Dynamical interplay of disc thickness and the interstellar gas: implication for the longevity of spiral density waves

Abstract

A typical galactic disc has a finite thickness and in addition to stars it also contains a finite amount of interstellar gas. Here, we investigate the physical impact of the finite thickness of a galactic disc on the disc stability against the non-axisymmetric perturbations and on the longevity of the spiral density waves, with and without the presence of gas. The longevity is quantified via group velocity of density wavepackets. The galactic disc is first modelled as a collisionless stellar disc with finite height and then more realistically as a gravitationally-coupled stars plus gas system (with different thickness for stars and gas). For each case, we derive the appropriate dispersion relation in the WKB approximation, and study the dynamical effect of the disc thickness on the life-time of spiral density waves via a parametric approach. We find the generic trend that the effective reduction in disc self-gravity due to disc thickness makes it more stable against the non-axisymmetric perturbations, and shortens the life-span of the spiral density waves. Further, the interstellar gas and the disc thickness are shown to have a mutually opposite dynamical effect on the disc stability as well as the longevity of the spiral density waves. While the gas supports the non-axisymmetric features for a longer time, the disc thickness has an opposite, quenching effect. Consequently, the net change is set by the relative dominance of the opposite effects of the interstellar gas and the disc thickness.

Spiral arms in broad-line regions of active galactic nuclei

“…It should be noted that radiation pressure from the disk may puff up the height of this region (e.g., Emmering et al 1992;Murray et al 1995;Chiang & Murray 1996;Czerny & Hryniewicz 2011;Elvis 2017;Baskin & Laor 2018) and provide a covering factor that is large enough to explain the BLR observations, which will ease the restriction of polytropic index. However, a thicker disk may potentially provide stability against non-axisymmetric perturbation and reduce the lifetimes of the spiral arms (Ghosh & Jog 2021). An observation noted in Horne et al (2021) indicates azimuthal structures in BLRs, which adds a constraint that the lifetime of the arms cannot be too short.…”

Spiral Arms in Broad-line Regions of Active Galactic Nuclei. I. Reverberation and Differential Interferometric Signals of Tightly Wound Cases

“…It should be noted that radiation pressure from the disk may puff up the height of this region (e.g., Emmering et al 1992;Murray et al 1995;Chiang & Murray 1996;Czerny & Hryniewicz 2011;Elvis 2017;Baskin & Laor 2018) and provide a covering factor that is large enough to explain the BLR observations, which will ease the restriction of polytropic index. However, more thick disk may potentially stabilize against non-axisymmetric perturbation and reduce the lifetime of spiral arms (Ghosh & Jog 2021). The observation in Horne et al (2021) has indicated the azimuthal structures in BLRs, which gives a constraint that the lifetime of the arms cannot be too short.…”