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Context. The origin and nature of spiral arms remain unclear. Star-forming regions and young stars are generally strongly associated with the spiral structure, but there are few quantitative predictions from simulations about the contribution of stars of different ages. Aims. We aim to quantify the interplay between spiral arms and different populations. Methods. To this end, we used a hydrodynamical simulation of an isolated disc galaxy displaying a dynamic multi-armed spiral structure. Inspired by cosmological structure metrics, we developed a new method, the local dimension, that robustly delineates arms across populations and through space and time. Results. We find that all stars, including those as old as 11 Gyr, support the arms. The spiral strength decreases with stellar age up to 2 Gyr, and it remains nearly constant for older stars. However, the scaling between arm strength and age (or velocity dispersion) depends on the strength of the global spiral structure at each time. Almost all stars formed in arms remain within them for no more than 140–180 Myr, whereas old stars leave arms about three times faster. Even if the youngest populations dominate in the production of the spiral torques at early times, all populations contribute equally at later times. Conclusions. Our results highlight the power of the local dimension for studying complex spiral structures and show that spiral arms are present in all stellar populations. Since we see spiral arms in populations with velocity dispersions up to 90 km/s, which are comparable to those of the Milky Way, we predict that old Galactic populations could also exhibit spiral structure.
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Mapping luminous hot stars in the Galaxy
Luminous hot stars ( M K s ≲ 0 mag and T eff ≳ 8000 K) dominate the stellar energy input to the interstellar medium throughout cosmological time, are used as laboratories to test theories of stellar evolution and multiplicity, and serve as luminous tracers of star formation in the Milky Way and other galaxies. Massive stars occupy well-defined loci in colour–colour and colour–magnitude spaces, enabling selection based on the combination of Gaia EDR3 astrometry and photometry and 2MASS photometry, even in the presence of substantive dust extinction. In this paper we devise an all-sky sample of such luminous OBA-type stars, which was designed to be complete rather than very pure, providing targets for spectroscopic follow-up with the SDSS-V survey. To estimate the purity and completeness of our catalogue, we derive stellar parameters for the stars in common with LAMOST DR6 and we compare the sample to other O and B-type star catalogues. We estimate ‘astro-kinematic’ distances by combining parallaxes and proper motions with a model for the expected velocity and density distribution of young stars; we show that this adds useful constraints on the distances and therefore luminosities of the stars. With these distances we map the spatial distribution of a more stringently selected subsample across the Galactic disc, and find it to be highly structured, with distinct over- and under-densities. The most evident over-densities can be associated with the presumed spiral arms of the Milky Way, in particular the Sagittarius-Carina and Scutum-Centaurus arms. Yet, the spatial picture of the Milky Way’s young disc structure emerging in this study is complex, and suggests that most young stars in our Galaxy ( t age < t dyn ) are not neatly organised into distinct spiral arms. The combination of the comprehensive spectroscopy to come from SDSS-V (yielding velocities, ages, etc.) with future Gaia data releases will be crucial in order to reveal the dynamical nature of the spiral arms themselves.
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- Award ID(s):
- 2034429
- PAR ID:
- 10298458
- Date Published:
- Journal Name:
- Astronomy & Astrophysics
- Volume:
- 650
- ISSN:
- 0004-6361
- Page Range / eLocation ID:
- A112
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1051/0004-6361/202039726
