Rome, 28 July 2021. The Milky Way, the galaxy where the Sun lives along with a few hundred billion stars, consists of a disc where most of these stars are located, and is characterised by a spiral shape. Ever since the 1950s, astronomers have been trying to constrain the number and geometry of the spiral arms, which has not been a trivial endeavour due to our position within the disc itself. This is finally changing thanks to Gaia, the European Space Agency’s (ESA) satellite that is creating a cosmic census of the position, distance and motion of almost two billion stars with unprecedented precision.

The new map, based on the latest Gaia data released last December, indicates that the geometric shape of the Milky Way’s spiral arms is different from the prediction of most earlier models. The results concern primarily the Perseus arm, one of the main structures in the galactic spiral, and the Local arm, a less pronounced structure where the Sun is located, among other stars.

“The Local arm appears to be larger than previously thought, reaching an extension of at least 26,000 light years. The Perseus arm, instead, exhibits a different geometry from that of many previous models, with a larger pitch angle”, explains Eloisa Poggio, first author of the paper published in Astronomy & Astrophysics, researcher at the Côte d'Azur Observatory, in France, and INAF associate in Turin.

The research relies on the largest sample of young stars – around 600,000 – ever used to map the spiral arms within about 16,000 light-years from the Sun, a distance that amounts to roughly one sixth of the Milky Way’s diameter of around 100,000 light-years. Analysing the spatial distribution of young stars in the Galaxy’s disc, the team has created maps of regions that are richer or less rich in stars with respect to the average density. The denser areas, also referred to as overdensities, appear to be organised in a coherent fashion, tracing the spiral arm segments in the vicinity of the Solar System.

“This study would not have been possible before Gaia”, adds co-author Ronald Drimmel, INAF researcher in Turin. “Gaia provides us with precise measurements of the positions for an unprecedented amount of data, with such a large number of objects that it enables us to have enough statistical information to apply the method of overdensity mapping in the disc”.

The team compiled the map using primarily young and bright stars, which trace the areas where the star formation activity is more intense, such as the spiral arms. In addition to these, they also analysed the distribution of other young stellar components, such as a class of variable stars called Cepheids and open star clusters.

“We know that the Milky Way is a spiral galaxy”, underlines Poggio. “However, even if this might sound paradoxical, we do not know exactly how many spiral arms there are in our Galaxy, where they are located exactly, and what is their shape, because the Solar System is embedded in the Galactic disc, making large-scale mapping much more complicated. Nevertheless, having a map of the spiral arms is key for several reasons: one example is to study the different physical phenomena that take place in the disc with respect to them”.

The new map of the galactic spiral in our cosmic neighbourhood is a first step towards the understanding of these structures, whose origin and dynamical nature have been long – and still are – heavily debated.

“Our study contributes to outlining an ever more detailed map of the Milky Way’s spiral structure”, Drimmel concludes. “With future data from Gaia, we plan to extend this map to farther distances and to compare the position of the spiral arms obtained from the stellar motions in the disc”.

For further information:

Galactic spiral structure revealed by Gaia EDR3, by E. Poggio, R. Drimmel, T. Cantat-Gaudin, P. Ramos, V. Ripepi, E. Zari, R. Andrae, R. Blomme, L. Chemin, G. Clementini, F. Figueras, M. Fouesneau, Y. Frémat, A. Lobel, D. J. Marshall, T. Muraveva and M. Romero-Gómez, is published online in the journal Astronomy & Astrophysics.

Gaia is a European Space Agency (ESA) mission launched on 19 December 2013. Italy has an important scientific participation in the mission with the National Institute for Astrophysics (INAF) and the Italian Space Agency (ASI) participating in the Data Processing and Analysis Consortium (DPAC). INAF is involved via its facilities in Bologna, Catania, Florence, Naples, Padua, Rome, Teramo and Turin (where the national management is based); ASI participates with the ASI Space Science Data Center (SSDC) and with the DPCT, the data processing center in Turin, the only one based in Italy out of the six in Europe, entirely dedicated to astrometric validation and hosting all mission data for a total of 1.5 petabytes, or 1.5 million gigabytes.

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