The Young Starburst Nucleus of the Wolf-Rayet LINER Galaxy NGC 6764

Abstract

NIR K band imaging spectroscopy of the central 8'' (1.3 kpc) in the Wolf-Rayet LINER galaxy NGC6764 shows that the most recent star formation is most likely still unresolved at sub-arcsecond resolution (< 100pc). The continuum source has a size of about 1.5''. The H_2 emission is resolved showing a rotating ring/disk of ~1.2'' diameter. An analysis of the nuclear stellar light using population synthesis models in conjunction with NIR spectral synthesis models suggests following star formation history: Two starbursts with decay times of 3 Myr occurred 3 - 5 Myr and 15 - ~50 Myr ago. Continuous star formation over at least 1Gyr can also explain the observed parameter. However, the mass relocation and consumption involved as well as the different spatial distribution of the lines associated with the star formation strongly favor the 'two starburst' scenario. In that scenario, up to 35% of the total observed Br_gamma flux could still be due to the AGN. In contrast to other starburst galaxies (e.g. M82, NGC 7552), the younger starburst in NGC6764 is very likely located closer to the nucleus and surrounded by the older starburst. One possible explanation can be that the stellar bar still transports gas down to radii close to the nucleus. This suggests that the massive star formation activity is directly competing with the AGN for the fuel. The 44 ksec. HRI ROSAT data show the presence of a X-ray source (probably an AGN) which varies by more than a factor of 2 over a time scale of 7 days. This implies the presence of a compact source with a discrete or at most 1000 AU source size. In addition, we find an extended X-ray component which looks similar to published VLA radio continuum maps.