Growing Snow in Pasadena

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Growing Snow in Pasadena
Credit: Ken Libbrecht


All snowflakes are broadly categorized as either columns, which are long and thin, or plates, which are flat. Needles are considered a type of column and form only when the temperature is near 23 degrees Fahrenheit and the humidity level is high.

Credit: Ken Libbrecht


Columns like these—whether hollow or solid—typically appear at around 23 degrees Fahrenheit. Unlike needles, they can appear at varying levels of humidity and also at temperatures below 8 degrees Fahrenheit.

Credit: Ken Libbrecht

Stellar Dendrites

Plate-like snowflakes, such as this stellar dendrite, typically appear under conditions of very high humidity—but only when the temperature is near 5 degrees Fahrenheit. For the record, Libbrecht employs no Photoshop trickery to create his colorful images. Instead, he shines colored lights in from different angles behind the crystal. The ice acts like a complex lens and bends the light to accentuate structural details.


Credit: Ken Libbrecht

Stellar Plates

Snowflakes such as this stellar plate are common at lower levels of humidity, usually at temperatures between 26 degrees Fahrenheit and freezing, or below −14 degrees Fahrenheit. Why plates fail to form between these temperature ranges—or, more generally, why snow crystals grow into such different shapes at different temperatures—remains a scientific mystery.


Credit: Ken Libbrecht

Sectored Plates

Libbrecht created this sectored plate snowflake in his lab when the weather outside was frightful—for snowflakes, anyway. "It was summer and about 95 degrees outside when this crystal was growing," he said. Inside the lab, this flake grew at temperatures between 5 and 8 degrees Fahrenheit, and varying levels of humidity.

Credit: Ken Libbrecht

Capped Columns

The capped column starts out as a columnar crystal growing near 21 degrees Fahrenheit. If the crystal then falls into colder air, plates may grow from both ends of the column. The final crystal looks like a spool of thread with a hexagonal center column.


Credit: Ken Libbrecht

Bullet Rosettes

The capped bullet rosette forms in much the same way as a capped column but has at least three sections sprouting from a common center. This one grew columns at 21 degrees Fahrenheit, then Libbrecht lowered the temperature to 8 degrees Fahrenheit to create conditions favorable to the formation of plates on the ends.


For a city so proud of its mild, sunny winters that it created the Rose Parade to tout its climate, Pasadena—or, at least, Caltech—has become, somewhat paradoxically, renowned for its snowflakes.

Every year, when flurries begin falling across the country, members of the media begin flocking to speak with Caltech physicist and snowflake guru Ken Libbrecht, who literally wrote the book—or, rather, books—on the science and beauty of the frozen crystals. In his lab, he creates countless snowflakes to investigate and explain how subtle changes in temperature, pressure, and humidity give rise to an impressively varied menagerie of intricate snowflake forms.

In recent years, we have showcased his research in articles that explain the physics of snowflake formation in clouds and even how to grow your own snowflakes at home. This year, we are pleased to share some of his most recent micrographs documenting the fascinating and beautiful results of his investigations.

Displayed below are some of Libbrecht's images that show representative samples of some common types of snowflakes: needles, stellar dendrites, stellar plates, columns, sectored plates, capped columns, and bullet rosettes.

To learn more, visit Libbrecht's comprehensive site. It features sections on the science, aesthetics, and history of snowflakes—and it also answers the age-old question of whether any two snowflakes are alike. (Spoiler alert: they aren't.)

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Prime Numbers, Quantum Fields, and Donuts: An Interview with Xinwen Zhu

In 1994, British mathematician Andrew Wiles successfully developed a proof for Fermat's last theorem—a proof that was once partially scribbled in a book margin by 17th-century mathematician Pierre de Fermat but subsequently eluded even the best minds for more than 300 years. Wiles's hard-won success came after digging into a vast web of mathematical conjectures called the Langlands program. The Langlands program, proposed by Canadian mathematician Robert Phelan Langlands in the 1960s, acts as a bridge between seemingly unrelated disciplines in mathematics, such as number theory—the study of prime numbers and other integers—and more visual disciplines such as geometry.

