Researchers from the University of Connecticut have created the first 3D maps of star-forming gas clouds in one of the most extreme environments in our galaxy, and have studied previous flaring events from our galaxy’s supermassive black hole, Sagittarius A* (Sgr A*).  

The center of the Milky Way is an extreme environment where gas temperatures, densities, and turbulence are about 10 times higher than the rest of the galaxy. In this central region inflowing gas can sometimes find its way into the supermassive black hole at the very center. When Sgr A* feeds on this material, it emits X-ray flares that propagate outwards in all directions. These flares interact with molecular clouds – gas clouds where stars form – in our galaxy’s center through the process of fluorescence. As the X-ray light travels, it illuminates slices of the molecular clouds over time, like an X-ray scan.  

University of Connecticut Physics Researcher Danya Alboslani ’24 (CLAS) and postdoctoral researcher Samantha Brunker, part of the Milky Way Laboratory led by Associate Professor of Physics Cara Battersby, developed a new X-ray tomography method to make 3D maps of two galactic center molecular clouds dubbed the “Stone” and the “Sticks” clouds. These maps are the first ever renderings of galactic center molecular clouds in three spatial dimensions. Alboslani detailed this research in her presentation “X-ray echoes from Sgr A* reveal the 3D structure of molecular clouds in the Galactic Center” at the 245th meeting of the American Astronomical Society (AAS) in National Harbor, Maryland on January 14th. Two manuscripts have been submitted to AAS Journals and will be available on arXiv January 14th at 20:00 EST. 

“We can study processes in the Milky Way’s Central Molecular Zone (CMZ) and use our findings to learn about other extreme environments. While many distant galaxies have similar environments, they are too far away to study in detail. By learning more about our own galaxy, we also learn about these distant galaxies that cannot be resolved with today’s telescopes,” says Alboslani. 

Alboslani explains that Sgr A* has gone through periods of intense activity in the past when it emitted X-ray flares. We did not have X-ray telescopes when those X-ray flares first came to Earth, however the X-ray light interacted with molecular clouds in the CMZ.

“The cloud absorbs the X-rays that are coming from Sgr A* then re-emits X-rays in all directions. Some of these X-rays are coming towards us, and there is this very specific energy level, the 6.4 electron volt neutral iron line, that has been found to correlate with the dense parts of molecular gas,” says Alboslani. “If you imagine a black hole in the center producing these X-rays which radiate outwards and eventually interact with a molecular cloud in the CMZ, over time, it will highlight different parts of the cloud, so what we’re seeing is a scan of the cloud.”

Since the galactic center is filled with large amounts of dust, the visible light may be obscured, but the X-rays emitted by Sgr A* during intense accretion events can be seen.  

Alboslani’s paper focuses on the Stone cloud, while Brunker’s paper studies the Sticks cloud. “The overall morphological agreement, and in particular, the association of the densest regions in both X-ray and molecular line data is striking and is the first time it has been shown on such a small scale,” says Brunker. 

Alboslani and Brunker used data from NASA’s Chandra X-ray Observatory spanning two decades to create their 3D models of the Stone and Sticks molecular clouds. Battersby explains that while we typically only see two spatial dimensions of objects in space, the X-ray tomography method allows us to measure the third dimension of the cloud because we see the X-rays illuminate individual slices of the cloud over time. “We can use the time delay between illuminations to calculate the third spatial dimension because X-rays travel at the speed of light,” Battersby explains. 

Alboslani and Brunker also used data from the Submillimeter Array and Herschel Space Observatory to compare the structures seen in the X-ray echoes to those seen in other wavelengths. Because X-ray data is not collected continuously, there are some structures seen in submillimeter wavelengths that are not seen in X-rays. However, they use these “missing” structures to constrain the duration of the X-ray flare event illuminating the Stone cloud.   

“We can estimate the sizes of the molecular structures that we do not see in the X-ray,” says Brunker, “and from there we can place constraints on the duration of the X-ray flare by modeling what we would be able to observe for a range of flare lengths. The model that reproduced observations with similar sized ‘missing structures’ indicated that the X-ray flare couldn’t have been much longer than 4-5 months.” 

Pioneering approaches like this method to model molecular clouds in 3D are helping researchers glean more information about the conditions that lead to star formation.

“While we learn a lot about molecular clouds from data collected in 2D, the added third dimension allows for a more detailed understanding of the physics of how new stars are born,” says Battersby. “Additionally, these observations place key constraints on the global geometry of our galaxy’s center as well as the past flaring activity of Sgr A*, central open questions in modern astrophysics.” 

 Another exciting aspect of this work is that it presents a new way to use archival data, says Alboslani, 

“New instruments and telescopes launched in the future will provide astronomers with higher resolution, allowing us to study objects in more detail. However, we can also look back in time and study objects over longer periods to extract new information-which is what this project does.” 

This work was made possible with funding from the National Aeronautics and Space Administration (Grant number 80NSSC22K1125), the University of Connecticut Summer Undergraduate Research Fund Award and Research Travel Award from the Office of Undergraduate Research, and Centre National d’Etudes Spatiales (CNES).