A NaI absorption map of the small-scale structure in the interstellar gas toward M15

Using the DensePak fiber-optic array on the KPNO WIYN telescope, we have ob tained high signal-to-noise ratio echelle spectra of the Na I D wavelength region toward the central 27 " x 43 " of the globular cluster M15 at a spat ial resolution of 4 ". The spectra exhibit significant interstellar Na I ab sorption at LSR velocities of 3 km s(-1) (local interstellar medium [LISM] component) and 68 km s(-1) (intermediate-velocity cloud [IVC] component). B oth components vary appreciably in strength on these scales. The derived Na I column densities differ by a factor of 4 across the LISM absorption map and by a factor of 16 across the IVC map. Assuming distances of 500 and 150 0 pc for the LISM and IV clouds, respectively, these maps show evidence of significant ISM structure down to the minimum scales of 2000 and 6000 AU pr obed in these absorbers. The smallest scale N(Na I) variations observed in the M15 LISM and IVC maps are typically comparable to or higher than the va lues found at similar scales in previous studies of interstellar Na I struc ture toward binary stars. The physical implications of the small and larger scale Na I features observed in the M15 maps are discussed in terms of var iations in the H I column density as well as in the Na ionization equilibri um.