However, to get the ideas he needed for his history-making proof, Wiles only scratched the surface of the Langlands program. Now Xinwen Zhu, an associate professor of mathematics at Caltech, is digging deeper, looking for further applications of this so-called unifying theory of mathematics—and how it can be used to relate number theory to disciplines ranging from quantum physics to the study of donut-shaped geometric surfaces.

Zhu came to Caltech from Northwestern University in September. Originally from Sichuan, China, he received his bachelor's degree from Peking University in 2004 and his doctorate from UC Berkeley in 2009.

He recently spoke with us about his work, the average day of a mathematician, and his new life in California.


Can you give us a general description of your research?

My work is in mathematics, related to what's called Langlands program. It's one of the most intrinsic parts of modern mathematics. It relates number theory—specifically the study of prime numbers like 2, 3, 5, 7, and so on—to topics as diverse as geometry and quantum physics.


Why do you want to connect number theory to geometry and quantum physics?

Compared to number theory, geometry is more intuitive. You can see a shape and understand the mathematics that are involved in making that shape. Number theory is just numbers—in this case, just prime numbers. But if we combine the two, then instead of thinking about the primes as numbers, we can visualize them as points on a Riemann surface—a geometric surface kind of represented by the shape of a donut—and the points can move continuously. Think of an ant on a donut—the ant can move freely on the surface. This means that a point on the donut has some intrinsic connections with the points nearby. In number theory it is very difficult to say that any relationship exists between two primes, say 5 and 7, because there are no other primes between them, but there are points between any two points. It is still very difficult to envision, but it gives us a more intuitive way to think about the numbers.

We want to understand certain things about prime numbers—for example, the distribution of primes among all natural numbers. But that's difficult when you're working with just the numbers; there are very few rules, and everything is unpredictable. The geometric theory here adds a sort of geometric intuition, and the application to quantum field theory adds a physical intuition. Thinking about the numbers and equations in these contexts can give us new insights. I really don't understand exactly how physicists think, but physicists are very smart because they have this intuition. It's just sort of their nature. They can always make the right guess or conjecture. So our hope is to use this sort of intuition to come back to understand what happens in number theory.


Mathematicians don't really have lab spaces or equipment for experiments, so what does a day at the office look like for you?

Usually I just think. And unfortunately, it's usually without any result, but that's fine. Then, after months and months, one day there is an idea. And that's how we do math. We read papers sometimes to keep our eyes on what the newest development is, but it's probably not as important as it is for other disciplines. Of course, one can also get new ideas and stimulation from reading, so we keep our eyes on what's going on this week.


A two-part question: How did you get first get interested in math in general, and how did you get interested in this particular field that you're in now?

My interest in math began when I was a child. People can usually count numbers at a pretty early age, but I was interested in math and could do calculations a little bit quicker and a bit younger than others. It came naturally to me. Also, my grandfather was a chemist and physicist, and he always emphasized the importance of math.

But to be honest, I didn't really know anything about this aspect of the Langlands program until I was in graduate school at Berkeley. My adviser, Edward Frenkel, brought me into this area.


What are you most excited about in terms of your move to Caltech?

I think this is, of course, a fantastic place. The undergraduates here are very strong, and the graduate school is also very good, so I'm also very excited to work with all of those young people. Also, the physics department here is very good, and as I said, quantum field theory has recently provided promising new ways to think about these old problems from number theory. Caltech professors Anton Kapustin and Sergei Gukov have played central roles in revealing these connections between physics and the Langlands problem.


Is there anything else that you're looking forward to about living in Pasadena?

I'm from Sichuan [province in China], and one thing that I miss is the food. It's hot and spicy, and now it's also kind of popular in the U.S. And there are very good Szechwan restaurants in the San Gabriel Valley. Actually, maybe the best Szechwan food in the U.S. is right here.


