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

• Published in: Monthly notices of the Royal Astronomical Society 2016-10, Vol.462 (2), p.1826-1833
• Main Authors: Herpich, F., Mateus, A., Stasińska, G., Cid Fernandes, R., Vale Asari, N.
• Format: Article
• Language: English
• Subjects: Active galactic nuclei; Astrophysics; Color; Colour; Galaxies; Liners; Physics; Sky surveys (astronomy); Space telescopes; Star & galaxy formation; Star formation; Star maps; Stars; Stars & galaxies
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stw1742

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 α 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 Hα equivalent widths, W H α, 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 α, pointing to the presence of an AGN. (4) Finally, a few LINERs tagged as retired by their W H α 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.