Formation of disk galaxies with long-lived and non-stationary spiral arms

This work represents a development of the study of the collapses of purely self-gravitating systems  (see https://physics.aps.org/articles/v12/s19) to the case in which a dissipational gas component is also present. These latter systems show much richer morphological and kinematical structures that may have important observational implications to understand the kinematic and dynamics of the Milky Way as revealed by ongoing surveys such as the Gaia Mission. 

 by Francesco Sylos Labini, Luis Diego Pinto, Roberto Capuzzo-Dolcetta 

Understanding the origin and evolution of spiral structures has proved to be one of the hardest problems in astrophysics. In this work we have studied the formation of long-lived, but non-stationary, spiral arms as consequence of the violent collapse of an isolated over-density, characterized by a mixture of purely self-gravitating matter and gas that may dissipate energy via radiative cooling.  The rapid variation of the system gravitational potential during the collapse leads particles that interact only via Newtonian gravity to form a quasi-stationary thick disk, whose dynamics is dominated by an axial rotational velocity, surrounded by far out-of-equilibrium spiral arms. On the other hand, the gas component is subjected to compression shocks and radiative cooling so that it develops a much flatter disk shape, where rotational motions are coherent, and the velocity dispersion is small. Around such gaseous disk long-lived, but non-stationary, spiral arms form that show a surprisingly complex velocity field (Figure 1).

 

Figure 1 Top (top) and side (bottom) view of the gas particles disk. The color code is proportional to the density. Distances are in kiloparsec and time is in Giga years.

Spiral arms are made of gaseous particles that move coherently because they have acquired a specific phase-space correlation during the gravitational collapse phase: this represents a signature of the violent origin of the arms and implies both the motion of matter and the transfer of energy.

From an observational point of view, the ongoing Gaia space-mission has recently detected several phase-space structures in the Milky Way velocity field that can be naturally understood in a model in which the Galaxy has a non-stationary nature of the type we discuss in this work: the forthcoming data of the Gaia mission should  clarify these issues.

The physical mechanism studied in this work is different from the slow and more gentle dynamical evolution that takes place in standard cosmological models. In that case structure formation proceeds through a hierarchical bottom-up aggregation process instead than through a top-down monolithic collapse. In the standard scenario the disk is embedded in the gravitational potential field of a halo structure, i.e. a system close to spherical with an almost isotropic velocity dispersion which contains most of the mass: the gaseous matter forms a disk where rotational motions dominate and whose dynamical properties are determined by the more massive halo structure.  On the contrary, in the system we have studied, the rotating disk is embedded in the gravitational field of a more massive  and thicker disk that is also rotation dominated (Figure 2). Note that the halo typically extends much more (> 100 kpc) than the thick disk (< 30 kpc) in our model and thus it is much more massive.

The complex velocity field characterizing the system formed from a monolithic collapse implies that it is not possible to infer in a simple manner estimates of gravitating mass from the measurement of a velocities. In this situation the estimates of the quantity of dark matter in galaxies, based on the assumption that emitters motion is maximally rotational, must be questioned as the measurement of a velocity larger than that obtained from the luminous matter estimate may imply that the system has not reach a simple stationary state, as in the cases considered in this work.

Figure 2. Halo-disk system formed in a standard cosmological model (left panels): top view (upper) and side view (bottom); the halo particles are purely self-gravitating and form an ellipsoid with an isotropic velocity dispersion (green/blue) while the gas particles (red/orange) form a rotating disk. Purely self-gravitating particles (middle panels) and gas particles (right panels) originated from a monolithic collapse: the former form a rotating thick disk and the latter a rotating thin disk. The color code is proportional to the density. Distances are in kiloparsec and time is in Giga years.

From a cosmological point of view a monolithic collapse may take place if  power spectrum  of initial mass fluctuations differently from the standard case, has a sharp cut-off suppressing fluctuations at small enough scales. Such scales are not probed by the cosmic microwave background radiation, and thus do not have to satisfy strong observational constraints but may be significant for galaxy formation.

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