Spiral density waves in the outer galactic gaseous discs

Abstract

Deep HI observations of the outer parts of disc galaxies demonstrate the frequent presence of extended, well-developed spiral arms far beyond the optical radius. To understand the nature and the origin of such outer spiral structure, we investigate the propagation in the outer gaseous disc of large-scale spiral waves excited in the bright optical disc. Using hydrodynamical simulations, we show that non-axisymmetric density waves, penetrating in the gas through the outer Lindblad resonance, can exhibit relatively regular spiral structures outside the bright optical stellar disc. For lowamplitude structures, the results of numerical simulations match the predictions of a simple WKB linear theory. The amplitude of spiral structure increases rapidly with radius. Beyond ≈ 2 optical radii, spirals become nonlinear (the linear theory becomes quantitatively and qualitatively inadequate) and unstable to Kelvin-Helmholtz instability. In numerical simulations, in models for which gas is available very far out, spiral arms can extend out to 25 disc scale-lengths. A comparison between the properties of the models we have investigated and the observed properties of individual galaxies may shed light into the problem of the amount and distribution of dark matter in the outer halo.

Synthetic HI observations of spiral structure in the outer disk in galaxies

“…For the perturbed (evolved) disk state the flat rotation curve is not so clearly evident because non-linear spiral perturbations strongly affect the velocity field: rather regular variations of the LOS velocity, with amplitude 30 − 50 km s −1 , mark the location of the spiral arms. Such systematic variations, if found in 21 cm observations, would add strong support to the dynamical scenario we are proposing (Bertin & Amorisco 2010;Khoperskov & Bertin 2015). The position-velocity diagrams displayed in Fig.…”

“…In Sects. 2 and 3 we briefly summarize the general set-up and results of a separate paper (Khoperskov & Bertin 2015) where the present dynamical scenario is described in full detail. In particular, the dynamical model and numerical approach are summarized in Sect.…”

Spiral patterns beyond the optical radius: numerical simulations and synthetic HI observations

“…The amplitude of the line-of-sight velocity perturbations increases with radius, marking the location of the spiral arms. Such systematic variations, if found in 21 cm observations, would give strong support to the physical scenario that we are considering (Bertin & Amorisco 2010, Khoperskov & Bertin 2015a.…”

“…In the present paper we focus on the case in which the outer gaseous disk is dominated by a two-armed spiral pattern. Other examples with more complex morphologies can be found in a full-length journal article (Khoperskov & Bertin 2015a). We consider density-wave perturbations of the hydrodynamical quantities at the inner boundary (cyan thin circular annulus in Fig.…”

Molecular Gas in the Outskirts

“…This Chapter focuses on observations, but here we briefly highlight theoretical works that are related to molecular gas in the outskirts. The majority of the relevant theoretical studies have concentrated on the origin of gas in the outskirts (e.g., Dekel and Birnboim 2006;Sancisi et al 2008;Sánchez Almeida et al 2014;Mitra et al 2015) and star formation in the outskirts (Bush et al 2008(Bush et al , 2010Ostriker et al 2010;Krumholz 2013;Sánchez Almeida et al 2014; see also Roškar et al 2010;Khoperskov and Bertin 2015). Krumholz (2013) is particularly relevant because he extended earlier work to develop an analytic model for the atomic and molecular ISM and star formation in outer disks.…”