Dynamical cooling of galactic discs by molecular cloud collisions -- Origin of giant clumps in gas-rich galaxy discs

Different from Milky-Way-like galaxies, discs of gas-rich galaxies are clumpy. It is believed that the clumps form because of gravitational instability. However, a necessary condition for gravitational instability to develop is that the disc must dissipate its kinetic energy effectively, this energy dissipation (also called cooling) is not well-understood. We propose that collisions (coagulation) between molecular clouds dissipate the kinetic energy of the discs, which leads to a dynamical cooling. The effectiveness of this dynamical cooling is quantified by the dissipation parameter $D$, which is the ratio between the free-fall time $t_{\rm ff}\approx 1/ \sqrt{G \rho_{\rm disc}}$ and the cooling time determined by the cloud collision process $t_{\rm cool}$. This ratio is related to the ratio between the mean surface density of the disc $\Sigma_{\rm disc}$ and the mean surface density of molecular clouds in the disc $\Sigma_{\rm cloud}$. When $D <1/3$ (which roughly corresponds to $\Sigma_{\rm disc} < 1/3 \Sigma_{\rm cloud}$), cloud collision cooling is inefficient, and fragmentation is suppressed. When $D > 1/3$ (which roughly corresponds to $\Sigma_{\rm disc} > 1/3 \Sigma_{\rm cloud}$), cloud-cloud collisions lead to a rapid cooling through which clumps form. On smaller scales, cloud-cloud collisions can drive molecular cloud turbulence. This dynamical cooling process can be taken into account in numerical simulations as a subgrid model to simulate the global evolution of disc galaxies.Comment: MNRAS accepte

A 500 pc filamentary gas wisp in the disk of the Milky Way

Star formation occurs in molecular gas. In previous studies, the structure of the molecular gas has been studied in terms of molecular clouds, but has been overlooked beyond the cloud scale. We present an observational study of the molecular gas at 49.5 degree <l<52.5 degree and -5.0 km/s <v_lsr <17.4 km/s. The molecular gas is found in the form of a huge (>= 500 pc) filamentary gas wisp. This has a large physical extent and a velocity dispersion of ~5 km/s. The eastern part of the filamentary gas wisp is located ~130 pc above the Galactic disk (which corresponds to 1.5-4 e-folding scale-heights), and the total mass of the gas wisp is >= 1 X 10^5 M_sun. It is composed of two molecular clouds and an expanding bubble. The velocity structure of the gas wisp can be explained as a smooth quiescent component disturbed by the expansion of a bubble. That the length of the gas wisp exceeds by much the thickness of the molecular disk of the Milky Way is consistent with the cloud-formation scenario in which the gas is cold prior to the formation of molecular clouds. Star formation in the filamentary gas wisp occurs at the edge of a bubble (G52L nebula), which is consistent with some models of triggered star formation.Comment: Accepted for publication in A&

Turbulent entrainment origin of protostellar outflows

Protostellar outflow is a prominent process that accompanies the formation of stars. It is generally agreed that wide-angled protostellar outflows come from the interaction between the wind from a forming star and the ambient gas. However, it is still unclear how the interaction takes place. In this work, we theoretically investigate the possibility that the outflow results from interaction between the wind and the ambient gas in the form of turbulent entrainment. In contrast to the previous models, turbulent motion of the ambient gas around the protostar is taken into account. In our model, the ram-pressure of the wind balances the turbulent ram-pressure of the ambient gas, and the outflow consists of the ambient gas entrained by the wind. The calculated outflow from our modelling exhibits a conical shape. The total mass of the outflow is determined by the turbulent velocity of the envelope as well as the outflow age, and the velocity of the outflow is several times higher than the velocity dispersion of the ambient gas. The outflow opening angle increases with the strength of the wind and decreases with the increasing ambient gas turbulence. The outflow exhibits a broad line width at every position. We propose that the turbulent entrainment process, which happens ubiquitously in nature, plays a universal role in shaping protostellar outflows.Comment: 15 pages, accepted for publication in A&

Mathesis of star formation – from kpc to parsec scales

In this thesis I present a series of studies aiming to understand the formation of stars from gas in the Milky Way. Generally speaking, I will progress from larger to smaller scales. The kilo-parsec scale (1000 parsec ~ 10^21 cm) is the scale at which dynamics of the molecular clouds is coupled to dynamics of the Milky Way disk. Here we present an observational study of molecular gas at 49.5 deg Molecular clouds (1 - 100 parsec) are the nurseries of the stars. There are many indications that molecular clouds are turbulence-dominated objects. However, it is not clear what role gravity plays. We propose a new method (G-virial) to quantify the role of gravity in molecular clouds. Our new method takes the gravitational interactions between all pixels in 3D position-position-velocity data cube into account, and generates maps of the importance of gravity in 3D position-position-velocity space. With our method we demonstrate that gravity plays an importance role in the individual regions in the Perseus and Ophiuchus molecular cloud, and find that high values of G-virial are reached in cluster-bearing regions. We also demonstrate the capability of our method in finding regions and quantifying the properties of the regions in the clouds. Protostellar outflow ~ 1 pc is a prominent process accompanying the formation of stars. In this work, we theoretically investigate the possibility that the outflow results from interaction between the wind and the ambient gas in the form of turbulent entrainment. In our model, the ram-pressure of the wind balances the turbulent ram-pressure of the ambient gas, and the outflow consists of the ambient gas entrained by the wind. We demonstrate that the outflow phenomena can be naturally generated through this process, and discuss the potential usage of outflows as a probes of the dynamical state of the turbulent molecular gas

G-virial: Gravity-based structure analysis of molecular clouds

We present the G-virial method (available at http://gxli.github.io/G-virial/) which aims to quantify (1) the importance of gravity in molecular clouds in the position-position-velocity (PPV) space, and (2) properties of the gas condensations in molecular clouds. Different from previous approaches that calculate the virial parameter for different regions, our new method takes gravitational interactions between all the voxels in 3D PPV data cubes into account, and generates maps of the importance of gravity. This map can be combined with the original data cube to derive relations such as the mass-radius relation. Our method is important for several reasons. First, it offers the the ability to quantify the centrally condensed structures in the 3D PPV data cubes, and enables us to compare them in an uniform framework. Second, it allows us to understand the importance of gravity at different locations in the data cube, and provides a global picture of gravity in clouds. Third, it offers a robust approach to decomposing the data into different regions which are gravitationally coherent. To demonstrate the application of our method we identified regions from the Perseus and Ophiuchus molecular clouds, and analyzed their properties. We found an increase in the importance of gravity towards the centers of the individual molecular condensations. We also quantified the properties of the regions in terms of mass-radius and mass-velocity relations. Through evaluating the virial parameters based on the G-virial, we found that all our regions are almost gravitationally bound. Cluster-forming regions appear are more centrally condensed.Comment: Accepted by A&