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Register a new userGas flow in barred potentials
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Added by
Mattia Carlo Sormani
Publication date
2015
fields
Physics
and research's language is
English
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We use a Cartesian grid to simulate the flow of gas in a barred Galactic potential and investigate the effects of varying the sound speed in the gas and the resolution of the grid. For all sound speeds and resolutions, streamlines closely follow closed orbits at large and small radii. At intermediate radii shocks arise and the streamlines shift between two families of closed orbits. The point at which the shocks appear and the streamlines shift between orbit families depends strongly on sound speed and resolution. For sufficiently large values of these two parameters, the transfer happens at the cusped orbit as hypothesised by Binney et al. over two decades ago. For sufficiently high resolutions the flow downstream of the shocks becomes unsteady. If this unsteadiness is physical, as appears to be the case, it provides a promising explanation for the asymmetry in the observed distribution of CO.
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Spiral arms that emerge from the ends of a galactic bar are important in interpreting observations of our and external galaxies. It is therefore important to understand the physical mechanism that causes them. We find that these spiral arms can be understood as kinematic density waves generated by librations around underlying ballistic closed orbits. This is even true in the case of a strong bar, provided the librations are around the appropriate closed orbits and not around the circular orbits that form the basis of the epicycle approximation. An important consequence is that it is a potentials orbital structure that determines whether a bar should be classified as weak or strong, and not crude estimates of the potentials deviation from axisymmetry.
We run hydrodynamical simulations of a 2D isothermal non self-gravitating inviscid gas flowing in a rigidly rotating externally imposed potential formed by only two components: a monopole and a quadrupole. We explore systematically the effects of varying the quadrupole while keeping fixed the monopole and discuss the consequences for the interpretation of longitude-velocity diagrams in the Milky Way. We find that the gas flow can constrain the quadrupole of the potential and the characteristics of the bar that generates it. The exponential scale length of the bar must be at least $1.5rm, kpc$. The strength of the bar is also constrained. Our global interpretation favours a pattern speed of $Omega=40,rm km s^{-1} {kpc}^{-1}$. We find that for most observational features, there exist a value of the parameters that matches each individual feature well, but is difficult to reproduce all the important features at once. Due to the intractably high number of parameters involved in the general problem, quantitative fitting methods that can run automatic searches in parameter space are necessary.
We investigate stationary gas flows in a fixed, rotating barred potential. The gas is assumed to be isothermal with an effective sound speed c_s, and the equations of motion are solved with smoothed particle hydrodynamics (SPH). Since the thermal energy in cloud random motions is negligible compared to the orbital kinetic energy, no dependence of the flow on c_s is expected. However, this is not the case when shocks are involved. For low values of c_s an open, off-axis shock flow forms that is characteristic for potentials with an inner Lindblad resonance (ILR). Through this shock the gas streams inwards from x_1 to x_2-orbits. At high sound speeds the gas arranges itself in a different, on-axis shock flow pattern. In this case, there is no gas on x_2-orbits, demonstrating that the gas can behave as if there were no ILR. The critical effective sound speed dividing the two regimes is in the range of values observed in the Milky Way. We give a heuristic explanation for this effect. A possible consequence is that star formation may change the structure of the flow by which it was initiated. Low-mass galaxies should predominantly be in the on-axis regime. A brief comparison of our SPH results with those from a grid-based hydrodynamic code is also given.
We select a sample of galaxies from the Sloan Digital Sky Survey Data Release 7 (SDSS-DR7) where galaxies are classified, through visual inspection, as hosting strong bars, weak bars or as unbarred galaxies, and make use of HI mass and kinematic information from the Arecibo Legacy Fast ALFA (ALFALFA) survey catalog, to study the stellar, atomic gas and dark matter content of barred disk galaxies. We find, in agreement with previous studies, that the bar fraction increases with increasing stellar mass. A similar trend is found with total baryonic mass, although the dependence is not as strong as with stellar mass, this due to the contribution of gas. The bar fraction shows a decrease with increasing gas mass fraction. This anticorrelation between the likelihood of a galaxy hosting a bar with the gas richness of the galaxy results from the inhibiting effect the gas has in the formation of bars. We also find that for massive galaxies with stellar masses larger than 10$^{10} M_{odot}$, at fixed stellar mass, the bar fraction decreases with increasing global halo mass (i.e. halo mass measured up to a radius of the order of the HI disk extent).
We identify an effective proxy for the analytically-unknown second integral of motion (I_2) for rotating barred or tri-axial potentials. Planar orbits of a given energy follow a tight sequence in the space of the time-averaged angular momentum and its amplitude of fluctuation. The sequence monotonically traces the main orbital families in the Poincare map, even in the presence of resonant and chaotic orbits. This behavior allows us to define the Calibrated Angular Momentum, the average angular momentum normalized by the amplitude of its fluctuation, as a numerical proxy for I_2. It also implies that the amplitude of fluctuation in L_z, previously under-appreciated, contains valuable information. This new proxy allows one to classify orbital families easily and accurately, even for real orbits in N-body simulations of barred galaxies. It is a good diagnostic tool of dynamical systems, and may facilitate the construction of equilibrium models.
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