Small-scale interstellar NaI structure toward M92

We have used integral field echelle spectroscopy with the DensePak fiber-op tic array on the KPNO WIYN telescope to observe the central 27" x 43" of th e globular cluster M92 in the Na I D wavelength region at a spatial resolut ion of 4". Two interstellar Na I absorption components are evident in the s pectra at LSR velocities of 0 km s(-1) (cloud 1) and -19 km s(-1) (cloud 2) . Substantial strength variations in both components are apparent down to s cales limited by the fiber-to-fiber separations. The derived Na I column de nsities differ by a factor of 4 across the cloud 1 absorption map and by a factor of 7 across the cloud 2 map. Using distance upper limits of 400 and 800 pc for cloud 1 and cloud 2, respectively, the absorption maps indicate structure in the interstellar medium (ISM) down to scales of 1600 and 3200 AU. The fiber-to-fiber Na I column density differences toward M92 are compa rable to those found in a similar study of the ISM toward the globular clus ter M15. Overall, the structures in the interstellar components toward M92 have significantly lower column densities than those toward M15. We interpr et these low column density structures as small-scale turbulent variations in the gas and compare them with the larger scale, higher column density va riations toward M15, which may be the hallmarks of actual H I structures.