The nucleus of M33

We have obtained high-resolution surface photometry and velocity dispersion measurements of the semistellar nucleus of M33 to study its structure and to look for a central black hole. The images were obtained with the DAO/CFHT HR Camera; this uses a tip-tilt mirror to correct the motion component of seeing. Our best images have resolution σ * =0″.186 in the B band and σ * =0″.177 in R (σ * = Gaussian dispersion radius of the point spread function). The nucleus is unresolved. Its true core radius is r c ≤ 0″.10 ≃ 0.39 pc; this is as small as in core-collapsed globular clusters. The true central surface brightness is μ 0 < 11.3 R mag arcsec −2 , and the central density is ρ 0 ≥ 5 × 10 5 M ○ . pc −3 . Except for M32 and the nucleus of our Galaxy, this is the most compact galactic core known