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

We present high spatial resolution (similar to0.6) near-infrared broad-band JHK images and Br gamma 2.1661 mum and H-2 1-0 S(1) 2.122 mum emission line images of the nuclear regions in the interacting starburst galaxies NGC 520, NGC 1614 and NGC 7714. The near-infrared emission line and radio morphologies are in general agreement, although there are differences in details. In NGC 1614, we detect a nuclear double structure in Br gamma, in agreement 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 mechanism of the molecular gas is fluorescence by intense UV radiation from clusters of hot young stars, while shock excitation carl be ruled out. The starburst regions in all galaxies exhibit small Br gamma equivalent widths. Assuming a constant star formation model, even with a lowered upper mass cutoff of 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 instantaneous burst of star formation with M-u = 100 M., occurring similar to6-7 Myr ago, 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.