The circumnuclear starburst in NGC 7552: First results from near-infrared spectral synthesis

We present new near- and mid-infrared high angular resolution spectroscopic and imaging data of the nuclear region of NGC 7552. This galaxy is a face-on barred spiral galaxy with several circumnuclear rings. Our near-infrared images have subarcsecond resolution and reveal the spatial distribution of the J (1.25 μm), H (1.60 μm), K (2.20 μm) continuum and most of the prominent near-infrared K-band lines (Bry, He I, H₂ in emission and ¹²CO, ¹³CO in absorption). Our N-band (10.5 μm) 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 radio 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 spectra to perform a spectral synthesis for the very nucleus as well as for selected regions in it. As a result we obtain a first insight into the different star formation histories of the nucleus and the 1 kpc circumnuclear ring. We also demonstrate that the star formation rate and efficiency are intimately associated with the dynamics of the stars and gas in the nucleus of NGC 7552. The combination of the starburst model with the spectral synthesis demonstrates for the first time the feasibility of this approach in the near-infrared. This approach validates 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 stellar clusters. These clusters were formed in a decaying starburst about 10⁷ yr ago with an upper mass cutoff well above 50 M_⊙. The K-band light from the nucleus most likely contains comparable contributions from an constant, old (≈ 10¹⁰ yr) and a decaying, young (≈ 10⁷ yr) stellar population.