Aside from your research and professional interests, do you have any other hobbies?

Yes, I've been playing the game Go for more than 20 years. It's a board game that is kind of like chess. It's interesting, and it's very complicated. Many years ago, you'd play with a game set and one opponent, but now you can also play it online. And that's good for me because after moving from place to place, it's hard to consistently find someone to play with.

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India Becomes TMT Partner

Leaders of the Thirty Meter Telescope (TMT) project announced on Tuesday that the government of India has signed on as a full partner in the construction of what will be the world's largest ground-based telescope.

A collaboration of institutions in the United States, including Caltech and the University of California, along with institutions from Canada, Japan, India, and China, are working together to build the TMT observatory on Mauna Kea in Hawaii. TMT is expected to begin operation in the early 2020s.

Members of the TMT International Observatory Board call India's commitment to the project "most welcome and essential" and say that it "will enhance science collaborations for the next generation."

"India's commitment is an exciting step in making our shared vision of the Thirty Meter Telescope a reality," says Ed Stone, the David Morrisroe Professor of Physics and Executive Director of the TMT International Observatory. "We look forward to a close collaboration with Indian colleagues in launching a new age of astronomy."

To learn more about TMT—as well as Caltech's involvement in the project—visit our TMT Updates page.


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Marvin L. "Murph" Goldberger


Marvin L. "Murph" Goldberger, Caltech president and professor of theoretical physics, emeritus, passed away on November 26, 2014. He was 92.

A Chicago native, Goldberger received his bachelor of science degree from the Carnegie Institute of Technology (now Carnegie Mellon University) in 1943 and, in 1948, his PhD in physics from the University of Chicago, where he later served as a professor of physics. In 1957, he was named Higgins Professor of Mathematical Physics at Princeton University, where he remained until 1978, when he was named Caltech's fifth president and a professor of theoretical physics.

During his tenure as president, Goldberger helped to spearhead the development of the first 10-meter telescope at the W. M. Keck Observatory in Hawaii—that telescope and its twin are the largest optical telescopes in the world—and he worked to secure the support of the Arnold and Mabel Beckman Foundation to build the Beckman Institute. "Murph played a key role in helping me convince Arnold Beckman to fund the Caltech proposal for an institute that would develop technology to support research in chemistry and biology," says Harry Gray, Arnold O. Beckman Professor of Chemistry and founding director of the Beckman Institute.

While Goldberger was in office, Caltech's endowment more than doubled. In addition, the Institute's teaching standards were revised and the curriculum was restructured, and the undergraduate houses were renovated.

"The loss of Murph Goldberger is one that affects us deeply here at Caltech, but also reverberates throughout the nation's scientific leadership and the international physics community," says Caltech president Thomas F. Rosenbaum. "Murph was a man whose excitement about physics was contagious, and his enjoyment of life and caring for individuals deeply felt. He held to a vision to push the Institute to new heights of discovery and educational distinction, realized through decisive interventions."

Goldberger left Caltech in 1987 to assume the directorship of the Institute for Advanced Study in Princeton. In 1991, he returned to California to become a professor of physics at UCLA; in 1993, he moved to UC San Diego, where he was a professor of physics and later dean of the university's Division of Natural Sciences.

Goldberger, a particle physicist who worked on the Manhattan Project, derived, with theoretical physicist Sam Bard Treiman, the so-called Goldberger-Treiman relation, which gives a quantitative connection between the strong and weak interaction properties of the proton and neutron.

The recipient of numerous awards and academic honors, including the Dannie Heineman Prize for Mathematical Physics, the Presidential Award of the New York Academy of Sciences, the Leonard I. Beerman Peace and Justice Award, and honorary degrees from Carnegie Mellon, the University of Notre Dame, Hebrew Union College, the University of Judaism, and Occidental College, Goldberger was a member of the National Academy of Sciences, the American Physical Society, the American Association for the Advancement of Science, the American Academy of Arts and Sciences, the American Philosophical Society; served as cochairman of the National Research Council; and was a member of the Council on Foreign Relations, the Institute on Global Conflict and Cooperation International Advisory Board, and the President's Science Advisory Committee.

