Citizen science project confirms that galaxies become more active when they collide and merge

The Galaxy Cruise project, led by the National Astronomical Observatory of Japan (NAOJ), has been on a cosmic journey since 2019, collaborating with citizen astronomers to unveil the secrets of galaxies.

By leveraging deep, high-quality images from the Subaru Telescope and the precise classifications of galaxies provided by citizen astronomers, professional researchers have definitively confirmed that galaxies become more active during collisions and mergers with other galaxies. This groundbreaking discovery is detailed in the inaugural scientific paper from Galaxy Cruise, titled “Galaxy Cruise: Deep Insights into Interacting Galaxies in the Local Universe,” published online in the Publications of the Astronomical Society of Japan on September 26, 2023.

The universe boasts a rich tapestry of galaxies, from red elliptical ones to blue spirals, and even those without well-defined shapes. This diversity is believed to stem from the interactions and mergers between galaxies over cosmic timescales. However, the precise roles of these collisions and mergers have long eluded astronomers due to the rarity of interacting galaxies, making them challenging to locate.

To surmount this challenge, Galaxy Cruise enlisted the help of citizen astronomers to identify interacting galaxies within the deep images captured by the Hyper Suprime-Cam (HSC) on the Subaru Telescope. While these citizen astronomers may lack professional training, they are well-equipped after undergoing a training course that imparts the fundamentals of galaxy morphology, granting them a ticket to embark on the Galaxy Cruise.

A remarkable 10,000 citizen astronomers set sail, captivated by the galaxy’s diversity. They collectively contributed over 2 million classifications within the first 2.5 years of Galaxy Cruise, a feat unattainable by professional astronomers alone.

Figure 2: Spiral galaxies identified by the citizen astronomers. They all show beautiful spiral arms. Credit: NAOJ

Dr. Masayuki Tanaka, affectionately known as the “Captain” of Galaxy Cruise, meticulously analyzed the citizen scientists’ classifications and found that they were remarkably accurate. The quality of the HSC images played a pivotal role in this precision, revealing details in galaxies previously categorized as elliptical, but upon closer examination, exhibiting clear spiral arms. Tanaka notes that “The classification accuracy of Galaxy Cruise surpasses previous studies.”

This accuracy extended to the identification of interacting galaxies, the project’s primary focus. Thanks to the HSC’s high sensitivity and angular resolution, Galaxy Cruise succeeded in capturing faint, often overlooked features such as tidal tails that indicate galaxy interactions.

Figure 3: Violent mergers. The galaxies are significantly distorted by the strong tidal field, demonstrating how violent mergers can be. Credit: NAOJ

Surprisingly, the citizen astronomers uncovered that many galaxies previously considered “normal” actually displayed signs of interaction, and they also identified galaxies currently undergoing violent mergers, as depicted in Figure 3, showcasing their distorted shapes and complex structures. These violent mergers, a rarity, highlight the power of visual classifications by a large number of citizen astronomers.

The sample of interacting galaxies unveiled an elevated level of star formation activity compared to normal galaxies, and supermassive black holes were also found to be more active. This heightened activity was most pronounced in violent mergers, emphasizing that the final coalescence of merger events triggers the most substantial internal activity in galaxies. These significant findings were documented in Galaxy Cruise’s first scientific paper, marking a milestone for both astronomers and the participating citizen scientists.

Dr. Tanaka explained, “Researchers have often reached contradictory results when attempting to understand star formation and black hole activities in merging galaxies, likely due to the difficulties in identifying mergers and variations in their definitions and analyses. Galaxy Cruise’s visual classification approach, combined with high-quality HSC images, has allowed for a more robust merger sample, conclusively demonstrating that mergers enhance internal galaxy activities.”

The Galaxy Cruise classification catalog has been made available to the public, offering a valuable resource for professional astronomers worldwide. This catalog is poised to guide them toward new discoveries.

This story underscores the vital role of citizen scientists in advancing scientific knowledge. It serves as an invitation for others to join the ongoing voyage of Galaxy Cruise and contribute to solving the mysteries of galaxies alongside Captain Tanaka and his team.

Source: Subaru Telescope

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