Near-infrared line imaging of the starburst galaxies NGC 520, NGC 1614 andNGC 7714

We present high spatial resolution (similar to0."6) near-infrared broad-ban d JHK images and Br gamma 2.1661 mum and H-2 1-0 S(1) 2.122 mum emission li ne images of the nuclear regions in the interacting starburst galaxies NGC 520, NGC 1614 and NGC 7714. The near-infrared emission line and radio morph ologies are in general agreement, although there are differences in details . In NGC 1614, we detect a nuclear double structure in Br gamma, in agreeme nt with the radio double structure. We derive average extinctions of A(K) = 0.41 and A(K) = 0.18 toward the nuclear regions of NGC 1614 and NGC 7714, respectively. For NGC: 520, the extinction is much higher, A(K) = 1.2-1.6. The observed H-2/Br gamma ratios indicate that the main excitation mechanis m of the molecular gas is fluorescence by intense UV radiation from cluster s of hot young stars, while shock excitation carl be ruled out. The starbur st regions in all galaxies exhibit small Br gamma equivalent widths. Assumi ng a constant star formation model, even with a lowered upper mass cutoff o f M-u = 30 M., results in rather old ages (10-40 Myr), in disagreement with the clumpy near-infrared morphologies. We prefer a model of an instantaneo us burst of star formation with M-u = 100 M., occurring similar to6-7 Myr a go, in agreement with previous determinations and with the detection of W-R features in NGC 1614 and NGC 7714. Finally, we note a possible systematic difference in the amount of hot molecular gas between starburst and Seyfert galaxies.