Revisiting the stellar velocity ellipsoid–Hubble-type relation: observations versus simulations

Abstract

The stellar velocity ellipsoid (SVE) in galaxies can provide important information on the processes that participate in the dynamical heating of their disc components (e.g. giant molecular clouds, mergers, spiral density waves, bars). Earlier findings suggested a strong relation between the shape of the disc SVE and Hubble type, with later-type galaxies displaying more anisotropic ellipsoids and early-types being more isotropic. In this paper, we revisit the strength of this relation using an exhaustive compilation of observational results from the literature on this issue. We find no clear correlation between the shape of the disc SVE and morphological type, and show that galaxies with the same Hubble type display a wide range of vertical-to-radial velocity dispersion ratios. The points are distributed around a mean value and scatter of σ z /σ R = 0.7 ± 0.2. With the aid of numerical simulations, we argue that different mechanisms might influence the shape of the SVE in the same manner and that the same process (e.g. mergers) does not have the same impact in all the galaxies. The complexity of the observational picture is confirmed by these simulations, which suggest that the vertical-to-radial axis ratio of the SVE is not a good indicator of the main source of disc heating. Our analysis of those simulations also indicates that the observed shape of the disc SVE may be affected by several processes simultaneously and that the signatures of some of them (e.g. mergers) fade over time.

The stellar velocity dispersion in nearby spirals: radial profiles and correlations

“…Parameter B is constant due to the assumption of a constant β z by K17 and A typically decreases with increasing R from a maximum ∼ 1. There is a large variation in B between galaxies, ranging between 0.4 and 1.3, which is larger than found in previous studies and typically used in models (Shapiro et al 2003;Leroy et al 2008;Romeo & Fathi 2015, 2016Pinna et al 2018).…”

“…These detailed local observations show the anisotropy between the radial, azimuthal and vertical stellar velocity dispersion components such that σ R > σ φ > σ z . The ratios of these components (anisotropy parameters) are often thought of as the velocity ellipsoid (e.g., Schwarzschild 1907) and are crucial to quantifying the anisotropy and understanding its causes (e.g., Spitzer & Schwarzschild 1951;Jenkins & Binney 1990;Shapiro, Gerssen, & van der Marel 2003;Gerssen, & Shapiro 2012;Pinna et al 2018). In particular, σ z /σ R has a minimum of 0.3 due to the bending instability (Rodionov & Sotnikova 2013) and is used to constrain these "disc heating" processes.…”

A mass-velocity anisotropy relation in galactic stellar disks

“…They also analyzed a compilation of σ z σ R from photometry and spectroscopy-based observations and came to the conclusion that no correlation exists between the morphological type and vertical anisotropy parameter, in contrast with claims of Gerssen & Shapiro Griffin (2012). The study of a range of simulated galaxy mass larger than that of Pinna et al (2018) would be useful to investigate whether the new magnitude and mass-anisotropy correlations shown in this work are present in cosmological numerical models as well. If present, such simulations could likely identify which of the secular and hierarchical mechanisms participates the most in the building of that relation.…”

“…During the refereeing step of this article, Pinna et al (2018) published a detailed analysis of the impact of many dynamical processes on σ z σ R from idealized and cosmological simulations. They found that the vertical-to-radial axis of the velocity ellipsoid can be produced by a multitude of disk heating mechanisms Fig.…”

Combining stellar populations with orbit-superposition dynamical modelling: the formation history of the lenticular galaxy NGC 3115

“…A number of studies have identified the conditions at birth as the dominant effect in determining a population's present-day velocity dispersion, again both from resolved observations (Leaman et al 2017) and simulations (Bird et al 2013;Ma et al 2017). Finally, the comparison between observations and simulations in Pinna et al (2018) has identified the underlying complexity and inherent degeneracy in discriminating between these scenarios. They claim that many of the effects described above likely play a role to a varying degree, and that the imprint of some mechanisms fade over time, further complicating any attempt to constrain the physical cause of disk heating.…”