J, H, and K' images obtained with the Canada-France-Hawaii Telescope adaptive optics system are used to investigate the star-forming histories of the central regions of the Sc galaxies NGC 247 and NGC 2403. The brightest resolved red stars within 15 00 of the nucleus of each galaxy are red supergiants (RSGs), indicating that the central few hundred parsecs of these galaxies experienced star formation within the last similar to0.1 Gyr. While there are indications of galaxy-to-galaxy differences in the recent (t less than or equal to 0.1 Gyr) star-forming histories, a comparison of the K luminosity functions of stars near the centers of NGC 2403 and M33 indicates that, when averaged over gigayear timescales, the star-forming histories of the inner disks of these galaxies have been remarkably similar. This is consistent with suggestions that the long-term evolution of disks is defined by local characteristics such as mass density. It is demonstrated that NGC 247 and NGC 2403, like M33, harbor nuclear star clusters with stellar contents that differ from the surrounding central light concentrations, which in turn have near-infrared spectral energy distributions that are similar to old stellar systems. The nucleus of NGC 2403 is significantly bluer than that of the other two galaxies. If the nuclei of NGC 247, NGC 2403, and M33 are subject to similar amounts of extinction, then this indicates that NGC 2403 harbors the youngest nuclear star cluster in the sample. The K-band surface brightnesses near the centers of NGC 247 and NGC 2403 are 1-2 mag arcsec(-2) lower than in M33. Finally, it is noted that young or intermediate-age nuclear star clusters are a common occurrence in nearby spirals, indicating that nuclear star formation in these objects is either continuous or episodic on timescales of 0.1 1 Gyr. This is consistent with models that have been proposed to explain the properties of the Galactic center. References: 99
展开▼
