Gas velocity patterns in simulated galaxies: observational diagnostics of spiral structure theories

Abstract

There are two theories of stellar spiral arms in isolated disc galaxies that model stellar spiral arms with different longevities: quasi-stationary density wave theory, which characterises spirals as rigidly rotating, long-lived patterns (i.e. steady spirals), and dynamic spiral theory, which characterises spirals as differentially rotating, transient, recurrent patterns (i.e. dynamic spirals). In order to discriminate between these two spiral models observationally, we investigated the differences between the gas velocity patterns predicted by these two spiral models in hydrodynamic simulations. We found that the azimuthal phases of the velocity patterns relative to the gas density peaks (i.e. gaseous arms) differ between the two models, as do the gas flows; nevertheless, the velocity patterns themselves are similar for both models. Such similarity suggests that the mere existence of streaming motions does not conclusively confirm the steady spiral model. However, we found that the steady spiral model shows that the gaseous arms have radial streaming motions well inside the co-rotation radius, whereas the dynamic spiral model predicts that the gaseous arms tend to have tangential streaming motions. These differences suggest that the gas velocity patterns around spiral arms will enable distinction between the spiral theories.

Young stars as tracers of a barred-spiral Milky Way

“…That is: the minimum of Vr is associated with the potential minimum, whereas V φ experiences a strong turning point. These patterns are different to those seen in transient/dynamic spiral arm models (see Baba et al 2016 for a comparison between the two arm models, in particular their figure 4).…”

Evidence of Ongoing Radial Migration in NGC 6754: Azimuthal Variations of the Gas Properties

“…Azimuthal variations in the velocity distribution across a classical density wave-like spiral arm have been suggested by some authors (Minchev & Quillen 2008;Chemin et al 2016;Pasetto et al 2016). However, the gas motion should show a clear offset from the spiral arm density peak, and the offset should strongly depend on the radius, as demonstrated in Baba et al (2016). Also, to our knowledge, they do not find azimuthal metallicity variations.…”

Kinetic Tomography. II. A Second Method for Mapping the Velocity Field of the Milky Way Interstellar Medium and a Comparison with Spiral Structure Models

“…This is one reason for the greater diversity of v d fields in the dynamic spiral structure simulations relative to those in the SDW simulations. The two main qualitative predictions of the dynamic spiral structure model are that dense gas in growing spiral arms should be located at the center of converging flows and that spiral arms should, at some point, dissipate (Baba et al 2016). When a spiral arm is in the dissipation phase, the gas in that arm should be located in diverging parts of the velocity field.…”