The nature of LINERs

We present J-band (1.15-1.35 μm) spectroscopy of a sample of nine galaxies showing some degree of LINER activity (classical LINERs, weak-[O I] LINERs, and transition objects), together with H-band spectroscopy for some of them. A careful subtraction of the stellar continuum allows us to obtain reliable [Fe II] 1.2567 μm/Paβ line ratios. We conclude that different types of LINERs (i.e., photoionized by a stellar continuum or by an active galactic nucleus) cannot be easily distinguished based solely on the [Fe II] 1.2567 μm/Paβ line ratio. The emission-line properties of many LINERs can be explained in terms of an aging starburst. The optical line ratios of these LINERs are reproduced by a model with a metal-rich H II region component photoionized with a single stellar temperature T_* = 38,000 K, plus a supernova remnant (SNR) component. The [Fe II] line is predominantly excited by shocks produced by SNRs in starbursts and starburst-dominated LINERs, while Paβ tracks H II regions ionized by massive young stars. The contribution from SNRs to the overall emission-line spectrum is constrained by the [Fe II] 1.2567 μm/Paβ line ratio. Although our models for aging starbursts are constrained only by these infrared lines, they consistently explain the optical spectra of the galaxies also. The LINER-starburst connection is tested by predicting the time dependence of the ratio of the ionizing luminosity (L_ion) to the supernova rate (SNr), L_ion/(SNr). We predict the relative number of starbursts to starburst-dominated LINERs (aging starbursts) and show that it is in approximate agreement with survey findings for nearby galaxies.