We present a new systematic way of setting up galactic gas discs based on the assumption of detailed hydrodynamic equilibrium. To do this, we need to specify the density distribution and the velocity field which supports the disc. We first show that the required circular velocity has no dependence on the height above or below the mid‐plane so long as the gas pressure is a function of density only. The assumption of discs being very thin enables us to decouple the vertical structure from the radial direction. Based on that, the equation of hydrostatic equilibrium together with the reduced Poisson equation leads to two sets of second‐order non‐linear differential equations, which are easily integrated to set up a stable disc. We call one approach ‘density method’ and the other one ‘potential method’. Gas discs in detailed balance are especially suitable for investigating the onset of the gravitational instability. We revisit the question of global, axisymmetric instability using fully three‐dimensional disc simulations. The impact of disc thickness on the disc instability and the formation of spontaneously induced spirals is studied systematically with or without the presence of the stellar potential. In our models, the numerical results show that the threshold value for disc instability is shifted from unity to 0.69 for self‐gravitating thick discs and to 0.75 for combined stellar and gas thick discs. The simulations also show that self‐induced spirals occur in the correct regions and with the right numbers as predicted by the analytic theory.

The interaction between a supermassive black hole (SMBH) and the surrounding material is of primary importance in modern astrophysics. The detection of the molecular 2-pc circumnuclear disk (CND) immediately around the Milky Way SMBH, SgrA*, provides an unique opportunity to study SMBH accretion at subparsec scales. Our new wide-field CS(J = 2 − 1) map toward the Galactic center (GC) reveals multiple dense molecular streamers originated from the ambient clouds 20-pc further out, and connecting to the central 2 parsecs of the CND. These dense gas streamers appear to carry gas directly toward the nuclear region and might be captured by the central potential. Our phase-plot analysis indicates that these streamers show a signature of rotation and inward radial motion with progressively higher velocities as the gas approaches the CND and finally ends up co-rotating with the CND. Our results might suggest a possible mechanism of gas feeding the CND from 20 pc around 2 pc in the GC. In this paper, we discuss the morphology and the kinematics of these streamers. As the nearest observable Galactic nucleus, this feeding process may have implications for understanding the processes in extragalactic nuclei.

We (Li et al. 2009; Paper-I) compared the magnetic field directions inferred from polarimetry data obtained from 100-pc scale inter-cloud media (ICM) and from sub-pc scale molecular cloud cores. The highly correlated result led us to conclude that cloud turbulence must be sub-Alfvenic. Here we extend the study with 0.01-pc cores observed by interferometers. The inferred field directions at this scale significantly deviate from that of the surrounding ICM. An obvious question to ask is whether this high-resolution result contradicts the sub-Alfvenic picture concluded earlier. We performed MHD simulations of a slightly super-critical (magnetic criticality = 2) clouds with Alfvenic Mach number MA = 0.63 , which can reproduce the Paper-I results, and observed the development 1 towards smaller scales. Interestingly, all subregions hosting cores with nH2 > 10 5 /cc (the typical density observed by interferometers) possess MA = 2-3. Not too surprisingly, these slightly super-Alfvenic cores result in B-field orientation offsets comparable to the interferometer observations. The result suggests that gravity can concentrate (and maybe also contribute to, which takes more study to confirm) turbulent energy and create slightly super-Alfvenic cores out from sub-Alfvenic clouds. The results of our simulations also agree with the observed velocity-scale (Kauffmann et al. 2013), mass-scale (Lombardi et al. 2010) and field strength-density (Li et al. 2015; Crutcher et al. 2010) relations. MA ≡ < V/vA> , where V and vA are, respectively, local 3D velocity dispersion and Alfven velocity; <…> 1 means the average within the entire simulated volume (e.g. Burkhart et al. 2009).