SDSS IV MaNGA-spatially resolved diagnostic diagrams: A proof that many galaxies are LIERs

We study the spatially resolved excitation properties of the ionized gas in a sample of 646galaxies using integral field spectroscopy data from the Sloan Digital Sky Survey IV MappingNearby Galaxies at Apache Point Observatory (MaNGA) programme.Making use ofBaldwin-Philips-Terlevich diagnostic diagrams we demonstrate the ubiquitous presence of extended(kpc scale) low-ionization emission-line regions (LIERs) in both star-forming and quiescentgalaxies. In star-forming galaxies LIER emission can be associated with diffuse ionizedgas, most evident as extraplanar emission in edge-on systems. In addition, we identify twomain classes of galaxies displaying LIER emission: 'central LIER' (cLIER) galaxies, wherecentral LIER emission is spatially extended, but accompanied by star formation at largergalactocentric distances, and 'extended LIER' (eLIER) galaxies, where LIER emission isextended throughout the whole galaxy. In eLIER and cLIER galaxies, LIER emission isassociated with radially flat, low Ha equivalent width of line emission (<3 Å) and stellarpopulation indices demonstrating the lack of young stellar populations, implying that lineemission follows tightly the continuum due to the underlying old stellar population. TheHa surface brightness radial profiles are always shallower than 1/r² and the line ratio [O III]λ5007/[O II] λλ3727,29 (a tracer of the ionization parameter of the gas) shows a flat gradient.This combined evidence strongly supports the scenario in which LIER emission is not dueto a central point source but to diffuse stellar sources, the most likely candidates being hot,evolved (post-asymptotic giant branch) stars. Shocks are observed to play a significant role inthe ionization of the gas only in rare merging and interacting systems.