From AAS NOVA: “When Dark Matter Gets Fuzzy”

AASNOVA

From AAS NOVA

14 August 2020
Susanna Kohler

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This composite image reveals the central region of our galaxy at X-ray (green and blue) and radio (red) wavelengths. A new study uses the Central Molecular Zone to constrain dark matter models. [X-Ray:NASA/CXC/UMass/D. Wang et al.; Radio:NRF/SARAO/MeerKAT]

NASA/Chandra X-ray Telescope

SKA SARAO Meerkat telescope(s), 90 km outside the small Northern Cape town of Carnarvon, SA

What model of Dark Matter best describes our universe? A new study uses a unique region in our own galaxy to constrain one particular model: that of fuzzy dark matter.

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There are many models describing the composition and behavior of dark matter, and how its evolution has affected the structure of our universe. [AMNH]

Observations of our universe tell us that only 15% of the universe’s matter is the ordinary baryonic matter that we’re able to see. The remaining 85% is dark matter — mysterious material that has shaped the structure and evolution of our universe via its gravitational interactions, but that doesn’t give off any light.

Because we can’t directly observe it, dark matter is still a relative unknown — and there are many different hypothesized models that describe its nature. Is dark matter hot? Cold? Composed of subatomic particles? Or macroscopic objects like primordial black holes? There’s a model for all of these options, and the best way to test them is to compare their predictions to the actual structure that we observe.

Constraints from an Odd Structure

One such constraining structure is a unique region in our own galaxy: the Central Molecular Zone, or CMZ. This extremely dense, rich collection of orbiting molecular gas lies in the very center of the Milky Way and spans just a few hundred light-years in diameter. Observations suggest that the molecular gas clouds orbit in a ring or a disk with a twisted 3D shape, but the thick dust that shrouds the galactic center limits what we can learn about the CMZ directly.

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Plot of gas surface density from a simulation showing the formation of the CMZ — seen as the high-density gas ring at the heart of the plot — in the center of the Milky Way. This simulation included a nuclear bulge only, with no dark-matter core from the fuzzy dark matter model. [Li et al. 2020]

The CMZ’s shape is not its only mystery, however: we also don’t fully understand what caused this odd structure to develop. Past studies of the birth of our galaxy’s structure from a thin disk suggest that formation of the CMZ relies on a combination of the Milky Way’s barred gravitational potential and an especially dense nuclear region.

In a new publication led by Zhi Li (Shanghai Jiao Tong University, China), a team of scientists has now used this picture to constrain a dark matter model that relies on light dark-matter particles concentrated at the center of the galaxy.

Adding Fuzziness to the Milky Way

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Zoomed-in plot of gas surface density from a simulation showing the formation of the CMZ in the center of the Milky Way. This simulation included both a nuclear bulge and a dark-matter core from the fuzzy dark matter model. [Adapted from Li et al. 2020]

Li and collaborators conduct a series of cosmological simulations that model the formation of the Milky Way from a thin disk in a realistic gravitational potential. In some of these simulations, the authors include only a dense nuclear bulge at the center of the galaxy. In others, they also add a galaxy core consistent with the predictions of fuzzy dark matter, a model that describes the universe’s dark matter as very light bosons that exhibit wave behavior on some scales.

The authors show that the structure and dynamics of the CMZ can be reproduced well with only an exceedingly compact nuclear bulge. But the combination of a smaller nuclear bulge and a fuzzy-dark-matter core also neatly reproduces observations, leaving the door open for this dark-matter model.

So is our dark matter fuzzy or not? We can’t tell yet, but Li and collaborators outline some future observations — like pinning down the mass-to-light ratio in the galactic center — that will help us answer this question and better understand what’s going on with that invisible 85% of our universe’s matter.

Citation

“Testing the Prediction of Fuzzy Dark Matter Theory in the Milky Way Center,” Zhi Li et al 2020 ApJ 889 88.

https://iopscience.iop.org/article/10.3847/1538-4357/ab6598

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Dark Matter Background
Fritz Zwicky discovered Dark Matter in the 1930s when observing the movement of the Coma Cluster., Vera Rubin a Woman in STEM denied the Nobel, did most of the work on Dark Matter.

Fritz Zwicky from http:// palomarskies.blogspot.com

Coma cluster via NASA/ESA Hubble

In modern times, it was astronomer Fritz Zwicky, in the 1930s, who made the first observations of what we now call dark matter. His 1933 observations of the Coma Cluster of galaxies seemed to indicated it has a mass 500 times more than that previously calculated by Edwin Hubble. Furthermore, this extra mass seemed to be completely invisible. Although Zwicky’s observations were initially met with much skepticism, they were later confirmed by other groups of astronomers.

Thirty years later, astronomer Vera Rubin provided a huge piece of evidence for the existence of dark matter. She discovered that the centers of galaxies rotate at the same speed as their extremities, whereas, of course, they should rotate faster. Think of a vinyl LP on a record deck: its center rotates faster than its edge. That’s what logic dictates we should see in galaxies too. But we do not. The only way to explain this is if the whole galaxy is only the center of some much larger structure, as if it is only the label on the LP so to speak, causing the galaxy to have a consistent rotation speed from center to edge.

Vera Rubin, following Zwicky, postulated that the missing structure in galaxies is dark matter. Her ideas were met with much resistance from the astronomical community, but her observations have been confirmed and are seen today as pivotal proof of the existence of dark matter.

Astronomer Vera Rubin at the Lowell Observatory in 1965, worked on Dark Matter (The Carnegie Institution for Science)


Vera Rubin measuring spectra, worked on Dark Matter (Emilio Segre Visual Archives AIP SPL)


Vera Rubin, with Department of Terrestrial Magnetism (DTM) image tube spectrograph attached to the Kitt Peak 84-inch telescope, 1970. https://home.dtm.ciw.edu

The Vera C. Rubin Observatory currently under construction on the El Peñón peak at Cerro Pachón Chile, a 2,682-meter-high mountain in Coquimbo Region, in northern Chile, alongside the existing Gemini South and Southern Astrophysical Research Telescopes.

LSST Data Journey, Illustration by Sandbox Studio, Chicago with Ana Kova

See the full article here .


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