He also was active in international security and arms-control issues and served on the Supercollider Site Evaluation Committee of the National Academies of Sciences and Engineering and as a member of the Media Resource Service Advisory Committee of the Scientists' Institute for Public Information. In addition, he was a member of JASON, an independent group that advises the United States government on matters of science and technology policy.

Goldberger was predeceased by his wife, Mildred Goldberger, in 2006. He is survived by his sons, Joel and Sam, and three granddaughters, Nicole, Natalie, and Natasha.

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Tuesday, December 2, 2014
Guggenheim 101 (Lees-Kubota Lecture Hall)

PUSD: Annual Open Enrollment

New Center Supports Data-Driven Research

With the advanced capabilities of today's computer technologies, researchers can now collect vast amounts of information with unprecedented speed. However, gathering information is only one half of a scientific discovery, as the data also need to be analyzed and interpreted. A new center on campus aims to hasten such data-driven discoveries by making expertise and advanced computational tools available to Caltech researchers in many disciplines within the sciences and the humanities.

The new Center for Data-Driven Discovery (CD3), which became operational this fall, is a hub for researchers to apply advanced data exploration and analysis tools to their work in fields such as biology, environmental science, physics, astronomy, chemistry, engineering, and the humanities.

The Caltech center will also complement the resources available at JPL's Center for Data Science and Technology, says director of CD3 and professor of astronomy George Djorgovski.

"Bringing together the research, technical expertise, and respective disciplines of the two centers to form this joint initiative creates a wonderful synergy that will allow us opportunities to explore and innovate new capabilities in data-driven science for many of our sponsors," adds Daniel Crichton, director of the Center for Data Science and Technology at JPL.

At the core of the Caltech center are staff members who specialize in both computational methodology and various domains of science, such as biology, chemistry, and physics. Faculty-led research groups from each of Caltech's six divisions and JPL will be able to collaborate with center staff to find new ways to get the most from their research data. Resources at CD3 will range from data storage and cataloguing that meet the highest "housekeeping" standards, to custom data-analysis methods that combine statistics with machine learning—the development of algorithms that can "learn" from data. The staff will also help develop new research projects that could benefit from large amounts of existing data.

"The volume, quality, and complexity of data are growing such that the tools that we used to use—on our desktops or even on serious computing machines—10 years ago are no longer adequate. These are not problems that can be solved by just buying a bigger computer or better software; we need to actually invent new methods that allow us to make discoveries from these data sets," says Djorgovski.

Rather than turning to off-the-shelf data-analysis methods, Caltech researchers can now collaborate with CD3 staff to develop new customized computational methods and tools that are specialized for their unique goals. For example, astronomers like Djorgovski can use data-driven computing in the development of new ways to quickly scan large digital sky surveys for rare or interesting targets, such as distant quasars or new kinds of supernova explosions—targets that can be examined more closely with telescopes, such as those at the W. M. Keck Observatory, he says.

Mary Kennedy, the Allen and Lenabelle Davis Professor of Biology and a coleader of CD3, says that the center will serve as a bridge between the laboratory-science and computer-science communities at Caltech. In addition to matching up Caltech faculty members with the expertise they will need to analyze their data, the center will also minimize the gap between those communities by providing educational opportunities for undergraduate and graduate students.

"Scientific development has moved so quickly that the education of most experimental scientists has not included the techniques one needs to synthesize or mine large data sets efficiently," Kennedy says. "Another way to say this is that 'domain' sciences—biology, engineering, astronomy, geology, chemistry, sociology, etc.—have developed in isolation from theoretical computer science and mathematics aimed at analysis of high-dimensional data. The goal of the new center is to provide a link between the two."

Work in Kennedy's laboratory focuses on understanding what takes place at the molecular level in the brain when neuronal synapses are altered to store information during learning. She says that methods and tools developed at the new center will assist her group in creating computer simulations that can help them understand how synapses are regulated by enzymes during learning.

