Iron Snails: Nonequilibrium Dynamics and Spiral Abundance Patterns
Galaxies are not in a dynamical steady state: they continually undergo perturbations, e.g., from infalling dwarf galaxies and dark matter substructure. After a dynamical perturbation, stars phase mix toward a new steady state; in so doing, they generally form spiral structures, such as spiral densit...
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2025-01-01
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| author | Neige Frankel David W. Hogg Scott Tremaine Adrian Price-Whelan Jeff Shen |
| author_facet | Neige Frankel David W. Hogg Scott Tremaine Adrian Price-Whelan Jeff Shen |
| author_sort | Neige Frankel |
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| description | Galaxies are not in a dynamical steady state: they continually undergo perturbations, e.g., from infalling dwarf galaxies and dark matter substructure. After a dynamical perturbation, stars phase mix toward a new steady state; in so doing, they generally form spiral structures, such as spiral density waves in galaxy disks and the Gaia Snail observed in the vertical phase-space density in the solar neighborhood. Structures in phase-space density can be hard to measure accurately, because spatially varying selection effects imprint their own patterns on the density. However, stellar labels such as metallicity, or other element abundances, or stellar masses and ages, can be measured even in the face of complex or unknown spatial selection functions. We show that if the equilibrium galaxy has phase-space gradients in these labels, any perturbation that could raise a spiral wave in the phase-space density will raise a spiral wave in the distribution of labels as well. We work out the relationship between the spiral patterns in the density and in the labels. As an example, we analyze the Gaia Snail, and show that its amplitude and dynamical age as derived from elemental abundances (mainly [Mg/Fe]) follow similar patterns to those derived from the phase-space density. Our best model dates the Snail’s perturbation to about 400 Myr ago, although we find significant variations with angular momentum in the best-fit age. Conceptually, the ideas presented here are related to orbital torus imaging, chemical tagging, and other methods that use stellar labels to trace dynamics. |
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| language | English |
| publishDate | 2025-01-01 |
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| spelling | doaj-art-17e84a2be3274189a7587bb01101f61d2025-08-20T02:38:15ZengIOP PublishingThe Astrophysical Journal1538-43572025-01-0198718110.3847/1538-4357/add5eaIron Snails: Nonequilibrium Dynamics and Spiral Abundance PatternsNeige Frankel0https://orcid.org/0000-0002-6411-8695David W. Hogg1https://orcid.org/0000-0003-2866-9403Scott Tremaine2https://orcid.org/0000-0002-0278-7180Adrian Price-Whelan3https://orcid.org/0000-0003-0872-7098Jeff Shen4https://orcid.org/0000-0001-6662-7306Canadian Institute for Theoretical Astrophysics, University of Toronto , 60 St. George Street, Toronto, ON M5S 3H8, Canada ; frankel@cita.utoronto.ca; David A. Dunlap Department of Astronomy and Astrophysics, University of Toronto , 50 St. George Street, Toronto, ON M5S 3H4, CanadaMax Planck Institute for Astronomy , Königstuhl 17, D-69117 Heidelberg, Germany; Center for Cosmology and Particle Physics, Department of Physics, New York University , 726 Broadway, New York, NY 10003, USA; Flatiron Institute , 162 Fifth Avenue, New York, NY 10010, USACanadian Institute for Theoretical Astrophysics, University of Toronto , 60 St. George Street, Toronto, ON M5S 3H8, Canada ; frankel@cita.utoronto.ca; School of Natural Sciences, Institute for Advanced Study , Princeton, NJ 08540, USACenter for Cosmology and Particle Physics, Department of Physics, New York University , 726 Broadway, New York, NY 10003, USA; Flatiron Institute , 162 Fifth Avenue, New York, NY 10010, USADepartment of Astrophysical Sciences, Princeton University , 4 Ivy Lane, Princeton, NJ 08544, USAGalaxies are not in a dynamical steady state: they continually undergo perturbations, e.g., from infalling dwarf galaxies and dark matter substructure. After a dynamical perturbation, stars phase mix toward a new steady state; in so doing, they generally form spiral structures, such as spiral density waves in galaxy disks and the Gaia Snail observed in the vertical phase-space density in the solar neighborhood. Structures in phase-space density can be hard to measure accurately, because spatially varying selection effects imprint their own patterns on the density. However, stellar labels such as metallicity, or other element abundances, or stellar masses and ages, can be measured even in the face of complex or unknown spatial selection functions. We show that if the equilibrium galaxy has phase-space gradients in these labels, any perturbation that could raise a spiral wave in the phase-space density will raise a spiral wave in the distribution of labels as well. We work out the relationship between the spiral patterns in the density and in the labels. As an example, we analyze the Gaia Snail, and show that its amplitude and dynamical age as derived from elemental abundances (mainly [Mg/Fe]) follow similar patterns to those derived from the phase-space density. Our best model dates the Snail’s perturbation to about 400 Myr ago, although we find significant variations with angular momentum in the best-fit age. Conceptually, the ideas presented here are related to orbital torus imaging, chemical tagging, and other methods that use stellar labels to trace dynamics.https://doi.org/10.3847/1538-4357/add5eaGalaxy stellar disksStellar dynamicsGalaxy abundances |
| spellingShingle | Neige Frankel David W. Hogg Scott Tremaine Adrian Price-Whelan Jeff Shen Iron Snails: Nonequilibrium Dynamics and Spiral Abundance Patterns The Astrophysical Journal Galaxy stellar disks Stellar dynamics Galaxy abundances |
| title | Iron Snails: Nonequilibrium Dynamics and Spiral Abundance Patterns |
| title_full | Iron Snails: Nonequilibrium Dynamics and Spiral Abundance Patterns |
| title_fullStr | Iron Snails: Nonequilibrium Dynamics and Spiral Abundance Patterns |
| title_full_unstemmed | Iron Snails: Nonequilibrium Dynamics and Spiral Abundance Patterns |
| title_short | Iron Snails: Nonequilibrium Dynamics and Spiral Abundance Patterns |
| title_sort | iron snails nonequilibrium dynamics and spiral abundance patterns |
| topic | Galaxy stellar disks Stellar dynamics Galaxy abundances |
| url | https://doi.org/10.3847/1538-4357/add5ea |
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