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Barred galaxies contain substructures such as a pair of dust lanes and nuclear rings, with the latter being sites of intense star formation. We study the substructure formation as well as star formation in nuclear rings using numerical simulations. We find that nuclear rings form not by the Lindblad resonances, as previously thought, but by the centrifugal barrier that inflowing gas along dust lanes cannot overcome. This predicts a smaller ring in a more strongly barred galaxy, consistent with observations. Star formation rate (SFR) in a nuclear ring is determined primarily by the mass inflow rate to the ring. In our models, the SFR typically shows a short strong burst associated with the rapid gas infall and stays very small for the rest of the evolution. When the SFR is low, ages of young star clusters exhibit an azimuthal gradient along the ring since star formation takes place mostly near the contact points between the dust lanes and the nuclear ring. When the SFR is large, on the other hand, star formation is widely distributed throughout the whole length of the ring, with no apparent age gradient of star clusters. Since observed ring star formation appears long-lived with episodic bursts, our results suggest that the bar region should be replenished continually with fresh gas from outside.

Galactic disks consist of both stars and gas. The stars gravitationally influence the gas either in disks at large or within spiral arms, leading to the formation of giant clouds and turbulence driving in the gas. In featureless disks as in flocculent galaxies, swing amplification operating in a combined star-gas disk is efficient to form bound condensations and feed a significant level of random gas motions. This occurs when the gaseous Toomre parameter is less than 1.4 for the stellar parameters similar to the solar neighbourhood conditions. In disks with spiral features, on the other hand, spiral-arm spurs and associated giant clouds develop as a consequence of magneto-Jeans instability in which magnetic tension counterbalances the stabilizing Coriolis force. Spiral shocks are inherently unstable when the vertical dimension is taken into account, exhibiting flapping motions of the shock front. This naturally converts the kinetic energy in galaxy rotation into random kinetic energy of the gas. The resulting turbulent motions are supersonic and persist despite strong shock dissipation. Thermal instability occurring in gas flows across spiral arms prompts phases transitions that produce a significant fraction of thermally-unstable, intermediate-temperature gas in the postshock expansion zones.

We study star formation occurring in nuclear rings of barred-spiral galaxies by using hydrodynamic simulations with the prescriptions of star formation and feedback included. In models without spiral arms, the star formation rate (SFR) in a ring exhibits a strong primary burst at early time and declines to small values at late time. The early burst is caused by a rapid gas infall due to the bar growth, consuming most of the gas inside the bar regions. On the other hand, models with spiral arms show multiple starburst activities at late time caused by arm-induced gas inflows, provided that the arm pattern speed is slower than that of the bar. The SFR in models with spirals is larger by a factor of ~ 1.4–4.0 than that in the bar-only models, with larger values corresponding to stronger and slower arms. In all models, young star clusters in nuclear ring show an azimuthal age gradient only when the SFR is small, such that younger clusters tend to locate closer to the contact points between the ring and dust lanes.

We discuss recent advances in cloud formation via gravitational instability under the action of self-gravity, magnetic fields, rotational shear, active stars, and/or stellar spiral arms. When shear is strong and the spiral arms are weak, applicable to flocculent galaxies at large, swing amplification exhibits nonlinear threshold behavior such that disks with a Toomre parameter Q < Qc experience gravitational runaway. For most realistic conditions, local models yield Qc ~ 1.4, similar to the observed star formation thresholds. When shear is weak, on the other hand, as in galactic central parts or inside spiral arms, magneto-Jeans instability is very powerful to form spiral-arm substructures including gaseous spurs and giant clouds. The wiggle and Parker instabilities proposed for cloud formation appear to be suppressed by strong non-steady motions inherent in vertically-extended spiral shocks, suggesting that gravitational instability is a primary candidate for cloud formation.

Star formation is self-regulated by its feedback that drives turbulence and heats the gas. In equilibrium, the star formation rate (SFR) should be directly related to the total (thermal plus turbulent) midplane pressure and hence the total weight of the diffuse gas if energy balance and vertical dynamical equilibrium hold simultaneously. To investigate this quantitatively, we utilize numerical hydrodynamic simulations focused on outer-disk regions where diffuse atomic gas dominates. By analyzing gas properties at saturation, we obtain relationships between the turbulence driving and dissipation rates, heating and cooling rates, the total midplane pressure and the total weight of gas, and the SFR and the total midplane pressure. We find a nearly linear relationship between the SFR and the midplane pressure consistent with the theoretical prediction., 2 pages, 1 figure. To appear in the proceeding of the IAU GA XXVIII, Special Session 12: Modern Views of the Interstellar Medium

To study the effects of magnetic fields on the properties of bar substructures, we run two-dimensional, ideal MHD simulations of barred galaxies under the influence of a non-axisymmetric bar potential. In the bar regions, magnetic fields reduce density compression in the dust-lane shocks, while removing angular momentum further from the gas at the shocks. This evidently results in a smaller and more distributed ring, and a larger mass inflows rate to the galaxy center in models with stronger magnetic fields. In the outer regions, an MHD dynamo due to the combined action of the bar potential and background shear operates, amplifying magnetic fields near the corotation resonance. In the absence of spiral arms, the amplified fields naturally shape into trailing magnetic arms with strong fields and low density. The reader is refereed to Kim & Stone (2012) for a detailed presentation of the simulation outcomes.