"The ability to simulate molecular mechanisms in detail and then test predictions of the simulations with experiments will revolutionize our understanding of highly interconnected control mechanisms in cells," she says. "To some, this seems like science fiction, but it won't stay fictional for long. Caltech needs to lead in these endeavors."

Assistant Professor of Biology Mitchell Guttman says that the center will also be an asset to groups like his that are trying to make sense out of big sets of genomic data. "Biology is becoming a big-data science—genome sequences are available at an unprecedented pace. Whereas it took more than $1 billion to sequence the first genome, it now costs less than $1,000," he says. "Making sense of all this data is a challenge, but it is the future of biomedical research."

In his own work, Guttman studies the genetic code of lncRNAs, a new class of gene that he discovered, largely through computational methods like those available at the new center. "I am excited about the new CD3 center because it represents an opportunity to leverage the best ideas and approaches across disciplines to solve a major challenge in our own research," he says.

But the most valuable findings from the center could be those that stem not from a single project, but from the multidisciplinary collaborations that CD3 will enable, Djorgovski says. "To me, the most interesting outcome is to have successful methodology transfers between different fields—for example, to see if a solution developed in astronomy can be used in biology," he says.

In fact, one such crossover method has already been identified, says Matthew Graham, a computational scientist at the center. "One of the challenges in data-rich science is dealing with very heterogeneous data—data of different types from different instruments," says Graham. "Using the experience and the methods we developed in astronomy for the Virtual Observatory, I worked with biologists to develop a smart data-management system for a collection of expression and gene-integration data for genetic lines in zebrafish. We are now starting a project along similar methodology transfer lines with Professor Barbara Wold's group on RNA genomics."

And, through the discovery of more tools and methods like these, "the center could really develop new projects that bridge the boundaries between different traditional fields through new collaborations," Djorgovski says.

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Caltech Rocket Experiment Finds Surprising Cosmic Light

Using an experiment carried into space on a NASA suborbital rocket, astronomers at Caltech and their colleagues have detected a diffuse cosmic glow that appears to represent more light than that produced by known galaxies in the universe.

The researchers, including Caltech Professor of Physics Jamie Bock and Caltech Senior Postdoctoral Fellow Michael Zemcov, say that the best explanation is that the cosmic light—described in a paper published November 7 in the journal Science—originates from stars that were stripped away from their parent galaxies and flung out into space as those galaxies collided and merged with other galaxies.

The discovery suggests that many such previously undetected stars permeate what had been thought to be dark spaces between galaxies, forming an interconnected sea of stars. "Measuring such large fluctuations surprised us, but we carried out many tests to show the results are reliable," says Zemcov, who led the study.

Although they cannot be seen individually, "the total light produced by these stray stars is about equal to the background light we get from counting up individual galaxies," says Bock, also a senior research scientist at JPL. Bock is the principal investigator of the rocket project, called the Cosmic Infrared Background Experiment, or CIBER, which originated at Caltech and flew on four rocket flights from 2009 through 2013.

In earlier studies, NASA's Spitzer Space Telescope, which sees the universe at longer wavelengths, had observed a splotchy pattern of infrared light called the cosmic infrared background. The splotches are much bigger than individual galaxies. "We are measuring structures that are grand on a cosmic scale," says Zemcov, "and these sizes are associated with galaxies bunching together on a large-scale pattern." Initially some researchers proposed that this light came from the very first galaxies to form and ignite stars after the Big Bang. Others, however, have argued the light originated from stars stripped from galaxies in more recent times.

CIBER was designed to help settle the debate. "CIBER was born as a conversation with Asantha Cooray, a theoretical cosmologist at UC Irvine and at the time a postdoc at Caltech with [former professor] Marc Kamionkowski," Bock explains. "Asantha developed an idea for studying galaxies by measuring their large-scale structure. Galaxies form in dark-matter halos, which are over-dense regions initially seeded in the early universe by inflation. Furthermore, galaxies not only start out in these halos, they tend to cluster together as well. Asantha had the brilliant idea to measure this large-scale structure directly from maps. Experimentally, it is much easier for us to make a map by taking a wide-field picture with a small camera, than going through and measuring faint galaxies one by one with a large telescope." 

