An Ultradeep Survey for Irregular Satellites of Uranus: Limits to Completeness

We present a deep optical survey of Uranus' Hill sphere for small satellites.The Subaru 8-m telescope was used to survey about 3.5 square degrees with a 50%detection efficiency at limiting red magnitude m = 26.1 mag. This magnitudecorresponds to objects that are about 7 km in radius (assuming an albedo of0.04). We detected (without prior knowledge of their positions) all previouslyknown outer satellites and discovered two new irregular satellites (S/2001 U2and S/2003 U3). The two inner satellites Titania and Oberon were also detected.One of the newly discovered satellites (S/2003 U3) is the first known irregularprograde of the planet. The population, size distribution and orbitalparameters of Uranus' irregular satellites are remarkably similar to theirregular satellites of gas giant Jupiter. Both have shallow size distributions(power law indices q~2 for radii > 7 km) with no correlation between the sizesof the satellites and their orbital parameters. However, unlike those ofJupiter, Uranus' irregular satellites do not appear to occupy tight distinctdynamical groups in semi-major axis versus inclination phase space. Twogroupings in semi-major axis versus eccentricity phase space appear to bestatistically significant.Comment: 10 Figures (2 color), 3 Tables, to appear in AJ January 200