The dark side of the Milky Way revealed by pulsar acceleration measurements

It is well known that the expansion of the universe is accelerating due to a mysterious dark energy. In galaxies, stars also experience acceleration, although this is due to a combination of dark matter and stellar density. In one new study to publish in Letters from the Astrophysical Journal, researchers have now obtained the first direct measure of the average acceleration occurring in our home galaxy, the Milky Way.

Directed by He loves Chakrabarti at the Institute for Advanced Study, with collaborators from the Rochester Institute of Technology, the University of Rochester, and the University of Wisconsin-Milwaukee, the team used pulsar data to clock the radial and vertical accelerations of stars inside and outside the galactic plane. Based on these new high-precision measurements and the known amount of visible matter in the galaxy, the researchers were then able to calculate the dark matter density of the Milky Way without making the usual assumption that the galaxy is in a state. stationary.

“Our analysis not only gives us the first measurement of the tiny accelerations undergone by the stars in the galaxy, but also opens the possibility of extending this work to understand the nature of dark matter, and ultimately dark energy on a larger scale. Said Chakrabarti, lead author of the article and current fellow and IBM Einstein Fellow at the Institute for Advanced Study.

Stars traverse the galaxy at hundreds of kilometers per second, but this study indicates that the change in their speed occurs at a literal snail speed – a few centimeters per second, which is roughly the same speed as a crawling baby. To detect this subtle movement, the research team relied on the ultra-precise ability to time-keep the pulsars that are widely distributed throughout the galactic plane and the halo – a diffuse spherical region that surrounds the galaxy.

“By exploiting the unique properties of pulsars, we were able to measure very small accelerations in the galaxy. Our work opens a new window into galactic dynamics, ”said co-author Philip chang from the University of Wisconsin-Milwaukee.

Extending outward about 300,000 light years from the galactic center, the halo may provide important clues to understanding dark matter, which makes up about 90% of the galaxy’s mass and is highly concentrated above and below. -below the starry galactic plane. Stellar movement in this particular region – a primary focus of this study – may be influenced by dark matter. Using the local density measurements obtained from this study, researchers will now have a better idea of ​​how and where to look for dark matter.

While previous studies assumed a state of galactic equilibrium to calculate the average density, this research is based on the natural unbalanced state of the galaxy. You could compare this to the difference between the surface of a pond before and after a stone is thrown. By taking the “ripples” into account, the team was able to get a more accurate picture of reality. Although in this case, rather than stones, the Milky Way is influenced by a turbulent history of galactic mergers and continues to be disturbed by outer dwarf galaxies like the Small and Large Magellanic Clouds. As a result, stars do not have flat orbits and tend to follow a path similar to that of a warped vinyl record, traversing above and below the galactic plane. One of the key factors that enabled this direct observational approach was the use of pulsar data compiled from international collaborations, including NANOGrav (North American Nanohertz Observatory for Gravitational Waves) which obtained data Green Bank and Arecibo telescopes.

This historical article develops the work of Jan H. Oort (1932); John bahcall (1984); Kuijken and Gilmore (1989); Holmberg and Flynn (2000); and Jo Bovy & Scott Tremaine (2012) to calculate the mean mass density in the galactic plane (Oort limit) and the local density of dark matter. IAS researchers, including Oort, Bahcall, Bovy, Tremaine, and Chakrabarti, have played an important role in advancing this area of ​​research.

“For centuries, astronomers have measured the position and speed of stars, but these only provide a snapshot of the complex dynamic behavior of the Milky Way galaxy,” said Scott Tremaine, Professor Emeritus at the Institute for Advanced Study. “The accelerations measured by Chakrabarti and his collaborators are directly caused by the gravitational forces of matter in the galaxy, visible and dark, and thus offer a promising new window into the distribution and composition of matter in the galaxy and the universe. “

This particular article will allow for a wide variety of future studies. Accurate measurements of accelerations will also soon be possible using the complementary radial velocity method developed by Chakrabarti earlier this year, which measures the evolution of star velocity with high accuracy. This work will also allow more detailed simulations of the Milky Way, improve constraints on general relativity and provide clues in the search for dark matter. Extensions of this method may ultimately allow us to directly measure cosmic acceleration as well.

Although a direct image of our original galaxy – similar to those of Earth taken by the Apollo astronauts – is not yet possible, this study provided essential new details to help envision the dynamic organization. of the galaxy from within.

Reference: “A Measurement of Galactic Plane Mass Density from Binary Pulsar Accelerations” by Sukanya Chakrabarti, Philip Chang, Michael T. Lam, Sarah J. Vigeland, Alice C. Quillen, accepted, Astrophyiscal Journal Letters.
arXiv: 2010.04018

Meeting: 237th Meeting of the American Astronomical Society