Title: Introducing the Rhea simulations of Milky Way-like galaxies I: Effect of gravitational potential on morphology and star formation
Authors: Junia Göller, Philipp Girichidis, Noé Brucy, Glen Hunter, Karin Kjellgren, Robin Tress, Ralf S. Klessen, Simon C. O. Glover, Patrick Hennebelle, Sergio Molinari, Rowan Smith, Juan D. Soler, Mattia C. Sormani, and Leonardo Testi
First Author’s Institution: Institute for Theoretical Astrophysics, Heidelberg University
Status: Submitted to Astronomy & Astrophysics [open]
The Narrative of Our Galaxy
The Milky Way is the galaxy in which our own planet resides, appearing from Earth as a soft, glowing band of light stretching across the night sky (Figure 1). Despite its striking beauty, the origins of galaxies like our own remain one of the most profound questions in astrophysics.
To the Ancient Greeks, the night sky was not merely a canvas of distant points of light, but a celestial narrative—one that intertwined their beliefs and their understanding of how the galaxy itself came to be. In one myth (explained in a footnote of today’s paper), the goddess Rhea was nursing her newborn son while protecting him from being devoured by her husband, Cronos. As she did, some of her milk flowed out across the heavens, forming the Milky Way. It was not just the Greeks; many cultures around the world, from South America, Northern Africa, to Central Asia, have also created folklores and names for our galaxy (check out this interesting article on Nautilus).
Today, we know that the Milky Way is made of hundreds of billions of stars, reservoirs of gas and dust, and a central supermassive black hole—all held together by gravity. It has spiral arms, and is a fairly typical galaxy. Nevertheless, exploring the structure and star formation of the Milky Way is challenging precisely because we are inside of it. Unlike viewing other galaxies, where the full shape and structure are visible from a distance, our position within the Milky Way limits our perspective. Observational astronomers have worked to overcome these challenges through multi-wavelength studies that probe many bands of light that can penetrate dust, uncover star formation, and detect black holes. However, observations still struggle to capture the full picture of the Milky Way.
The Rhea Simulations of Milky-Way Like Galaxies
Rhea is a new set of Milky Way-like idealized simulations that implement a detailed model for galaxy structure as well as star formation in an interstellar medium to study these exact questions: How does gravity shape galaxies like ours into the stunning spiral structures we observe? And how is galaxy structure related to the way stars form? In astrophysics, “idealized” simulations are computer simulations that test different physical processes under controlled, simplified settings. “Milky Way-like” galaxies typically refer to galaxies with properties similar to those of the Milky Way, often characterized by a prominent spiral structure and a stable, rotating disk.
To set the shape of the galaxy, the simulations follow an analytic formula for the gravitational potential, which describes how gravity influences the movement of matter within the galaxy. It acts like an invisible landscape, where stars and gas move in response to the “depth” of this potential. The deeper the gravitational potential well (i.e. near the galactic center), the stronger the gravitational pull. The Rhea simulations emulate the Milky Way by using a gravitational potential that includes a supermassive black hole similar to Sagittarius A*, a bulge near the galactic center, a stellar disk, and four spiral arms embedded in dark matter. In order to test their set-up, they compare it to a control simulation with a flat gravitational potential (Figure 2).
Where the Spirals Give Birth to Stars
Apart from the formation of a bar in the central regions of the galaxy, both simulations show similar spiral structures and average star formation rates. However, the differing gravitational potentials do affect where the star formation takes place. The Milky Way-like gravitational potential favors the formation of stars in the spiral arms, whereas stable spiral arms, where stars continue to form over time, did not appear with the flat potential (Figure 3). Moreover, the Milky Way-like case experienced a constant flow of gas toward the center of the galaxy, fueling denser star clusters. The Rhea simulations provide insight into the interplay between gravity, structure, and how the Milky Way’s spirals serve as a site for stellar birth.
Understanding the physical processes behind galaxy structure and formation is not only key to unlocking the past and fate of the cosmos, but also our place in it. As we refine these models and compare them with observations, we get closer to answering one of the mysteries of astrophysics: how our galaxy came to be, and what forces continue to shape its evolution.
Astrobite edited by Sparrow Roch.
Featured image credit: The Greek goddess Rhea against a backdrop of the Milky Way galaxy, adapted from Wikimedia Commons and NASA.
