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We present an empirical relation between the cold gas surface density (Σgas) and the optical extinction (AV) in a sample of 103 galaxies from the Extragalactic Database for Galaxy Evolution (EDGE) survey. This survey provides CARMA interferometric CO observations for 126 galaxies included in the Calar Alto Legacy Integral Field Area (CALIFA) survey. The matched, spatially resolved nature of these data sets allows us to derive the Σgas– AV  relation on global, radial, and kpc (spaxel) scales. We determine AV  from the Balmer decrement (H α/H β). We find that the best fit for this relation is |$\Sigma _{\rm gas}\,(\rm {M_\odot \,pc}^{-2}) \sim 26 \times {\rm \mathit{ A}_\mathit{ V}} \,(\rm mag)$|⁠ , and that it does not depend on the spatial scale used for the fit. However, the scatter in the fits increases as we probe smaller spatial scales, reflecting the complex relative spatial distributions of stars, gas, and dust. We investigate the Σgas/ AV  ratio on radial and spaxel scales as a function of |$\mathrm{EW(H\,\alpha)}$|⁠. We find that at larger values of |$\mathrm{EW({H\,\alpha })}$| (i.e. actively star-forming regions) this ratio tends to converge to twice the value expected for a foreground dust screen geometry (∼30 |$\mathrm{M_{\odot } \, pc^{-2} \, mag^{-1}}$|⁠). On radial scales, we do not find a significant relation between the Σgas/ AV  ratio and the ionized gas metallicity. We contrast our estimates of Σgas using AV  with compilations in the literature of the gas fraction on global and radial scales as well as with well-known scaling relations such as the radial star formation law and the Σgas–Σ* relation. These tests show that optical extinction is a reliable proxy for estimating Σgas in the absence of direct sub/millimeter observations of the cold gas. [ABSTRACT FROM AUTHOR]

The inner regions of barred galaxies contain substructures such as off-axis shocks, nuclear rings, and nuclear spirals. These substructures may affect star formation, and control the activity of a central black hole (BH) by determining the mass inflow rate. We investigate the formation and properties of such substructures using high-resolution, grid-based hydrodynamic simulations. The gaseous medium is assumed to be infinitesimally thin, isothermal, and non-self-gravitating. The stars and dark matter are represented by a static gravitational potential with four components: a stellar disk, a bulge, a central BH, and a bar. To investigate various galactic environments, we vary the gas sound speed, cs, as well as the mass of the central BH, MBH. Once the flow has reached a quasi-steady state, off-axis shocks tend to move closer to the bar major axis as cs increases. Nuclear rings shrink in size with increasing cs, but are independent of MBH, suggesting that the ring position is not determined by the Lindblad resonances. Rings in low-cs models are narrow since they are occupied largely by gas on x2-orbits and well decoupled from nuclear spirals, while they become broad because of large thermal perturbations in high-cs models. Nuclear spirals persist only when either cs is small or MBH is large; they would otherwise be destroyed completely by the ring material on eccentric orbits. The shape and strength of nuclear spirals depend sensitively on cs and MBH such that they are leading if both cs and MBH are small, weak trailing if cs is small and MBH is large, and strong trailing if both cs and MBH are large. While the mass inflow rate toward the nucleus is quite small in low-cs models because of the presence of a narrow nuclear ring, it becomes larger than 0.01 M⊙ yr-1 when cs is large, providing a potential explanation of nuclear activity in Seyfert galaxies. [ABSTRACT FROM AUTHOR]