Spiral Structure in the Circumnuclear Disk at the Center of NGC 4258

Observations of line emission from water masers near the center of the galaxyNGC 4258 have recently provided compelling evidence for rotating disk of gas,viewed nearly edge-on, surrounding a massive black hole (Miyoshi etal. 1995). We show that the disk is only marginally stable to radial perturbations - astability regime where weak non-axisymmetric disturbances tend to grow via theswing amplification effect, forming a ragged, multi-armed spiral patternsimilar to that observed in Sc galaxies. This suggests a natural explanation for the apparent clustering of thehigh-velocity emission sources into several distinct clumps, and for theobserved regularity in the distance intervals between them. The clumps ofsources appear at the intersections of the spiral arms and the radial line oflongest coherent gain path, and are thus spaced apart at the characteristiccrest-to-crest radial distance between the arms. This interpretation implies anH_2 density and disk thickness of about 1.8x10^{10} (Q/1.6)^{-1} cm^{-3}, and0.003(Q/1.6) pc at a radius of 0.2 pc, respectively, where the local value ofthe stability parameter is 1.2 < Q < 2.Comment: To Appear in ApJ (Letters), 10 pages, 1 figure, uuencoded + compressed postscrip