Post-95 Peking University doctoral students harvest “destined papers”

“China Science Daily” reporter Shen Chunlei

In September, he completed the Tour de Mont Blanc Supercross (UTMB), known to trail runners as the ultimate race. In the past eight months, he has carried his skis to many ski resorts around the world, from the observatories of Hawaii to the steep mountains of Alaska, leaving many eye-watering photos.


After running and climbing for 100 kilometers, Wang Kaixiang arrived at the UTMB finish arch in Chamonix, France. Courtesy of photographer Xiao Jin

He has won the team championship of the 2020-2021 National Collegiate Ski Challenge in the 2020-2021 snow season, the champion and runner-up of the Super Fixed-point Ski Open for many times, and is also one of the youngest finishers in the 100-kilometer cross-country race in China.

His name is Wang Kaixiang, and he is now a Ph.D. student in astrophysics in the Department of Astronomy at the School of Physics of Peking University. “This year’s birthday has to be celebrated two days later, and I’m waiting for a special birthday present. November 7 is Wang Kaixiang’s birthday, and he was a little excited to sell it in an interview with a reporter from China Science Daily. It turns out that the paper in which Kaixiang Wang is the first author and co-corresponding author will be published in Nature on November 9.

“This Nature article is the best birthday present I’ve ever received this year,” he said. In this paper, Wang Kaixiang sorted out the research work and achievements of the past five years, and revealed the mystery of the origin of ultra-dense dwarf galaxies that has puzzled astronomers for a long time.


Screenshot of the Nature paper

Fill in the gaps between galaxies and star clusters

“You astronomy, do you believe there are aliens?”

Wang Kaixiang often encounters such a question, and he usually asks rhetorically: “There are about tens of billions of stars like the sun in the Milky Way, and many stars have a planetary system like the solar system around them. And there are countless galaxies like the Milky Way in the vast universe. Since the solar system is not unique, do you think the Earth is unique?”

However, Wang Kaixiang’s research object is not aliens, but the dwarf galaxies and star clusters in the Virgo galaxy cluster, and this article published in Nature is also the result of Wang Kaixiang’s continuous focus and thinking for 5 years.

The public may be unfamiliar with the Virgo Cluster, but in fact, it has been on the hot search. In 2019, astronomers published the first ever image of a black hole, a supermassive black hole at the center of the M87 galaxy, which is the central galaxy of the Virgo cluster.

“The story told by this research is happening around M87. Wang Kaixiang said.

In astronomy, galaxies and star clusters are considered to be two distinct types of objects – galaxies are born and grow in dark matter halos and have a more complex history of star formation, while star clusters arise from giant molecular clouds in the interior of galaxies. Dwarf galaxies are those that are smaller, less massive, but they are often hundreds of times larger and more massive than ordinary star clusters.

Around 2000, astronomers discovered for the first time ultra-compact dwarf galaxies, which are somewhere between galaxies and star clusters, blurring the originally clear boundaries between galaxies and clusters.

Whether the nature of ultra-compact dwarf galaxies is galaxies or star clusters has been debated.

One side believes that ultra-dense dwarf galaxies originated from massive globular clusters, while the other side believes that ultra-compact dwarf galaxies are central nuclear clusters left over from the separation of outer star halos after ordinary nucleated dwarf galaxies have undergone accretion and merger. In addition, the astronomical community cannot rule out that ultra-dense dwarf galaxies represent the most compact type of galaxies, surrounded by a halo of dark matter, but this view is difficult to observe and verify.

Wang Kaixiang said that although more and more research results in recent years support that many ultra-dense dwarf galaxies may originate from stripped dwarf galaxies, astronomers have not yet clearly “seen” in their observations how this process that lasted for two or three billion years actually occurred.

Using the observations of the next-generation Virgo Cluster Survey (NGVS), Wang’s research team searched for about 600 ultra-compact dwarf galaxies in the Virgo Cluster, and after a very detailed structural analysis of these ultra-dense dwarf galaxies, it was found that about 15% of the ultra-dense dwarf galaxies were surrounded by very faint star halos, which exhibited unusual spatial distribution, color and kinematic characteristics, indicating that the predecessors of these objects came from more massive dwarf galaxies.


Wang Kaixiang runs under a solar eclipse. (This photo won the “2022 Astrophotographer Competition” held at the Beijing Planetarium)

“A Destined Paper”

This September, for Wang Kaixiang, is a little dreamy and a little unforgettable.

As a young cross-country ultramarathon runner, after several years of hard training and waiting, Wang Kaixiang finally came to Chamonix, France to participate in UTMB, standing on the pinnacle stage of the world’s extreme cross-country running, and completed the difficult race of 100 kilometers of the CCC group with an injury and a cumulative climb of 6,100 meters in the beautiful Alps.

