The nature of LINERs

We present J-band (1.15-1.35 mu m) spectroscopy of a sample of nine galaxie s showing some degree of LINER activity (classical LINERs, weak-[O I] LINER s, and transition objects), together with H-band spectroscopy for some of t hem. A careful subtraction of the stellar continuum allows us to obtain rel iable [Fe II] 1.2567 mu m/Pa beta line ratios. We conclude that different t ypes of LINERs (i.e., photoionized by a stellar continuum or by an active g alactic nucleus) cannot be easily distinguished based solely on the [Fe II] 1.2567 mu m/Pa beta line ratio. The emission-line properties of many LINER s 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 com ponent photoionized with a single stellar temperature T-* = 38,000 K, plus a supernova remnant (SNR) component. The [Fe nl line is predominantly excit ed by shocks produced by SNRs in starbursts and starburst-dominated LINERs, while Pa beta tracks H II regions ionized by massive young stars. The cont ribution from SNRs to the overall emission-line spectrum is constrained by the [Fe II] 1.2567 mu m/Pa beta line ratio. Although our models for aging s tarbursts are constrained only by these infrared lines, they consistently e xplain the optical spectra of the galaxies also. The LINER-starburst connec tion is tested by predicting the time dependence of the ratio of the ionizi ng luminosity (L-ion) to the supernova rate (SNr), L-ion/(SNr). We predict the relative number of starbursts to starburst-dominated LINERs (aging star bursts) and show that it is in approximate agreement with survey findings f or nearby galaxies.