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An unexpected pattern in the movements of the stars in the Milky Way disk indicates a recent hit from another galaxy.
Two independent groups of astronomers have found a fascinating motif that is revealed in the movements of the stars of the Milky Way. The diagram suggests that a neighboring galaxy swept by ours over the last billion years.
The spiral of Gaia
The Gaia satellite has provided exquisite and unprecedented measurements of distances and motions to 1 billion stars, providing a high resolution map of our galaxy. Since the release of the second version of data in April, there has been a deluge of exciting results, ranging from the discovery of new nearby groups and movements of distant galaxies. Now, astronomers are applying the data to our own galaxy.
This animation shows how a snail shell is formed over time in a graph representing the altitude of the stars above / below the plane of the Milky Way disk relative to their vertical velocity. At first, all stars are concentrated in one region of this graph. With time, each star circles clockwise and the global distribution begins to curve, possibly showing a spiral shape as we see in the Gaia data.
Teresa Antoja
Teresa Antoja (University of Barcelona, Spain) and her colleagues discovered for the first time an unexpected pattern when they selected 6 million nearby stars in the Gaia dataset with distances and distances. precise speeds. Looking at the stars through space, astronomers have not seen anything unusual. But a spiral shape appears when they look at the movements of the stars, especially when comparing the movements in the plane of the disk of the Milky Way to movements perpendicular to the disc. The spiral shape that appeared in a parcel of stellar velocities is known as spiral phase. The discovery appears in Nature.
Most stars in the galaxy are in the disk, orbiting the galactic center. The fact that their approximately circular orbits correspond to upward and downward movements, producing the spiral shape, means that something has hit the disc out of equilibrium. The spiral represents a large scale gravitational disturbance, so something with the size and mass of a small galaxy was the most likely culprit. When our galaxy first formed, these types of encounters were probably common, but the resulting disturbances gradually softened over hundreds of millions of years. Seeing a spiral phase, one may think that something has hit the disk of the Milky Way recently.
A recent close encounter
Although Gaia has set a new standard for understanding stars' distances and movements, it has not yet published detailed chemical compositions for many stars. Stars are mainly hydrogen and helium, but traces of heavier elements can reveal the age of stars and other properties. Spectroscopic ground surveys, such as the Galactic Archeology with HERMES survey (GALAH) of the Australian Astronomical Observatory, have measured these chemical properties at high resolution for hundreds of thousands of stars. By combining the Gaia and GALAH data, Joss Bland-Hawthorn (University of Sydney, Australia) and his colleagues studied phase spiral in a complementary way, using not only stellar movements, but also the heavy element content of stars for better understand the process. model. This study appeared on the arXiv preprint of astronomy on September 7th.
The impression of this artist shows a disturbance of the speeds of the stars in our galaxy, a model created by the destruction of another galaxy smaller, hundreds of millions of years ago.
ESA / CC BY-SA 3.0 IGO
Each successive generation of stars contains higher amounts of heavier elements than helium, which astronomers call metals (much to the chagrin of chemists around the world). This makes the metal content, or metallicity, an approximate approximation of the age of the stars: the most metal rich stars are younger. Bland-Hawthorn used GALAH and Gaia data to identify two records in the Milky Way, one rich in metal and one young, the other poor in metals and older.
The spiral phase is more apparent in the metal-rich disc, which allowed astronomers to estimate when our galaxy met its intruder. The Bland-Hawthorn team estimated that there are about 500 million years of encounters, which is in line with the estimates of the Antoja group, between 300 and 900 million years ago. It turns out that the dwarf spheroidal galaxy of Sagittarius passed near the Milky Way, about 200 million to 1 billion years ago, making it a good candidate for galactic disruption. This meeting is still tearing the dwarf itself to shreds.
The discovery of the spiral phase of the galaxy could be the beginning. Gaia's precise measurements provide clues of other correlated motions. – Antoja, for example, highlights the "snail shells and ridges" that complete the spiral. Identifying and understanding the creation of these models will help astronomers understand how our galaxy has evolved over time and what other recent interactions it has had with its neighbors.
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