Wang Kaixiang on the track at UTMB
In the middle of the competition, Wang Kaixiang also communicated with the editor of Nature about the paper by email. A week after Chamonix returned to China after the successful completion of the competition, Wang Kaixiang received an email from Nature’s editorial department, informing him that his article had been officially accepted.
“This is my first Nature paper and the first paper I published during my PhD. Wang Kaixiang clearly remembers the day he submitted the paper, which happened to be the 10,000th day of his birth, so he called it “a paper that was destined to be written”.
Aren’t you anxious that you are not anxious when you are about to graduate with a Ph.D. and have not published a single article? In the face of this reporter’s question, Wang Kaixiang said calmly: “The first research on ultra-dense dwarf galaxies was a small topic that I practiced a few years ago when I was just studying for my Ph.D. degree. Later, as my research progressed, I discovered more and more interesting phenomena, had more questions in my mind, and tried to fully unravel the ins and outs of the evolution of ultra-dense dwarf galaxies. ”
To this end, Wang’s research team spent more than two years applying for the world’s largest telescope time to observe, process and analyze data. “The most important thing in this process is to be calm. He sighed.
Due to the epidemic, the mentor and most of the collaborators were far away, and Wang Kaixiang spent 3 years of “fighting alone”. At the same time, they also successfully applied for more than 50 hours of observation time in two seasons for the 8-meter Gemini North telescope in Hawaii. During this period, Wang Kaixiang has maintained close online communication with his mentor and worked remotely under the guidance of his mentor.
Wang’s mentor, Eric Peng, was an associate professor in the Department of Astronomy at Peking University’s School of Physics and now works at the National Laboratory for Optical and Infrared Astronomy (NOIRLab). He said that studying the origin of ultra-compact dwarf galaxies is of great significance for understanding the formation and evolution of galaxies.
During his visit to the United States in the second half of 2022, after combing through a large number of observations, Wang Kaixiang suddenly realized that he had observed the various stages of the formation of ultra-dense dwarf galaxies, like time slices, depicting the transformation process of dwarf galaxies from ordinary dwarf galaxies to ultra-compact dwarf galaxies (and even star clusters) in the dense environment of galaxy clusters.
After discussing and communicating with his supervisor, Wang Kaixiang decided to combine these findings into a paper and submit it to the editorial department of Nature. “The review of the paper was relatively smooth, and it took less than half a year. In this regard, Wang Kaixiang believes that what impresses reviewers is that “through comprehensive and systematic research, changes that have not been discovered by predecessors have been discovered, which is expected to lead to more follow-up research”.

Schematic diagram of the whole process of the formation of ultra-compact dwarf galaxies. Drawing by Wang Kaixiang

When searching the literature, Wang Kaixiang found that since the discovery of ultra-compact dwarf galaxies, including this paper, the relevant research results have been published in Nature three times, and each paper was published almost 10 years apart. Wang Kaixiang also hopes that his work can become a new starting point for his field and his research career, and open up new ideas and windows for future research.

“Astronomy is what I love”

Skiing, photography, cross-countrying… In Wang Kaixiang’s WeChat and Weibo spaces, the sharing of these contents occupies a large part, and it is easy for people to misunderstand: “Where does this young man have time to do scientific research?”
“I love to travel outdoors, I like to use satellite maps and travelogues to search for relatively obscure spots, and plunge headlong into the mountains to experience the pristine charm of nature, which often happens to be the most beautiful place for the stars. Before hiking and camping weren’t as popular as they are today, I started exploring the mountains on my own. Wang Kaixiang said.
The Twelve Apostles at moonset and the Milky Way Arch Bridge. In July this year, Wang Kaixiang was photographed on the Great Ocean Road in Australia.
In 2009, Wang Kaixiang saw a total solar eclipse for the first time and fell in love with astronomy. After entering high school, Wang Kaixiang became the initial core member of the school’s newly formed astronomy club and began to systematically learn astronomy. In 2012, he won the first prize in the first national astronomy Olympiad, and was later selected for the national team, representing China in the Asia-Pacific Astronomy Olympiad that year, winning a silver medal.
“Astronomy is what I love and I hope to continue to strive for. “What you see is my skiing, cross-country and photography all over the world, but when I post these photos, I may be in the middle of reading papers or analyzing data.” ”


Wang Kaixiang trains at a ski resort in Chongli District, Zhangjiakou City.

Of course, Wang Kaixiang will also encounter situations where his research work has not progressed for the time being, and he will choose to go up the mountain for a run or a snowslide. He believes that extreme sports can shape a person and make them more resilient physically and mentally.
Read papers, apply for telescopes, make observations, analyze and discuss data… This is the real state of Wang Kaixiang’s work in recent years.

The results of this Nature study use some of the world’s top large telescopes, including the 3.6-meter Canada-France-Hawaii Telescope, the 8-meter Northern Twin Telescope, and the Hubble Space Telescope (HST), to reveal the formation and evolution of some extreme galaxies in the universe. Wang Kaixiang told China Science Daily, “The application for these telescopes is very badI hope that in the near future, our country will also build more large-scale optical astronomical telescopes to help Chinese astronomers make more cutting-edge discoveries. ”


The snow-covered Maunakia Observatory in Hawaii. Wang Kaixiang in February 2023

Today, Wang Kaixiang is no longer the thin little boy lying on the balcony at home more than 10 years ago, looking at the stars with his binoculars. He is using his astronomical knowledge to explore the starry sky and tell the world the story of the universe he has discovered.

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