Small-Scale Interstellar N[CLC]a[/CLC] [CSC]i[/CSC] Structure toward M92

We have used integral field echelle spectroscopy with the DensePakfiber-optic array on the KPNO WIYN telescope to observe the central 27" x 43"of the globular cluster M92 in the Na I D wavelength region at a spatialresolution of 4". Two interstellar Na I absorption components are evident inthe spectra at LSR velocities of 0 km/s (Cloud 1) and -19 km/s (Cloud 2).Substantial strength variations in both components are apparent down to scaleslimited by the fiber-to-fiber separations. The derived Na I column densitiesdiffer by a factor of 4 across the Cloud 1 absorption map and by a factor of 7across the Cloud 2 map. Using distance upper limits of 400 and 800 pc for Cloud1 and Cloud 2, respectively, the absorption maps indicate structure in the ISMdown to scales of 1600 and 3200 AU. The fiber-to-fiber Na I column densitydifferences toward M92 are comparable to those found in a similar study of theISM toward the globular cluster M15. Overall, the structures in theinterstellar components toward M92 have significantly lower column densitiesthan those toward M15. We interpret these low column density structures assmall-scale turbulent variations in the gas and compare them to thelarger-scale, higher column density variations toward M15, which may be thehallmarks of actual H I structures.Comment: 9 pages, 2 figures, accepted for publication in ApJ Letter