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Alar Toomre connected the stability of a stellar galactic disk to the competition between self-gravity, rotation and random stellar motion. His 1964 analysis gave a local axisymmetric stability criterion, commonly expressed through the Toomre parameter. A dynamically cold disk can be more vulnerable to gravitational structure formation than one with larger velocity dispersion.^[ref]

With Juri Toomre, he also modeled the tidal bridges and tails of interacting galaxies. Their test-particle encounters showed how gravity alone can draw long structures out of rotating galactic disks.^[ref]

A stellar disk does not respond to a density wave as a collisional gas with a local pressure law. Its stars follow individual orbits, and their spread in radial motions partly averages away the gravitational response to short-wavelength perturbations. The appropriate calculation uses a perturbed distribution function rather than only a fluid sound speed.

A characteristic parameter is $\chi =(k{\sigma }_R/\kappa {)}^2$, comparing the radial epicycle scale with the perturbation wavelength. Orbital averaging reduces the response to perturbations whose wavelength is small relative to the stellar excursions. Merely replacing the fluid sound speed with stellar velocity dispersion misses that orbital averaging and can give the wrong stability threshold.^[ref]

Gravitational instability of an astrophysical disk occurs when a density enhancement attracts enough additional matter for self-gravity to overcome the disk's restoring motions. It can generate spiral density waves or, under suitable thermal conditions, lead to cooling-limited disk fragmentation. Gas pressure and orbital motion oppose collapse on different scales.

For local axisymmetric disturbances of an inviscid razor-thin disk with surface density of a disk $\Sigma$, barotropic sound speed $c_s$ and radial epicyclic frequency $\kappa$, the compressive dispersion relation isThe razor-thin disk Poisson kernel supplies the attractive $|k|$ term. Pressure dominates at large wavenumber, while epicyclic motion stabilizes sufficiently small wavenumbers. Minimizing over $|k|$ gives $|k{|}_{\ast }=\pi G\Sigma /c_s^2$ and ${\omega }_{\min }^2={\kappa }^2[1-Q^{-2}]$, where the gas Toomre parameter is $Q=c_s\kappa /(\pi G\Sigma )$. Thus $Q<1$ admits exponential local axisymmetric growth; $Q=1$ is marginal. Collisionless stellar disks require a different random-motion treatment and numerical normalization.^[ref]

Linear growth does not guarantee bound fragments. Compression and shocks heat the gas; continued collapse depends on radiative cooling, transport and the tidal environment. Cooling-limited disk fragmentation therefore asks a further nonlinear question beyond whether an infinitesimal disturbance grows.^[ref]

differential rotation winds a material arm because $d\Omega /dr\ne 0$. A spiral density wave instead has a pattern speed different from the material speed: stars and gas can pass through its crests. Collective self-gravity, orbital resonances and shear can sustain or repeatedly amplify non-axisymmetric patterns; a visible spiral need not be a single permanent rigid material structure.

For a razor-thin gas disk, local tightly wound disturbances satisfyRotation stabilizes long wavelengths and pressure stabilizes short ones. Minimizing the axisymmetric right-hand side over $|k|$ gives $|k{|}_{\ast }=\pi G\Sigma /c_s^2$ andfor local axisymmetric stability. A collisionless stellar disk instead has $Q_{\ast }={\sigma }_R\kappa /(3.36G{\Sigma }_{\ast })$; the radial random velocity dispersion is not literally a gas sound speed.^[ref]

Local axisymmetric stability does not guarantee global disk stability. Gas and stars gravitate together, thickness modifies the short-scale force, and a disk with $Q>1$ may still have spiral or bar responses. Dissipation in gas can generate shock waves and concentrate star formation near a spiral crest; stellar random motions tend to reduce the collective response.

Radial epicyclic motion. At fixed specific angular momentum, use ${\Phi }_{\mathrm{eff}}=\Phi +{\ell }^2/(2r^2)$ and $\ddot{r}=-{\Phi }_{\mathrm{eff}}^{\prime }$. A circular orbit at $r_c$ obeys ${\Phi }^{\prime }(r_c)={\ell }^2/r_c^3=r_c{\Omega }^2$. Expanding $r=r_c+\xi$ gives $\ddot{\xi }+{\kappa }^2\xi =0$, whereFor ${\kappa }^2>0$ the radial epicyclic frequency describes simple harmonic motion. The angular correction follows from conservation of angular momentum, $\dot{\varphi }\simeq \Omega (1-2\xi /r_c)$, and describes an epicyclic motion around the guiding circular orbit.

Stabilizing terms. In the given local fluid dispersion relation, ${\kappa }^2$ is rotational support, important at long wavelength; $a_0^2{k}^2$ is random-motion or pressure support, important at short wavelength. The term $-2\pi G\Sigma |k|$ is destabilizing self-gravity. Although the wording refers to stellar dispersion, this algebra uses the supplied fluid approximation; a collisionless stellar treatment has a reduction factor and a different numerical normalization.^[ref]

Axisymmetric threshold. For $n=0$, set $q=|k|\ge 0$. Completing the square givesThus $Q>1$ gives local axisymmetric oscillatory stability; $Q<1$ gives a growing band, withAt $Q=1$, the minimizing mode has zero frequency and is marginal, not a strictly restoring oscillation. Local axisymmetric stability does not establish stability to every global nonaxisymmetric mode.

long-wave and short-wave branches at marginal Toomre stability. For $Q=1$ and $n\ge 1$,The long-wavelength branch, using the minus sign, requiresThese endpoints are the inner and outer Lindblad resonance conditions. The stated dispersion relation does not restrict every wave solution to that interval. Its plus-sign short-wavelength branch has $q>0$ even outside it: for example, ${\Omega }_P=\Omega +2\kappa /n$ gives $q=3\kappa /a_0$ and satisfies the equation. Additional branch selection or resonant absorption physics would be needed for an absolute confinement claim. No such condition is supplied.

Toomre stability radius of a flat-rotation disk. With $\Omega =v_0/r$, $\kappa =\sqrt{2}{v}_0/r$. Using $G=4.3009\times 10^{-3}\mathrm{pc}(\mathrm{km}{\mathrm{s}}^{-1}{)}^2{M}_{\odot }^{-1}$,The supplied local model therefore predicts stable radii below about $4.19\mathrm{kpc}$, marginality at that radius, and axisymmetric instability farther out while its assumed constant surface density and thin-disk approximation remain applicable. An infinite constant-density, flat-rotation disk is an idealization, not a global self-consistent galaxy model.