SPECTRAL-ANALYSIS OF THE NUCLEAR STELLAR POPULATION AND GAS EMISSION IN NGC-6240

We analyse the nuclear stellar population and emission-line spectrum i n the range lambda lambda 3100-9700 Angstrom of the infrared luminous merger galaxy NGC 6240. Two spectral extractions from CCD frames are s tudied: one corresponding to the double nucleus and the other from a l ight peak 4 arcsec east of the nucleus. The double nucleus is Delta E( B-V)approximate to 0.30 mag more reddened and has a stronger interstel lar Na I D absorption as compared to the eastern peak; otherwise the t wo extractions present similar spectral properties, in particular the dereddened continuum distribution and emission-line ratios. The popula tion synthesis of the double nucleus, which has age and metallicity as free parameters and is based on spectral properties of a library of s tar clusters, indicates that the continuum flux fraction at 5870 Angst rom owing to an old bulge/halo component is approximate to 75 per cent , that of intermediate-age components (1 to 5 Gyr) is approximate to 5 per cent, and that associated with young ones (50 to 500 Myr) is appr oximate to 20 per cent. Currently star-forming regions do not contribu te more than 1-2 per cent to the continuum at 5870 Angstrom, and this amount is contributed only if they are differentially reddened from th e rest of the stellar components by Delta E(B-V)(i)>1.00. The age dist ribution of the nuclear stellar population, when compared to that foun d in normal nuclei, sets constraints on the age of the dynamical inter action that led to the present merger; the synthesis suggests that the interaction has been operating for approximate to 1 Gyr. We derive an internal reddening affecting the nuclear stellar population of E(B-V) (sp)=0.48. The population-subtracted optical emission-line spectrum is typical of a shock-heated gas. We derive an internal reddening for th e emitting gas clouds of E(B-V)(g)=0.60. We conclude that in the centr al 2-3 kpc of NGC 6240 the old (bulge) and 1-Gyr to 50-Myr stellar com ponents are homogeneously mixed, and they coexist with colliding gas c louds from the discs of the merging spirals. The continuum and line em ission flux from currently star-forming regions is apparently blocked from the optical spectrum by dust absorption, and their absorbed photo ns from the ultraviolet to the optical appear to be the source of the strong emission observed with IRAS.