What is the mass of the Milky Way galaxy?

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Mass of the Milky Way: Astronomical Approaches To Galactic Weight

When pondering the question, “How does one weigh a galaxy?”, one may quickly come to realize the monumental task at hand, especially when the galaxy in question is the one we inhabit – the Milky Way. It’s a feat that combines mathematical complexities, astronomical observations, and an understanding of cosmic physics. The process is not straightforward, but scientists have established a variety of methods to estimate our galaxy’s weight. One such recent endeavor, summarized in a study published on the arXiv pre-print server, has combined these approaches to present an optimized estimation of the Milky Way’s mass.

991px Milky Way Arch mass of the Milky Way,milky way
The Milky Way arching at a high inclination across the night sky, (this composited panorama was taken at Paranal Observatory in northern Chile); the Magellanic Clouds can be seen on the left; the bright object near top center is Jupiter in the constellation Sagittarius, and the orange glow at the horizon on the right is Antofagasta city with a jet trail above it; galactic north is downward.

Stellar Motions: Key To Unearthing The Galactic Mass

One of the primary methods employed by astronomers revolves around observing the motions of stars within the Milky Way. To simplify, we can think of this conceptually: just as planets in our solar system orbit the Sun, stars in a galaxy orbit the galactic center. The roughly circular path followed by most Milky Way stars, coupled with the gravity keeping them in orbit, can be used to gauge the mass within a star’s orbit.

However, this does not mean every star follows a perfect circle. Rather, we observe that, on average, they follow a loosely circular orbit. By plotting the speed versus the distance of known stars from the galactic center, we obtain a useful tool in astronomy known as the rotation curve. This was first used in observing the Milky Way and other galaxies and provided the earliest evidence of galaxies possessing more mass than what was visible, thus introducing the concept of dark matter.

1920px Milky Way multispectral SMIL.svg mass of the Milky Way,milky way
The Milky Way viewed at different wavelengths

The Galactic Halo And Globular Clusters: More Than Meets The Eye

However, the rotation curve method hits a limit when we consider the distances to stars we can measure. As it turns out, most of our galaxy’s mass isn’t in the center but extends into a galactic halo. Estimating the halo’s mass can be done via the rotation curve method, but the motion of globular clusters, which are bright, dense clusters of stars, can provide additional information.

Globular clusters, being gravitationally bound, move around the galaxy as a single entity and are found in a sphere encompassing the Milky Way. Their motion gives us a method to measure the mass of the galactic halo.

550px Milky way map mass of the Milky Way,milky way
Map of the Milky Way Galaxy with the constellations that cross the galactic plane in each direction and the known prominent components annotated, including main arms, spurs, bar, nucleus/bulge, notable nebulae, and globular clusters

Satellite Galaxies And Tidal Forces: The Outer Limits And Cosmic Disruptions

To further refine our measurements, astronomers observe the motion of satellite galaxies such as the Magellanic Clouds. Around 60 small galaxies lie within approximately 1.4 million light-years of the Milky Way. Although not all are in orbit around our galaxy, a significant number are, and their orbital motions can shed light on our galaxy’s total mass.

Apart from orbital motion, other methods also exist for determining the galaxy’s mass. One such method involves studying the tidal plumes of dwarf galaxies. In our galaxy’s history, certain globular clusters and dwarf galaxies came too close to the Milky Way’s central region and were disrupted by tidal forces. By examining the remnants of these galaxies, we can form an estimate of galactic mass.

Escaping Stars And The Local Group: The Final Frontier

Another unique approach entails observing stars that are escaping our galaxy, often caused by near misses with other stars resulting in velocities high enough to escape the Milky Way. As the escape velocity is reliant on the galactic mass, studying these escaping stars can provide an additional estimate.

Lastly, we can turn our attention to the local group of galaxies, which includes the Andromeda galaxy and its satellite galaxies. Being gravitationally isolated from more distant galaxy clusters allows us to consider the equilibrium state of the local group to estimate both its overall mass and that of the Milky Way.

1024px Galactic longitude mass of the Milky Way,milky way
Diagram of the Sun’s location in the Milky Way, the angles represent longitudes in the galactic coordinate system

A Collective Insight: The Best Value For Our Galactic Mass

Every method has its merits and accuracy levels, and none can give a final, definitive answer independently. In this latest work, the team used a statistical average of various methods to derive the best estimate for the mass of our galaxy. The result? A breathtaking figure of a trillion solar masses, with a margin of error of a few hundred billion solar masses.

The Complexity of Weighing Galaxies and The Role of Dark Matter

Dark Matter: The Unseen Majority

While the methods mentioned above help us estimate the mass of the Milky Way, it’s essential to acknowledge a significant part of this mass is invisible to us. The concept of dark matter, introduced by the rotation curve method, plays a pivotal role in our understanding of the universe’s mass distribution. It’s an elusive type of matter that doesn’t interact with electromagnetic radiation, making it essentially invisible.

Dark matter’s existence is inferred by its gravitational effects on visible matter, lensing of light around galaxies, and the anisotropies observed in the cosmic microwave background. It’s a significant player when it comes to weighing galaxies as dark matter contributes to the majority of the mass in most galaxies. Current estimates suggest that nearly 85% of the matter in the universe is dark matter. In the Milky Way, the existence of a dark matter halo encompassing the visible matter is a widely accepted concept.

Delving into Dwarf Galaxies: A Cosmic Goldmine

One of the most intriguing aspects of studying galaxy mass is the observation of dwarf galaxies. These smaller galaxies often serve as satellites to larger galaxies, like the Milky Way. Dwarf galaxies provide a wealth of information for astrophysicists. The tidal plumes and streams they leave behind when disrupted by larger galaxies are invaluable for understanding not just the mass of our galaxy, but also the physics of galaxy formation and the nature of dark matter.

The Sagittarius Dwarf Spheroidal Galaxy, for example, is one of the several known dwarf galaxies orbiting the Milky Way. As it gets torn apart by tidal forces, it leaves behind a stream of stars that trace out its path, giving researchers a valuable tool for studying the mass distribution within our galaxy and the mysterious nature of dark matter.

The Cosmic Dance: Galactic Interactions and Mass Estimation

The motion of galaxies and their gravitational interactions within a local group provide another means of estimating galactic mass. For example, the Milky Way and the Andromeda Galaxy (our nearest spiral galaxy neighbor) are part of the same local group. Studying the motion of these galaxies, along with the dynamics of the satellite galaxies within this group, enables scientists to estimate the combined mass of this local group. From this, the individual mass of the Milky Way can also be inferred.

These techniques are not restricted to the local group. They can be applied to other galaxies and their associated groups, thereby helping us understand the mass distribution in the universe.

Galaxy Mass Estimation: An Evolving Science

Despite the progress made so far, the task of accurately determining the mass of galaxies, including our Milky Way, remains a challenging endeavor. Each method has its limitations, and our understanding of concepts such as dark matter is still evolving. As such, the quest to weigh our galaxy continues to be an exciting area of research in astronomy, with new methods and refinements to existing methods being developed regularly.

Read Original Article: https://arxiv.org/abs/2305.18408

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