Limitations and rotation of the two-armed phase spiral in the Milky Way stellar disc

¹ Lund Observatory, Division of Astrophysics, Department of Physics, Lund University, Box 118, 221 00 Lund, Sweden
² School of Physics & Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK

Received: 5 July 2024
Accepted: 17 August 2024

Abstract

Context. The Milky Way’s history of recent disturbances is vividly demonstrated by a structure in the vertical phase-space distribution known as the Gaia phase spiral. A one-armed phase spiral has been seen widely across the Milky Way disc, while a two-armed one has only been observed in the solar neighbourhood.

Aims. This study aims to determine the properties of the two-armed phase spiral and to put it in a Galactic context, with the ultimate goal of understanding the structure and history of the Milky Way disc.

Methods. The Gaia DR3 data were used to trace and characterise the two-armed phase spiral. Special focus was put on the phase spiral’s spatial distribution, rotational behaviour, and chemical characteristics. To quantify the properties of the phase spiral, we used a model that fits a spiral pattern to the phase space distribution of the stars.

Results. We found that the two-armed phase spiral is detectable only within a narrow range of galactocentric distances and angular momenta in the solar neighbourhood, R = 8 ± 0.5 kpc, L_Z = 1450 ± 50 kpc km s⁻¹. Outside this region, the phase spiral is one-armed. The two-armed phase spiral rotates with the phase angle, in a similar way to the one-armed phase spiral, and changes axis ratio with phase angle. Additionally, stars within the phase-space overdensity caused by the two-armed phase spiral pattern have slightly higher mean metallicity than stars in the underdense regions of the pattern at equivalent galactocentric distances, angular momenta, and vertical orbit extents.

Conclusions. The two-armed phase spiral rotates with phase angle and its effect can be seen in metallicity, in a similar way to the one-armed phase spiral. However, the limited range over which it can be found, and its variation in shape are quite different from the one-armed version, suggesting it is a much more localised phenomenon in the Galactic disc.