Spatially Resolved Spectroscopic Properties of Low-Redshift Star-Forming Galaxies

I review the spatially resolved spectroscopic properties of low-redshift star-forming galaxies (and their retired counterparts) using results from the most recent optical integral field spectroscopy galaxy surveys. First, I briefly summarize the global spectroscopic properties of these galaxies, discussing the main ionization processes and the global relations described by the star-formation rates, gas-phase oxygen abundances, and average properties of their stellar populations (age and metallicity) in comparison with the stellar mass. Second, I present the local distribution of the ionizing processes down to kiloparsec scales, and I show how the global scaling relations found using integrated parameters (like the star-formation main sequence, mass-metallicity relation, and Schmidt-Kennicutt law) have local/resolved counterparts, with the global ones being, for the most part, just integrated/average versions of the local ones. I discuss the local/resolved star-formation histories (SFHs) and chemical-enrichment histories and their implications on the inside-out growth of galaxies. Third, I present the radial distributions of the surface densities of the properties explored globally and how they depend on the integrated galaxy properties. In conclusion, I find that the evolution of galaxies is mostly governed by local processes but clearly affected by global ones: Many global scaling relations present resolved counterparts (verified down to kiloparsec scales) that can explain them as well as the observed radial gradients in galaxies. These relations are consequences of the local SFHs, the narrow range of the depletion times, and a local metal enrichment. Deviations from these relations are due to dynamical and mixing processes, local exchange of gas (inflows, outflows, and fountains), depletion time differences, and/or differences in the resolved SFHs. Ionization happens at local scales that may be driven by different physical processes, and it cannot be clearly understood using purely integrated quantities. The dominant ionization in galaxies has a stellar origin.