Mysterious interstellar icy objects

Niigata, Japan - Organic molecules that serve as the building blocks of life are believed to form in space, but their exact formation sites and delivery mechanisms to planets remain a major mystery in astronomy and planetary science. One of the key elements in solving this mystery is the presence of ice in interstellar environments. In cold, dense, and shielded regions of the galaxy, atoms and molecules adhere to the surfaces of submicron-sized solid particles (dust), leading to the formation of interstellar ices. This process is similar to how snow forms in Earth’s clouds.

Astronomers from Niigata University and The University of Tokyo (Japan) conducted molecular gas observations of two enigmatic interstellar objects using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. These objects were previously discovered by the Japanese infrared satellite AKARI in 2021 and are known to be rich in interstellar ices containing water and organic molecules, though their properties remained unclear. Typically, interstellar ices are detected in dense regions of star-forming clouds, but these two objects do not belong to any known star-forming regions.

The research team used the ALMA telescope to observe them at a wavelength of approximately 0.9 mm. While infrared observations are effective for studying solid materials, radio observations are more useful for analyzing the motion and composition of associated gases. If these two interstellar objects were forming stars, ALMA’s high spatial resolution and sensitivity would detect various molecular emissions. Additionally, if a previously undetected molecular cloud were present in the direction of these objects, it would appear as spatially extended gas emissions in carbon monoxide.

However, the observations revealed something different from either of these expectations. At the positions of the two icy objects, only molecular emission lines of carbon monoxide and silicon monoxide were detected, exhibiting a very compact distribution of less than one arcsecond. Using the ALMA data, the team analyzed the distance, motion, size, and chemical composition of the molecular gas associated with these objects.

For example, based on the analysis of their line-of-sight velocities, it was suggested that the two objects are located approximately 30,000 to 40,000 light-years away from Earth. Additionally, the significant difference in their velocities indicates that these objects are kinematically independent and situated at different distances, despite being separated by only about 3 arcminutes on the celestial sphere and exhibiting similar colors, brightness, and interstellar ice features.

Interstellar objects with ices are usually embedded in large amounts of dust, causing them to shine brightly in the far-infrared to submillimeter wavelengths. However, the ALMA observations in this study did not detect submillimeter radiation from the two icy objects, revealing an unusual energy distribution that does not match the characteristics of previously known interstellar icy objects.

Furthermore, the ALMA observations revealed that the ratio of silicon monoxide to carbon monoxide in the two objects is significantly higher than what is typically observed in normal molecular clouds. Such abundant silicon monoxide is usually found only in regions where interstellar dust is being destroyed by intense shock waves, suggesting that the two objects are associated with an energy source that is strongly disturbing the gas.

The unique properties of the mysterious icy objects revealed by ALMA cannot be explained by the characteristics of any known objects associated with interstellar ices, such as newly formed stars, young stars with protoplanetary disks, evolved stars that exhibit intense mass loss, or bright stars located behind the dense molecular clouds.

"They may represent a new class of interstellar objects that provide an environment conducive to the formation of ices and organic molecules," says Takashi Shimonishi, an astronomer at Niigata University, Japan, and the lead author of the paper. "Future high-resolution observations of the associated gas using the ALMA telescope, along with more detailed studies of ices and dust with the James Webb Space Telescope, would shed light on the nature of these mysterious icy objects," hopes Takashi Shimonishi.

The findings were published in The Astrophysical Journal on February 25, 2025.

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