The 3D reconstruction reveals the activities of star formation from two dust clouds

Using tens of thousands of stars observed by the Gaia spacecraft, astronomers from MPIA and Chalmers have discovered 3D shapes of two large star-forming molecular clouds, the California Cloud and Orion A Cloud. comparable density. In 3D, however, they look pretty clear. In fact, their density is much different than their images projected in the plane of the sky suggest. This result solves the long-standing mystery of why these two clouds form stars at different speeds.

Cosmic clouds of gas and dust are the birthplaces of stars. In particular, stars form in the thickest pockets of such material. Temperatures drop to almost absolute zero and the densely packed gas collapses under its own weight, eventually forming a star. “The density, the amount of matter compressed in a given volume, is one of the crucial properties that determines the efficiency of star formation,” says Sarah Rezai Hoschbacht. She is an astronomer at the Max Planck Institute for Astronomy in Heidelberg, Germany and co-author of a new article published in The Astrophysical Journal Letters today.

In a pilot study described in this paper, Sarah Rezai Koshbacht and co-author Youni Kainulainen applied a method that allows them to reconstruct 3D morphologies of molecular clouds to two giant star-forming clouds. Kainulainen is a scientist from Chalmers University of Technology in Gothenburg, Sweden, who also worked at MPIA. Their targets were the Orion A Cloud and the California Cloud.

Measuring cloud density is usually difficult. “All we see when we observe objects in space is their two-dimensional projection onto an imaginary celestial sphere,” explains Youni Kainulainen. He is an expert in interpreting the influence of cosmic matter on stellar light and calculating the densities of such data. Kainulainen adds: “Conventional observations do not have the necessary depth. Therefore, the only density we can usually deduce from such data is the so-called column density.”

The column density is the mass added along the line of sight divided by the designed cross section. Therefore, these column densities do not necessarily reflect the actual densities of molecular clouds, which is problematic when linking cloud properties to star formation activity. Indeed, the images of the two clouds studied in this paper, which show heat dust emissions, apparently share similar structures and densities. However, their significantly different rates of star formation have puzzled astronomers for many years.

Instead, the new 3D reconstruction now shows that these two clouds are not so similar. Despite the filamentous appearance of 2D images, the California cloud is a flat and nearly 500 light-years-long sheet of material with a large balloon that extends below. Thus, no distance can be attributed to the California cloud, which has significant implications for the interpretation of its properties. From our point of view on Earth, it is oriented almost marginally, which only simulates a filamentous structure. As a result, the actual density of the sheet is much lower than the density of the column suggests, explaining the discrepancy between previous estimates of density and cloud rate.

And what does Orion A Cloud look like in 3D? The team confirmed its dense filamentous structure seen in 2D images. However, its actual morphology also differs from what we see in 2D. Orion A is quite complex, with additional condensation on the protruding ridge of gas and dust. On average, Orion A is much denser than the California cloud, which explains its more pronounced star-forming activity.

Sara Rezaei Khoshbakht, also affiliated with Chalmers University of Technology, developed the 3D reconstruction method during her doctorate. in MPIA. It involves analyzing the change in starlight as it passes through these clouds of gas and dust, measured by the Gaia spacecraft and other telescopes. Gaia is a project of the European Space Agency (ESA), whose main goal is to accurately measure the distances to over a billion stars in the Milky Way. These distances are crucial for the method of 3D reconstruction.

“We analyzed and crossed the light of 160,000 and 60,000 stars for the California and Orion A clouds, respectively,” said Sarah Rezai Koshbacht. The two astronomers reconstructed the morphologies and densities of clouds with a resolution of only 15 light years. “This is not the only approach astronomers use to extract spatial cloud structures,” added Rezai Hosbacht. “But ours gives stable and reliable results without digital artifacts.”

This study demonstrates its potential to improve research into the formation of stars in the Milky Way by adding a third dimension. “I think one important result of this work is that it has led to research that relies solely on column density thresholds to bring out the properties of star formation and compare them to each other,” concludes Sarah Rezai Koshbacht.

However, this work is only the first step in what astronomers want to achieve. Sarah Rezai Hoschbach is pursuing a project that will eventually lead to the spatial distribution of dust throughout the Milky Way and reveal its connection to star formation.