The many faces of LINER-like galaxies: a WISE view

We use the Sloan Digital Sky Survey (SDSS) and Wide-field Sky Survey Explorer surveys to investigate the real nature of galaxies defined as low-ionization nuclear emission-line regions (LINERs) in the BPT diagram. After establishing a mid-infrared colour W2-W3 = 2.5 as the optimal separator between galaxies with and without star formation, we investigate the loci of different galaxy classes in the W-H alpha versus W2-W3 space. We find that: (1) a large fraction of LINER-like galaxies are emission-line retired galaxies, i.e. galaxies which have stopped forming stars and are powered by hot low-mass evolved stars (HOLMES). Their W2-W3 colours show no sign of star formation and their Ha equivalent widths, W-H alpha, are consistent with ionization by their old stellar populations. (2) Another important fraction have W2-W3 indicative of star formation. This includes objects located in the supposedly 'pure AGN' zone of the BPT diagram. (3) A smaller fraction of LINER-like galaxies have no trace of star formation from W2-W3 and a high W-H alpha, pointing to the presence of an AGN. (4) Finally, a few LINERs tagged as retired by their W-H alpha but with W2-W3 values indicative of star formation are late-type galaxies whose SDSS spectra cover only the old 'retired' bulge. This reinforces the view that LINER-like galaxies are a mixed bag of objects involving different physical phenomena and observational effects thrusted into the same locus of the BPT diagram.