Spotlight：China to play important role in building more powerful telescopes to observe distant galaxiesby Chen Yanbei, Guo Shuang
The latest estimation that there might be 10 times more galaxies in the universe than previously thought requires more powerful telescopes to observe the "missing" galaxies and China can be a strong player in this field, astronomers said.
"Evidence for 10 times more galaxies, invisible to current telescopes, strongly motivates the development and construction of next-generation telescopes, such as space telescopes and giant ground-based facilities, as well as the invention of new observational strategies," Zheng Cai, Hubble Fellow at the University of California, told Xinhua on Sunday.
Chinese President Xi Jinping recently wrote to emphasize that astronomy is crucial to propelling scientific progress and innovation. And in the field of future telescopes, China is becoming "a strong player," added Cai.
Galaxies are millions or billions of stars that gather in halos of dark matter. Since light takes time to travel, as astronomers gaze into the universe, they also look back in time. By observing galaxies billions of light years away, astronomers get glimpses of the early universe, billions of years ago, and look for answers to big questions like how the universe was born.
The universe was born around 13.7 billion years ago, and galaxies started forming when the universe was about several hundred million years old. The early galaxies were much smaller than current ones, they would later collide with each other, forming bigger ones.
As light travels from distant galaxies to the earth, it not only becomes much fainter, but also has increased wavelength due to the cosmological redshift effect. It is very difficult to observe the most distant galaxies, especially those among the first born.
Since the 1990s, astronomers used the so-called "Hubble Deep Field," a small patch of sky that NASA（The U.S. National Aeronautics and Space Administration）'s Hubble Space Telescope observed with long exposure time, to study early galaxies. By direct counting, astronomers estimated about 200 billion galaxies in the universe.
However, this turned out to be a big underestimate, according to a research recently conducted by a team of astronomers led by Christopher Conselice of Britain's University of Nottingham.
The scientists re-analyzed observational data with more elaborate mathematical models. After studying mass distributions of galaxies at different distances, they inferred the existence of a large number of small galaxies at large distances that are too faint to be observed by current telescopes.
These "missing" galaxies must have existed to collide and form the bigger galaxies that astronomers did observe. They may increase the total number of galaxies in the universe by 10 times.
"Two upcoming Chinese telescope projects are ideal tools for observing distant galaxies," said Xue Suijian, professor and deputy director of the National Astronomical Observatories of China（NAOC）under Chinese Academy of Sciences.
They are "a two-meter space-station optical telescope, and a 12-meter ground-based optical telescope," he added.
These two telescopes are among a series of large-scale astronomical projects to be undertaken by China.
According to Xue, the two-meter telescope will be the most challenging and ambitious scientific experiment to be performed by Tiangong, China's space-station project.
Chen Xuelei, a professor at the NAOC, added that this telescope's sensitivity will be comparable to that of the Hubble Deep Field observation but will observe a much larger patch of the sky.
The 12-meter ground-based telescope will potentially start construction in 2019, most likely in Tibet, and will be a stepping stone toward international cooperation on a larger-scale, 30-meter-class telescope, called the Thirty Meter Telescope（TMT）International Observatory, a cooperative project among the United States, Canada, China, India and Japan.
"As a founding member of the TMT International Observatory, China is making significant high-technology contributions to this world-leading facility," said Mao Shude, professor and director of the Center for Astrophysics at China's Tsinghua University and a member of the TMT board of governors.
According to Mao, the distant galaxies are so faint that on the ground, a 30-meter class telescope like the TMT may be necessary to scrutinize their internal structures.
Unfortunately, the TMT project has been much delayed, causing uncertainties about its eventual site and construction schedule.
Despite this, Edward Stone, executive director of the TMT International Observatory, and David Morrisroe Professor of Physics at the California Institute of Technology, told Xinhua that he continues to look forward to U.S.-China cooperation in astronomy and astrophysics.h Besides, the James Webb Space Telescope（JWST）, currently under construction and scheduled to launch in October 2018, may discover some of the currently invisible galaxies, Sterl Phinney, professor of theoretical astrophysics at the California Institute of Technology, told Xinhua.
A joint project between NASA, the Canadian Space Agency and the European Space Agency, the JWST is not only much larger than the Hubble Space Telescope but will also be optimized for infrared light, allowing it to better deal with the cosmological redshift effect.