Cooray originally developed this approach for the longer infrared wavelengths observed by the European Space Agency's Herschel Space Observatory. "With its 3.5-meter diameter mirror, Herschel is too small to count up all the galaxies that make the infrared background light, so he instead obtained this information from the spatial structure in the map," Bock says. 

"Meanwhile, I had been working on near-infrared rocket experiments, and was interested in new ways to use this unique idea to study the extragalactic background," he says. The extragalactic infrared background represents all of the infrared light from all of the sources in the universe, "and there were some hints we didn't know where it was all coming from."

In other words, if you calculate the light produced by individual galaxies, you would find they made less than the background light. "One could try and measure the total sky brightness directly," Bock says, "but the problem is that the foreground 'Zodiacal light,' due to dust in the solar system reflecting light from the sun, is so bright that it is hard to subtract with enough accuracy to measure the extragalactic background. So we put these two ideas together, applying Asantha's mapping approach to new wavelengths, and decided that the best way to get at the extragalactic background was to measure spatial fluctuations on angular scales around a degree. That led to CIBER."

The CIBER experiment consists of three instruments, including two spectrometers to determine the brightness of Zodiacal light and measure the cosmic infrared background directly. The measurements in the recent publication are made with two wide-field cameras to search for fluctuations in two wavelengths of near infrared light. Earth's upper atmosphere glows brightly at the CIBER wavelengths. But the measurements can be done in space—avoiding that glow—in just the short amount of time that a suborbital rocket flies above the atmosphere, before descending again back toward the planet.

CIBER flew four missions in all; the paper includes results from the second and third of CIBER's flights, launched in 2010 and 2012 from White Sands Missile Range in New Mexico and recovered afterward by parachute. In the flights, the researchers observed the same part of the sky at a different time of year, and swapped the detector arrays as a crosscheck against data artifacts created by the sensors. "This series of flights was quite helpful in developing complete confidence in the results," says Zemcov. "For the final flight, we decided to get more time above the atmosphere and went with a non-recovered flight into the Atlantic Ocean on a four-stage rocket." (The data from the fourth flight will be discussed in a future paper.)

Based on data from these two launches, the researchers found fluctuations, but they had to go through a careful process to identify and remove local sources, such as the instrument, as well as emissions from the solar system, stars, scattered starlight in the Milky Way, and known galaxies. What is left behind is a splotchy pattern representing fluctuations in the remaining infrared background light. Comparing data from multiple rocket launches, they saw the identical signal. That signal also is observed by comparing CIBER and Spitzer images of the same region of sky. Finally, the team measured the color of the fluctuations by comparing the CIBER results to Spitzer measurements at longer wavelengths. The result is a spectrum with a very blue color, brightest in the CIBER bands.

"CIBER tells us a couple key facts," Zemcov explains. "The fluctuations seem to be too bright to be coming from the first galaxies. You have to burn a large quantity of hydrogen into helium to get that much light, then you have to hide the evidence, because we don't see enough heavy elements made by stellar nucleosynthesis"—the process, occurring within stars, by which heavier elements are created from the fusion of lighter ones—"which means these elements would have to disappear into black holes." 

"The color is also too blue," he says. "First galaxies should appear redder due to their light being absorbed by hydrogen, and we do not see any evidence for such an absorption feature."

In short, Zemcov says, "although we designed our experiment to search for emission from first stars and galaxies, that explanation doesn't fit our data very well. The best interpretation is that we are seeing light from stars outside of galaxies but in the same dark matter halos. The stars have been stripped from their parent galaxies by gravitational interactions—which we know happens from images of interacting galaxies—and flung out to large distances."

