1997A&A...327..901R

We present an analysis of three medium band width (FWHM∼400-450Å) images of the nuclear region of NGC 6240 taken with the pre-COSTAR Faint Object Camera (FOC) on board of the Hubble Space Telescope. The first filter band comprises the [OII]λ3727 doublet, the second measures the blue continuum around 4400Å and the third includes Hβ+[OIII]λλ4959,5007. The images show that the apparent two galactic nuclei B (north) and A (south) identified from the ground consist of compact subtructures on the tenth-arcsec scale plus faint extended emission on the arcsec scale. Within a diameter of 0.26" (∼120pc), most compact features exhibit a summed line luminosity L([OIII]+Hβ) in the range 10³⁹-10⁴⁰erg/s which is comparable to that of giant HII regions. Although the data are not complete enough for a direct decomposition into line and continuum flux, probable solutions yield a LINER like [OII]/[OIII]/Hβ line ratio for the northern compact subcomponent B1. In the southern nucleus A the faint subcomponents A2 and A3 can be classified as LINER like if they are more strongly reddened than B1 or as HII-region like if not. The brightest southern compact component A1, however, exhibits line ratios that are typical of (i) a Seyfert galaxy or (ii) a high-excitation HII region. This leads to the interesting alternative that there is either (i) a hidden AGN (which is in accord with recent ASCA observations) or (ii) a hidden cluster of young massive stars. The second possibility would imply localized ongoing star formation within an environment whose visible light is dominated by radiation from older stellar populations. However, considering recent models for fast autoionizing shocks computed by Dopita & Sutherland (1995ApJ...455..468D), A1 might also represent a shock-plus-precursor region.