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Bars may induce morphological features, such as rings, through their resonances. Previous studies suggested that the presence of “dark gaps,” or regions of a galaxy where the difference between the surface brightness along the bar major axis and that along the bar minor axis is maximal, can be attributed to the location of bar corotation. Here, using GALAKOS, a high-resolution N -body simulation of a barred galaxy, we test this photometric method’s ability to identify the bar corotation resonance. Contrary to previous work, our results indicate that “dark gaps” are a clear sign of the location of the 4:1 ultraharmonic resonance instead of bar corotation. Measurements of the bar corotation can indirectly be inferred using kinematic information, e.g., by measuring the shape of the rotation curve. We demonstrate our concept on a sample of 578 face-on barred galaxies with both imaging and integral field observations and find that the sample likely consists primarily of fast bars. [ABSTRACT FROM AUTHOR]

Gas interior to the bar of the Milky Way has recently been shown to be the closest example of a low-ionization (nuclear) emission region (LI(N)ER) in the universe. To better understand the nature of this gas, a sample of face-on galaxies with integral field spectroscopy is used to study the ionized gas conditions of 240 barred and 250 nonbarred galaxies, focusing on those that are most similar to the Milky Way. Strong optical line emission of [N ii ] λ6584, Hα, [O iii ] λ5007, and Hβ are used to diagnose the dominant ionization mechanisms of gas across galaxies and the Galaxy via Baldwin–Phillips–Terlevich diagrams. Barred galaxies show a strong suppression of star formation and an increase in composite and LI(N)ER-like spectra in their inner regions when compared with similar nonbarred counterparts. This effect is lessened in galaxies of very low () or very high () total stellar mass. Bar masks from Galaxy Zoo:3D show the bar's nonaxisymmetric effect on the ionized gas and help predict the face-on distribution of ionized gas conditions near the bar of the Milky Way. [ABSTRACT FROM AUTHOR]

Widespread, high altitude, diffuse ionized gas with scale heights of around a kiloparsec is observed in the Milky Way and other star-forming galaxies. Numerical radiation-magnetohydrodynamic simulations of a supernova-driven turbulent interstellar medium show that gas can be driven to high altitudes above the galactic mid-plane, but the degree of ionization is often less than inferred from observations. For computational expediency, ionizing radiation from massive stars is often included as a post-processing step assuming ionization equilibrium. We extend our simulations of an Milky Way-like interstellar medium to include the combined effect of supernovae and photoionization feedback from mid-plane OB stars and a population of hot evolved low mass stars. The diffuse ionized gas has densities below 0.1 |${\rm \,cm^{-3}}$|⁠ , so recombination time-scales can exceed millions of years. Our simulations now follow the time-dependent ionization and recombination of low density gas. The long recombination time-scales result in diffuse ionized gas that persists at large altitudes long after the deaths of massive stars that produce the vast majority of the ionized gas. The diffuse ionized gas does not exhibit the large variability inherent in simulations that adopt ionization equilibrium. The vertical distribution of neutral and ionized gas is close to what is observed in the Milky Way. The volume filling factor of ionized gas increases with altitude resulting in the scale height of free electrons being larger than that inferred from H  |$\alpha$| emission, thus reconciling the observations of ionized gas made in H  |$\alpha$| and from pulsar dispersion measurements. [ABSTRACT FROM AUTHOR]

Bars inhabit the majority of local-Universe disc galaxies and may be important drivers of galaxy evolution through the redistribution of gas and angular momentum within discs. We investigate the star formation and gas properties of bars in galaxies spanning a wide range of masses, environments, and star formation rates using the Mapping Nearby Galaxies at APO galaxy survey. Using a robustly defined sample of 684 barred galaxies, we find that fractional (or scaled) bar length correlates with the host's offset from the star formation main sequence. Considering the morphology of the Hα emission we separate barred galaxies into different categories, including barred, ringed, and central configurations, together with Hα detected at the ends of a bar. We find that only low-mass galaxies host star formation along their bars, and that this is located predominantly at the leading edge of the bar itself. Our results are supported by recent simulations of massive galaxies, which show that the position of star formation within a bar is regulated by a combination of shear forces, turbulence, and gas flows. We conclude that the physical properties of a bar are mostly governed by the existing stellar mass of the host galaxy, but that they also play an important role in the galaxy's ongoing star formation. [ABSTRACT FROM AUTHOR]