The model, Bock admits, "isn't perfect. In fact, the color still isn't quite blue enough to match the data. But even so, the brightness of the fluctuations implies this signal is important in a cosmological sense, as we are tracing a large amount of cosmic light production." 

Future experiments could test whether stray stars are indeed the source of the infrared cosmic glow, the researchers say. If the stars were tossed out from their parent galaxies, they should still be located in the same vicinity. The CIBER team is working on better measurements using more infrared colors to learn how the stripping of stars happened over cosmic history.

In addition to Bock, Zemcov, and Cooray, other coauthors of the paper, "On the Origin of Near-Infrared Extragalactic Background Light Anisotropy," are Joseph Smidt of Los Alamos National Laboratory; Toshiaki Arai, Toshio Matsumoto, Shuji Matsuura, and Takehiko Wada of the Japan Aerospace Exploration Agency; Yan Gong of UC Irvine; Min Gyu Kim of Seoul National University; Phillip Korngut, a postdoctoral scholar at Caltech; Anson Lam of UCLA; Dae Hee Lee and Uk Won Nam of the Korea Astronomy and Space Science Institute (KASI); Gael Roudier of JPL; and Kohji Tsumura of Tohoku University. The work was supported by NASA, with initial support provided by JPL's Director's Research and Development Fund. Japanese participation in CIBER was supported by the Japan Society for the Promotion of Science and the Ministry of Education, Culture, Sports, Science and Technology. Korean participation in CIBER was supported by KASI. 

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No Galaxy Too Small: An Interview with Evan Kirby

Assistant Professor of Astronomy Evan Kirby arrived on campus in August. Born and raised in New Orleans, Kirby earned his BS in 2004 from Stanford University; his undergraduate thesis involved trips to Pasadena to test an instrument built by JPL's Jamie Bock, now also a Caltech professor of physics, and the late Andrew Lange, the Marvin L. Goldberger Professor of Physics at Caltech. Kirby earned his MS and PhD degrees from UC Santa Cruz in 2006 and 2009.  His PhD thesis involved an analysis of the spectra of bright stars in dwarf galaxies orbiting the Milky Way. Then as a Caltech postdoc and Hubble Fellow from 2009 to 2012, he moved on to more distant stars in Andromeda and its satellite galaxies. As a Center for Galaxy Evolution Fellow at UC Irvine from 2012 to 2014, he shifted the focus of his spectral analyses from chemical makeups to stellar motions.


Q: What do you do?

A: I study the smallest galaxies we know about. The Milky Way and our nearest big neighbor galaxy, Andromeda, have pantheons of little galaxies in orbit around them. These galaxies are interesting because they are part of our cosmic story. The first galaxies to form were small ones, and over time they got smashed together to build up bigger ones. Tidal disruptions from our galaxy's gravity will eventually rip apart all the remaining dwarf galaxies orbiting us, and they will dissolve into the Milky Way—stars, dust, gas, and all. Similarly, Andromeda will swallow up its dwarfs.

Both sets of satellite galaxies are close enough that I can see each one's individual stars, instead of seeing the whole galaxy as a little smudge. This is important because I can record the spectrum of each bright star separately. A star's spectrum tells me its composition—how much iron is in that star, how much magnesium, how much calcium, and so on—and by compiling that information for each galaxy I can reconstruct its entire history.

The dwarf galaxies' histories tell us about our own; our galaxy formed at the same time and from the same material. It just got bigger faster.


Q: How big a telescope do you need to see a dwarf galaxy?

A: If you're in the southern hemisphere you can see the Milky Way's two biggest dwarfs, the Large and Small Magellanic Clouds, just by looking up at night. But the third biggest, the Sagittarius Dwarf Elliptical Galaxy, was only discovered in 1994 by a team of astronomers at the Cambridge (UK) Astronomical Survey Unit using a 47½-inch telescope modeled after our own 48-inch Samuel Oschin Telescope at Palomar Observatory. The other dwarf galaxies are a lot smaller and a lot fainter, so you need even bigger telescopes to find them.

