On the Milky Way spiral arms from open clusters in Gaia EDR3

Abstract

Context. The physical processes driving the formation of Galactic spiral arms are still under debate. Studies using open clusters favour the description of the Milky Way spiral arms as long-lived structures following the classical density wave theory. Current studies comparing the Gaia DR2 field stars kinematic information of the Solar neighbourhood to simulations, find a better agreement with short-lived arms with a transient behaviour. Aims. Our aim is to provide an observational, data-driven view of the Milky Way spiral structure and its dynamics using open clusters as the main tracers, and to contrast it with simulation-based approaches. We use the most complete catalogue of Milky Way open clusters, with astrometric Gaia EDR3 updated parameters, estimated astrophysical information and radial velocities, to re-visit the nature of the spiral pattern of the Galaxy. Methods. We use a Gaussian mixture model to detect overdensities of open clusters younger than 30 Myr that correspond to the Perseus, Local, Sagittarius and Scutum spiral arms, respectively. We use the birthplaces of the open cluster population younger than 80 Myr to trace the evolution of the different spiral arms and compute their pattern speed. We analyse the age-distribution of the open clusters across the spiral arms to explore the differences in the rotational velocity of stars and spiral arms. Results. We are able to increase the range in Galactic azimuth where present-day spiral arms are described, better estimating its parameters by adding 264 young open clusters to the 84 high-mass star forming regions used so far, thus increasing by a 314% the number of tracers. We use the evolution of the open clusters from their birth positions to find that spiral arms nearly co-rotate with field stars at any given radius, discarding a common spiral pattern speed for the spiral arms explored. Conclusions. The derivation of different spiral pattern speeds for the different spiral arms disfavours classical density waves as the main drivers for the formation of the Milky Way spiral structure, and is in better agreement with simulation-based approaches that tend to favour transient spirals. The increase in the number of known open clusters, as well as in their derived properties allows us to use them as effective spiral structure tracers, and homogenise the view from open clusters and field stars on the nature of the Galactic spiral arms.

Using classical Cepheids to study the far side of the Milky Way disk. II. The spiral structure in the first and fourth Galactic quadrants

“…As far as we know, neither the spiral pattern speed for the MW is well established over the full extent of the disk, nor the assumption that it rotates as a rigid-body has been definitely proven (for a detailed discussion, see Dobbs & Baba 2014;Pettitt et al 2020). Recently, based on the current positions and computed birthplaces of the most complete catalogue of Milky Way open clusters with Gaia EDR3 astrometry, Castro-Ginard et al (2021) found that different spiral arms have distinct angular velocities, nearly co-rotating with the disk and disfavouring the density wave scenario. The understanding of the spiral structure of our Galaxy and the mechanisms involved in its formation are not yet settled.…”

Kinematic footprint of the Milky Way spiral arms in Gaia EDR3

“…the Local arm) suggests that the the MW spiral arms are quite complex with many authors favouring the scenario of flocculent transient spiral structures (e.g. Quillen 2002;Quillen et al 2018;Castro-Ginard et al 2021).…”

“…Dias & Lépine (2005) found how young open clusters must be to trace spiral arms; finding that clusters with ages under ∼ 12 Myr are still located inside the spiral arms. More recently, Castro-Ginard et al (2021) detected overdensities of open clusters younger than 30 Myr and traced the spiral arms with an increased range in Galactic azimuth. Georgelin & Georgelin (1976) were the first to propose the existence of four major spiral arms in the MW based on their determinations of the positions of H ii regions and exciting stars.…”