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

Near-infrared (NIR) K-band imaging spectroscopy of the central 8" (1.3 kpc) in the Wolf-Rayet LINER galaxy NGC 6764 shows that the most recent star formation is most likely still unresolved at subarcsecond resolution (<100 pc). The K-band continuum source has a size of about 1.5" (240 pc). In addition to stellar CO and Na absorption lines as well as recombination lines of H and He, the K-band spectrum shows several strong emission lines from molecular hydrogen (H₂). The H₂ line emission is spatially and spectrally resolved showing a rotating ring/disk of ~1.2" (200 pc) diameter. An analysis of the nuclear (3^''~480 pc) 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 to about 50 Myr ago. Continuous star formation with a star formation rate of ~0.3 M_solar yr^-1 over at least 1 Gyr 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 Brγ flux could still be due to the active galactic nucleus (AGN), depending on the assumed age of the older starburst event. In contrast to other starburst galaxies (e.g., M82, NGC 7552), the younger starburst in NGC 6764 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. We also present the results of a 44 ks HRI ROSAT exposure. The HRI data show the presence of an X-ray source (probably an AGN) which varies by more than a factor of 2 over a timescale 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 the radio continuum emission presented in published VLA maps. Both data sets confirm the composite nature of the center of NGC 6764: the presence of a compact AGN as well as recent violent nuclear star formation.