Sticks and Stones: The Molecular Clouds in the Heart of the Milky Way

The Central Molecular Zone (CMZ) at the heart of the Milky Way holds a lot of gas. It contains about 60 million solar masses of molecular gas in complexes of giant molecular clouds (GMCs), structures where stars usually form. Because of the presence of Sag. A*, the Milky Way’s supermassive black hole (SMBH), the CMZ is an extreme environment. The gas in the CMZ is ten times more dense, turbulent, and heated than gas elsewhere in the galaxy.

How do star-forming GMCs behave in such an extreme environment?

Researchers have found a novel way to study two of the GMCs in the CMZ. The clouds are named “Sticks” and “Stones” and astronomers have used decades of X-ray observations from the Chandra X-ray Observatory to probe the 3D structures of the pair of clouds.

University of Connecticut Physics Researcher Danya Alboslani and postdoctoral researcher Dr. Samantha Brunker are both with the Milky Way Laboratory at the University of Connecticut. They’ve produced two manuscripts presenting their new X-ray tomography method and their results. Brunker is the lead author of “3D MC I: X-ray Tomography Begins to Unravel the 3-D Structure of a Molecular Cloud in our Galaxy’s Center,” and Alboslani is the lead author of “3D MC II: X ray echoes reveal a clumpy molecular cloud in the CMZ.” Brunker and Alboslani are also co-authors on each paper. Alboslani also presented her results at the recent 245th Meeting of the American Astronomical Society.

When gas from elsewhere in the galaxy reaches Sgr A*, it forms an accretion ring around the SMBH. As the gas heats up, it releases X-rays. These X-ray emission are only intermittent, and in the past, some of these episodes have been very intense. The X-ray travel outward in all directions, and while we didn’t have the capability to observe them, they interacted with GMCs near the CMZ. The clouds first absorbed them the re-emitted them in a phenomenon called fluorescence.

“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.”

The center of the galaxy is choked with dust that obscures our view of the region. Visible light is blocked, but the powerful X-rays emitted by Sgr A* during accretion events are visible.

Typically, astronomers only see two dimensions of objects in space. According to Battersby, their new X-Ray tomography method allows them to measure the GMCs’ third dimension. 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. It’s 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.

The Chandra X-Ray Observatory has been observing these X-rays for two decades, and as it observes them it sees different “slices” of the clouds, just like medical tomography. The slices are then built up into a 3D image. These are the first 3D maps of star-forming clouds in such an extreme environment.

The X-ray tomography method has one weakness. The X-ray observations aren’t continuous, so there are gaps. There are also some structures visible in submillimeter wavelengths that aren’t seen in X-rays. To get around that, the pair of researchers used data from the ALMA and the Herschel Space Observatory to compare the structures seen in the X-ray echoes to those seen in other wavelengths. The structures that are missing in X-rays but visible in submillimeter wavelengths can also be used to constrain the duratio of X-ray flares that illuminated the clouds.

“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.”

“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.

Detecting a third dimension of the clouds in this extreme environment could open new avenues of discovery.

“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.”

When it comes to how new stars from, there are many unanswered questions. While we know turbulence in GMCs can inhibit star formation, the exact mechanism is unkown. Astronomers are also uncertain how environmental factors affect star formation. There are many others and some of them can be answered by watching how GMCs behave in extreme environments.

There are also many questions regarding Sgr A*’s X-ray flaring. Astronomers aren’t certain how factors like magnetic reconnection events near the black hole and hot spots in the accretion flow affect X-ray flaring. They also aren’t certain why X-ray flaring occurs in random intervals. That’s just a sample of unanswered questions that could be addressed by studying GMCs in the galactic centre.

If all large galaxies contain SMBHs, which seems increasingly likely, then all large galaxies have CMZs that are extreme environments. The CMZs and the SMBHs are the heart of galaxies, and astrophysicists are keen to understand the processes that play out there, and if stars are able to form there.

“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 presents her results in this video from AAS 245. Her presentation begins at the 32:40 mark.

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