However, the 10-meter Keck Telescope on Mauna Kea is definitely my instrument of choice. Andromeda is about 2.5 million light-years away, and the Keck gets me out to about 4.5 million light-years. If I go much beyond Andromeda, I no longer see galaxies as individual stars. And if I turn a medium-sized telescope on Andromeda, the stars become too faint to take spectra.


Q: A galaxy named Segue 2 features prominently on your website. What's the story there?

A: Segue 2 was discovered in 2007 by a group of astronomers at the Institute of Astronomy at Cambridge. I took spectra of many of its stars, which told me how fast they were moving. And I found that Segue 2's velocity dispersion, which is a measure of its mass, was less than 2.2 kilometers per second. That's very, very small, and it implies that Segue 2 has about a thousand stars, and up to another few hundred thousand solar masses' worth of dark matter. By comparison, the Milky Way's velocity dispersion is 200 kilometers per second and its total mass, including dark matter, is somewhere around a trillion solar masses. The Large Magellanic Cloud's mass is 20 times less than that. And the smaller dwarf galaxies typically have a few tens of millions of solar masses. A few hundred thousand solar masses is tiny.


Q: You mentioned dark matter. Does your work tell us anything about the nature of dark matter itself?

A: Absolutely. The currently accepted paradigm is "cold dark matter." Back in the 1980s, theorists began making computer models of the early universe to see how clouds of cold dark matter would coalesce. The big clumps became galaxies like Andromeda and the Milky Way, and the smaller clumps, called subhaloes, became their satellites. The simulations predicted that the Milky Way should be surrounded by lots and lots of satellites having about one one-hundredth the mass of the Magellanic Clouds, and a big problem arose well over a decade ago when we couldn't see as many of them as we thought we should. Finding things like Segue 2 is helping to resolve the missing-satellite problem.

Things got worse about three years ago, when astronomers discovered that not only were a lot of the little satellites missing, a lot of the big satellites are also missing! My officemate at UC Irvine, who did a lot of work on this, calls it the "too-big-to-fail" problem. The cold dark matter theory predicts there should be a decent number of subhaloes about one-tenth the size of the Magellanic Clouds. That's too big to not form stars, and if a subhalo that big forms stars, we should see the resulting galaxies—all of them. But we've counted up all the ones we can see, and we're missing about 10 of them. Either they don't exist, or somehow they did fail to form stars. Both alternatives challenge our understanding of how these dwarf galaxies form.


Q: How did you get started on all this?

A: When I was a little kid, I always wanted to be an astronaut. I realized that was unrealistic, so I chose a slightly less unrealistic goal—to be an astronomer. I subscribed to Astronomy magazine and bought all these astrophysics textbooks. I didn't understand a word of them, but I thought I was so cool for reading them. I went to Stanford knowing that I wanted to study physics and astronomy. I thought I would be a theoretician, but then I realized that observing is way cooler. Going out to the telescope is far more romantic than sitting in front of the computer, and I discovered I loved working with my hands and building instruments.


Q: What do you do for fun?

A: I'm into road cycling. UC Santa Cruz was Mecca for that. I biked uphill to campus four miles every day, and it really got me in shape. It was efficient—you should exercise for half an hour every day, so instead of spending 30 minutes sitting on a bus, I spent 30 minutes sitting on a bicycle.

When I was a Hubble fellow here, I met a postdoc named Hai Fu who became my best friend. I told him I was into biking and he said, "I am too. Let's go on a ride—I know an easy one." So I got in his car that weekend, and he started driving east on 210. After about an hour, I asked, "Where are you taking me?" "Oh, I'm just going to Mount Baldy." Cycling up Mount Baldy, the highest peak in the San Gabriel Mountains, was his idea of an easy ride.

I also play the clarinet. I was pretty serious about it at one point, but never professionally serious. Science and music are both hard to get jobs in, and I knew I had a much better chance in science.

Douglas Smith
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