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

TL;DR

This study uses SDSS and WISE data to clarify the nature of LINER-like galaxies, revealing they are a diverse group including retired galaxies, star-forming galaxies, and AGN hosts, challenging their traditional classification.

Contribution

It introduces a mid-infrared colour W2-W3 as an effective separator and provides a detailed analysis of the physical nature of LINER-like galaxies using multi-wavelength data.

Findings

Many LINER-like galaxies are retired, star-forming, or AGN-hosting.

W2-W3 colour effectively distinguishes star-forming activity.

LINER-like galaxies are a heterogeneous group with different physical phenomena.

Abstract

We use the SDSS and WISE surveys to investigate the real nature of galaxies defined as 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_{Ha} 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_{Ha}, 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_{Ha}, pointing to the presence of an AGN. (4) Finally, a few LINERs tagged as retired by their W_{Ha} 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.