Galaxy Cruise, a public science project led by the National Astronomical Observatory of Japan (NAOJ), has been sailing the cosmic ocean with citizen astronomers since 2019 to unravel the mysteries of galaxies.

By combining deep, high-quality images from the Subaru Telescope with high-precision galaxy classification provided by citizen astronomers, professional researchers have conclusively confirmed that galaxies become more active as they collide and merge with other galaxies. This was reported in the first scientific paper on Galaxy Cruise. Citizen astronomers’ classifications are made publicly available so that astronomers around the world can use them for new discoveries.

This study was published online under the title “Galaxy Cruise: Insights into Interacting Galaxies in the Local Universe” on September 26, 2023, in the journal Proceedings of the Astronomical Society of Japan.

The universe is filled with a wide variety of galaxies; Some galaxies are red elliptical galaxies and some are blue spiral galaxies. There are also galaxies that do not have well-defined shapes. This diversity is thought to result from collisions and mergers of galaxies on cosmic time scales. However, the exact roles of collisions and mergers are not fully understood because interacting galaxies are rare objects and therefore difficult to find.

To overcome this problem, Galaxy Cruise enlisted the help of citizen astronomers to identify interacting galaxies from deep images taken by the Hyper Suprime-Cam (HSC) mounted on the Subaru Telescope.

Public astronomers are not professional astronomers by any means, but can they classify galaxies? So they can! Professional astronomers show them how to do it. Citizen astronomers are required to attend a training course to understand the basics of galaxy morphology. After completing the course, they receive a boarding pass for the Galaxy Cruise.

Many citizen astronomers participated in this project and were fascinated by the diversity of galaxies in the universe. In the first 2.5 years of the Galaxy Cruise, nearly 10,000 citizen astronomers explored the universe and made more than 2 million classifications. Making such a large number of classifications would not be possible by professional astronomers alone.

The “captain” of the Galaxy Cruise, Dr. Masayuki Tanaka carefully analyzed the classification of scientists and found that astronomers classify galaxies very well. Figure 2 shows the accuracy. The quality of HSC images is crucial for high classification accuracy; There are many galaxies that were classified as elliptical in previous studies but show clear spiral arms in deeper HSC images. “Galaxy Cruise’s classification accuracy surpasses previous studies,” says Tanaka.

The same goes for the interactive galaxies that are the focus of Galaxy Cruise. When galaxies collide and merge, they often have distorted shapes around them that have distinctive features such as tidal tails. These features are often scattered and faint and can be easily overlooked. However, HSC Galaxy Cruise successfully captured these faint details thanks to the high sensitivity and high angular resolution of its images.

Public astronomers have discovered that many of the “normal” (i.e., non-interacting) galaxies reported in previous studies actually show signs of interaction. Citizen astronomers have also identified galaxies currently undergoing violent mergers.

Figure 3 shows these violent mergers. All of them show significantly distorted shapes with a very complex structure. Such violent mergers are extremely rare, and statistical sampling of such galaxies demonstrates the power of visual classification by large numbers of citizen astronomers.

A sample of interacting galaxies showed that these galaxies exhibit higher levels of star-forming activity (Note 1) compared to normal galaxies. It also turned out that supermassive black holes are more active. Interestingly, this activity is most increased in violent conjunctions such as those shown in Figure 3.

The final merger is likely to be the moment when the galaxies’ internal activity becomes most intense. These interesting results were published in a scientific article. This is Galaxy Cruise’s first paper and is an important milestone not only for astronomers but also for citizen astronomers.

“A lot of effort has been devoted to understanding the process of star formation and black hole formation during galaxy mergers,” says Tanaka. “But researchers often come to conflicting conclusions. This is probably due to difficulties in identifying merging galaxies, differences in the definition of mergers, differences in galaxy analysis methods, etc. Galaxy Cruise’s approach to the problem is classically visual classification”.

“This is a time-consuming but powerful method of identifying mergers. When combined with high-quality HSC images, we were able to obtain a better sample of mergers than before, allowing us to conclusively confirm that mergers increase the internal activity of galaxies. This is an extremely exciting result and “This would not have been possible without the participation of so many citizen astronomers.”

With the publication of the article, the Galaxy Navigation Classification Catalog was also published. High-quality classifications will continue to be used by professional astronomers worldwide. I hope the catalog will lead them to new discoveries.

“People may think that scientific work is only for professional researchers. Not really. Citizens can certainly contribute, as Galaxy Cruise has shown. Galaxy Cruise is still on its voyage. Why don’t you join us? I look forward to meeting you on board and unraveling the mysteries of the galaxies together,” he concludes. Captain Tanaka. Source