We have mapped the distribution of young and old stars in the gaseous HI warp
of NGC 4565. We find a clear correlation of young stars (<600 Myr) with the
warp, but no coincident old stars (>1 Gyr), which places an upper limit on the
age of the structure. The formation rate of the young stars, which increased
~300 Myr ago relative to the surrounding regions, is (6.3 +2.5/-1.5) x 10^-5
M_sol/yr/kpc^2. This implies a ~60+/-20 Gyr depletion time of the HI warp,
similar to the timescales calculated for the outer HI disks of nearby spiral
galaxies. While some stars associated with the warp fall into the asymptotic
giant branch (AGB) region of the color magnitude diagram, where stars could be
as old as 1 Gyr, further investigation suggests that they may be interlopers
rather than real AGB stars. We discuss the implications of these age
constraints for the formation of HI warps, and the gas fueling of disk
galaxies.Comment: 12 pages, 9 figures. Accepted for publication in Ap
