star-forming region. There, stars form at a rapid rate, producing powerful supernovae and high-velocity winds that heat and expel the remaining gas, thus halting the formation of more stars.

The heaviest stars that initially stand out because they are the brightest are also blue because they are the hottest. But they also die off very quickly, and when they do, all that is left are the lighter, red stars.

"If you stop the flow of cold gas to form stars, that's it," Sell said. "The stars stop forming, and the galaxy rapidly evolves and may eventually become a red, dead elliptical galaxy. These starbursts are quite rare, however, so they may not grow into the typical giant elliptical galaxies seen in our nearby galactic neighborhood. They may, instead, be more compact."

Sell said the discovery forces astronomers to closely examine how galaxies are evolving instead of assuming really fast outflows must be caused by the massive black hole in the center. He said more work is needed to fully understand what is going on.

"We want to understand how much gas is being blown out, how it's being blown out and whether it's going to escape or fall back in," Sell said. "We also want to figure out what types of galaxies these turn into. We see a lot of elliptical galaxies nearby. It's not a new class of galaxies, but more that it may be a new subclass."

Provided by Texas Tech University