4.03.2025

Abstract
Like snow forming in clouds on a freezing winter day, interstellar ices form in
specific regions of the galaxy. 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 and are known to be rich in interstellar ices
containing water and organic molecules, though their properties remained
unclear. The ALMA observations have revealed the distance, motion, size, and
chemical composition of the molecular gas associated with these objects.
However, their characteristics do not match those of any previously known
interstellar objects where ices have been detected. This study suggests that
these two objects may represent a new class of interstellar icy objects that
provide an environment conducive to the formation of ices and organic molecules.
The findings were published in The Astrophysical Journal on February 25, 2025.
Background
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. The
interstellar medium contains submicron-sized solid particles, known as
interstellar dust, composed mainly of carbon, oxygen, silicon, magnesium, and
iron. In cold, dense, and shielded regions of the galaxy, atoms and molecules
adhere to the surfaces of these dust particles, leading to the formation of
interstellar ices. This process is similar to how snow forms in Earth’s clouds.
Since chemical reactions in solid states are far more efficient at producing
complex organic molecules than those in gaseous states, interstellar ices are
believed to be a crucial site for such molecular formation.
In 2021, using data from the infrared satellite AKARI, astronomers discovered
two interstellar objects that are bright in near- to mid-infrared wavelengths. These
objects, located in the Galactic plane toward the Scutum-Centaurus Arm, exhibit
deep absorption bands caused by ices and dust. Spectral analysis suggested
that the ices contain water, carbon dioxide, carbon monoxide, and organic
molecules such as methanol (Fig.1 right). 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. This discovery raised questions about their
nature.

Research Overview and Results
To investigate the properties of these mysterious interstellar icy objects, 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 (Figure 1, left). On the one hand,
the submillimeter thermal emission from interstellar dust, which was expected to
be detected in objects deeply embedded in interstellar dust like these two, was
not detected. Through these ALMA observations, various physical and chemical
properties of the mysterious icy objects have been revealed.

Figure 1. Left: Molecular emission lines from mysterious icy objects captured by the ALMA
telescope. The background image is an infrared composite color map, where 1.2-micron light
is shown in cyan and 4.5-micron light is in red, based on infrared data from 2MASS and
WISE. Top right: Infrared spectrum of the upper icy object in the left image, observed by
the AKARI satellite, showing absorption bands due to ices and dust. Bottom right: The
location of the two icy objects in the Galactic plane (image modified from the GAIA data).
Credit: ALMA (ESO/NAOJ/NRAO), ESA/Gaia/DPAC, T. Shimonishi et al. (Niigata Univ.).

First, based on the kinematic distances estimated from the line-of-sight velocity
of the gas, 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. "This was an unexpected result,
as these peculiar objects are separated by only about 3 arcminutes on the
celestial sphere and exhibit similar colors, brightness, and interstellar ice features,
but they are not linked each other,
" says Takashi Shimonishi, an astronomer at
Niigata University, Japan, and the lead author of the paper.
Understanding the wavelength range in which an object emits radiation most
strongly is useful for investigating its physical properties. Interstellar objects
known to be associated with ices are embedded in large amounts of interstellar
dust, causing them to shine brightly in the far-infrared to submillimeter
wavelength range. However, as shown in Figure 2, 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.
Figure 2. Energy distribution of one of the mysterious icy objects (black)
compared with those of known interstellar icy objects. Interstellar ices are
detected in protostars (green), young stars with protoplanetary disks (cyan), and
mass-losing evolved stars (brown), but the spectral characteristics of the
mysterious icy object do not match any of these known sources.
Credit: T. Shimonishi et al. (Niigata Univ.)

The molecular composition of the gas surrounding these celestial bodies was
also analyzed. The ratio of silicon monoxide to carbon monoxide was found to be
unusually high, suggesting the presence of strong shock waves that have
disrupted interstellar dust. The detection of broad molecular emission lines further
indicates that these objects are associated with turbulent energy sources.
The molecular composition of the gas associated with the two objects was also
analyzed. The molecular abundance ratio of silicon monoxide to carbon
monoxide, estimated from the emission lines, was about 0.001, which is much
higher than what is typically observed in normal molecular clouds. Regions where
such abundant silicon monoxide is detected are limited to areas where interstellar
dust is being destroyed by intense shock waves. Along with the detection of broad
molecular emission lines from the two objects, which indicate that the gas is in
vigorous motion, it is suggested that there is some energy source associated with
the objects that is strongly disturbing the gas.
Observations also provided insights into their size. The team estimated the size
of the gas and dust clouds associated with the two objects by comparing ALMA’s
emission data with AKARI’s absorption data for gas, ice, and dust. As a result, it
was found that the two objects are compact, ranging from 100 to 1,000
astronomical units in size, which is much smaller compared to typical molecular
clouds or cores.
The unique properties of the mysterious icy objects revealed by the ALMA
observations 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. "I have been studying interstellar ices
for nearly 18 years, but these two objects puzzle me because they are unlike any
other known source of interstellar ices," says Takashi Shimonishi.
"They may
represent a new class of interstellar objects that provide an environment
conducive to the formation of ices and organic molecules".
Future Prospects
Compact and isolated interstellar objects that are surrounded by ices and dust,
possess an environment conducive to the formation of complex organic
molecules, are accompanied by a vigorous energy source that disturbs the gas,
and shine brightly only in the near- and mid-infrared. Their true nature remains
unknown at this time. "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.

Quelle: 

Niigata University

Publication Details:
Authors: Takashi Shimonishi, Takashi Onaka, Itsuki Sakon
Title: ALMA Observations of Peculiar Embedded Icy Objects
Journal: The Astrophysical Journal