Spectral analysis of the nuclear stellar population and gas emission in NGC 6240

Abstract

We analyse the nuclear stellar population and emission-line spectrum in the range λλ 3100-9700 Å of the infrared luminous merger galaxy NGC 6240. Two spectral extractions from CCD frames are studied: one corresponding to the double nucleus and the other from a light peak 4 arcsec east of the nucleus. The double nucleus is ∆E (B-V)~~ 0.30 mag more reddened and has a stronger interstellar Na I D absorption as compared to the eastern peak; otherwise the two extractions present similar spectral properties, in particular the dereddened continuum distribution and emission-line ratios. The population synthesis of the double nucleus, which has age and metallicity as free 'parameters and is based on spectral properties of a library of star clusters, indicates that the continuum flux fraction at 5870 Å owing to an old bulge/halo component is ~~ 7 5 per cent, that of intermediate-age components (1 to 5 Gyr) is ~~5 per cent, and that associated with young ones (50 to 500 Myr) is ~~ 20 per cent. Currently star-forming regions do not contribute more than 1-2 per cent to the continuum at 5870 Å, and this amount is contributed only if they are differentially reddened from the rest of the stellar components by ∆E (B - V)i > 1.00. The age distribution of the nuclear stellar population, when compared to that found in normal nuclei, sets constraints on the age of the dynamical interaction that led to the present merger; the synthesis suggests that the interaction has been operating for ~~ 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 the emitting gas clouds of E(B- V)g = 0.60. We conclude that in the central 2-3 kpc of NGC 6240 the old (bulge) and 1-Gyr to 50-Myr stellar components are homogeneously mixed, and they coexist with colliding gas clouds from the discs of the merging spirals. The continuum and line emission flux from currently star-forming regions is apparently blocked from the optical spectrum by dust absorption, and their absorbed photons from the ultraviolet to the optical appear to be the source of the strong emission observed with lRAS.