New gaia data reveals Milky Way is less massive than previously thought

The latest Gaia satellite catalog from the European Space Agency (ESA) has led to a groundbreaking discovery by an international team of astronomers from the Paris Observatory–PSL and the CNRS. They’ve achieved an unparalleled level of precision in measuring the Milky Way’s mass, which has far-reaching implications in the field of cosmology, especially in our understanding of the amount of dark matter within our galaxy.

In this groundbreaking study, it has been determined that the total mass of the Milky Way is approximately just two hundred billion times that of our Sun (2.06 x 10^11 solar masses). This represents a significant adjustment, with the new estimate being about four to five times lower than previous calculations.

The basis for this remarkable revelation lies in the third data release from the Gaia catalog, which was unveiled in 2022. This dataset comprises comprehensive information on a staggering 1.8 billion stars, offering insights into all three spatial dimensions and three velocity components, effectively providing a six-dimensional map of our Milky Way galaxy.

The bearable lightness of the Milky Way

Thanks to the Gaia data, scientists achieved a remarkable feat – they crafted the most precise rotation curve ever observed for a spiral galaxy, specifically our very own Milky Way. Prior to Gaia, establishing a robust rotation curve for our galaxy posed considerable challenges, unlike the relatively easier task for external spiral galaxies. This challenge was primarily due to our position within the Milky Way, making it exceptionally challenging to precisely discern the movements and distances of stars within the galactic disk.

In their groundbreaking study, which was published on September 27, 2023, in the prestigious journal Astronomy and Astrophysics, scientists unveiled an intriguing revelation about our galaxy’s rotation curve. Unlike the flat rotation curves typically observed in other large spiral galaxies, the Milky Way’s curve deviates from the norm.

Remarkably, as one ventures towards the outer edges of the galactic disk, this curve takes a sharp dip, conforming to the anticipated pattern known as the Keplerian decline.

Questioning cosmology

To establish a rotation curve for the Milky Way that displays a Keplerian decline, we must contextualize our galaxy within the broader framework of cosmology.

One of the pivotal breakthroughs in modern astronomy occurred when scientists realized that the rotational velocities of the expansive disks in spiral galaxies far exceeded what one would anticipate from a simple Keplerian decline. This revelation, dating back to the 1970s, was the result of the groundbreaking work of astronomers Vera Rubin, who focused on ionized gas observations, and Albert Bosma, who delved into the study of neutral gas. They demonstrated that the speed of rotation within spiral galaxies remains remarkably consistent, extending well beyond the boundaries of their visible optical disks.

This revelation had profound implications, as it led to the proposal of the existence of dark matter—a mysterious substance that exists alongside observable matter, distributed in a halo encompassing the outer regions of spiral galaxy disks. Without the presence of this elusive dark matter, the rotation curves would naturally follow a Keplerian decline, signifying the absence of substantial matter beyond the confines of the optical disk.

Source: Paris Observatory

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