How the Milky Way got its spiral arms
How the Milky Way got its spiral arms
Spiral galaxies are named after their most prominent and surprising features: their spiral arms. These filaments of well-coiled stars and dust are undoubtedly one of the main reasons why images of spiral galaxies, originally taken for research purposes, are often popularly part of desktop backgrounds or as central pages of table books of coffee. Intuition correctly links the spiral structure with the rotation movement of these galaxies, but the precise mechanism responsible for the formation of the spiral arms is still not well understood. The objective of today's article is to improve this image by comparing the maps generated from the second publication of data from the space telescope Gaia with the simulations.
Figure 1: The Whirlpool Galaxy and its spectacular spiral arms (credit: NASA / ESA).
Several theories have been proposed to explain how the spiral arms are formed. Perhaps the most intuitive of these, known as the material arm model, indicates that the spiral arms represent excessive densities in the galaxy that have wrapped around the galactic nucleus due to the rotation of the galaxy (see figure 2). A theory of competition states that spiral arms are transient features that result from the superposition of unstable density waves that cause stars to cluster or scatter intermittently.
Unfortunately, it is a difficult task to study the spiral arms in reality in order to prove these theories. Traditionally, astronomers have relied on comparing simulations of spiral galaxies with real observations; but even this only provides an incomplete picture. This is due to the difficulty of analyzing the orbits of individual stars in the spiral arms, even in our own galaxy.
Figure 2: Contours of the potential in the Spiral arm model material for a rotating galaxy clockwise. The time for each panel appears in the upper right, and is given in units without dimensions related to the radius and speed of the scale to 9 kiloparsecs (the distance marked by the dotted line in each panel). The negative times in the upper panels correspond to a rotating galaxy in a counter-clockwise direction (Credit: Figure 9 in the article).
All this has changed since Gaia began his observation journey. The space telescope of the European Space Agency has brought a revolution in galactic astrophysics, by providing accurate position and velocity data for approximately one billion stars in the Solar neighborhood. This incredible amount of data has allowed astronomers to conduct unprecedented studies that investigate the characteristics of our galaxy (for example, see here, here Y here). Today's article uses this information to evaluate the various theories of spiral arms.
The authors select a subset of stars from the Gaia data set found in the slim disk of the Milky Way and, therefore, would be expected to be better tracers of the spiral substructure. To ensure high accuracy, they discard stars beyond 200 parsecs from the Sun, after which they still end up with an impressive sample of more than 300,000 stars, most of which have distances measured with uncertainties greater than 1%. Then they produce several maps of these stars, with several projections of their coordinates (see figure 3). To produce some of these maps, the authors use the positions and speeds of the stars to calculate their Actions, quantities such as angular momentum that remain constant over time and, therefore, can be used to define the complete trajectory of the orbit of an individual star (as shown in the right panel of Figure 3). The action space proves to be particularly useful for revealing the substructures that may be responsible for the spiral configuration of the galaxy.
Figure 3: Two projections of the data of ~ 300,000 Gaia stars. The panel on the left shows the density of the stars in terms of their velocity components in the plane of the Milky Way disk (U and V), with respect to the Sun. The panel on the right represents the density in the space of action, with the z-component of the angular momentum on the x-axis and the radial action (which is also a constant for each orbit) on the y-axis. Both axes are given with respect to the z component of the angular momentum of the Sun. The substructure in both plots indicates deviations from a smooth distribution function, such as overdensities that appear as spiral arms (Figure 3 in the article).
After having mapped the real data of Gaia, the authors try to reproduce these maps using each of the theories proposed for the formation of spiral arms. In each case, they begin with the simulation of a smooth distribution of stars and introduce a disturbance predicted by one of the theories they try to prove. Then they produce similar maps from their simulations and compare them with the real data.
The panel on the left of Figure 4 shows the projection of the action space for the transient model (in which the spiral arms are instabilities of short duration). The panel on the right of that figure shows the same projection for the real Gaia data (same as the panel on the right of Figure 3), this time in color to highlight five characteristics similar to the one predicted in the simulation. Additional similarities between this simulation and the Gaia data also appear in other projections, suggesting that the transient model may be the correct explanation for the spiral arms of the Milky Way. The simulations for the theory of the material arm do not coincide with the data as well as with the model transiently, although the authors can not discard them completely. They also produce a similar simulation for a third model, known as the model of mass grouping dress (dressed mass clump model), and in this case they are able to discard it due to the inconsistencies between the simulations and the real data.
Figure 4: Comparison of a characteristic caused by the transient mode model (left panel) with the actual Gaia distribution (right panel), with similar characteristics in the Gaia data highlighted in the right panel (Figures 7 and 13 in the article).
So, where does this leave us? The authors prefer the transient arm model, arguing that the data seem to suggest that the spiral arms of our galaxy are short-lived features that appear and disappear intermittently. Although the current analysis is not entirely conclusive and does not rule out all alternative theories, however, it serves as a great demonstration of the benefits of maps in the action space. It also serves as another reminder of the extraordinary amount of new science that allowed the second release of Gaia data, which continues to inspire cutting-edge research almost half a year after its release. Stay tuned!
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