THE CIRCUMNUCLEAR STARBURST IN NGC-7552 - FIRST RESULTS FROM NEAR-INFRARED SPECTRAL-SYNTHESIS

We present new near-and mid-infrared high angular resolution spectrosc opic and imaging data of the nuclear region of NGC 7552. This galaxy i s a face-on barred spiral galaxy with several circumnuclear rings. Our near-infrared images have subarcsecond resolution and reveal the spat ial distribution of the J (1.25 mu m), H (1.60 mu m), K (2.20 mu m) co ntinuum and most of the prominent near-infrared K-band lines (Br gamma , He I, H-2 in emission and (CO)-C-12, (CO)-C-13 in absorption). Our N -band (10.5 mu m) continuum image has an angular resolution of about 1 '' and corresponds very well in structure with the JHK color maps and a published 6 cm radio continuum map. The data are combined with radi o and far-infrared data available in the literature and interpreted in the framework of a starburst model. The result of the starburst model is used in connection with published near-infrared template star spec tra to perform a spectral synthesis for the very nucleus as well as fo r selected regions in it. As a result we obtain a first insight into t he different star formation histories of the nucleus and the 1 kpc cir cumnuclear ring. We also demonstrate that the star formation rate and efficiency are intimately associated with the dynamics of the stars an d gas in the nucleus of NGC 7552. The combination of the starburst mod el with the spectral synthesis demonstrates for the first time the fea sibility of this approach in the near-infrared. This approach validate s the estimates of extinction and the contribution from hot dust made from JHK color diagrams and emission line ratios. It also proves in a consistent way that the emission from the ring originates in young ste llar clusters. These clusters were formed in a decaying starburst abou t 10(7) yr ago with an upper mass cutoff well above 50 M.. The K-band light from the nucleus most likely contains comparable contributions f rom an constant, old (approximate to 10(10) yr) and a decaying, young (approximate to 10(7) yr) stellar population.