SPIRAL STRUCTURE IN THE CIRCUMNUCLEAR DISK AT THE CENTER OF NGC-4258

Observations of line emission from water masers near the center of the galaxy NGC 4258 have recently provided compelling evidence for the pr esence of a rotating disk of gas, viewed nearly edge-on, surrounding a massive black hole. We show that this disk is very likely to be only marginally stable to radial perturbations-a stability regime where wea k, nonaxisymmetric disturbances grow via the ''swing amplification'' e ffect, leading to the formation of a ragged, multiarmed spiral pattern similar to that observed in Sc galaxies. This suggests a natural expl anation for the apparent clustering of the high-velocity emission sour ces into several distinct clumps and for the observed regularity in th e distance intervals between them. The clumps of maser sources appear at the intersections of the spiral arms and the radial lines of longes t coherent gain path (the diameter through the disk perpendicular to t he line of sight) and are thus spaced apart at the characteristic cres t-to-crest radial distance between the arms. This interpretation impli es a disk thickness of similar to 0.003((Q) over bar/1.6) pc at a radi us of similar to 0.2 pc, where the local value of the stability parame ter is 1.2 less than or similar to (Q) over bar less than or similar t o 2. The H-2 density is similar to 1.8 x 10(10)((Q) over bar/1.6)(-1) cm(-3), assuming that H-2 dominates the mass density in that region. T he disk mass is less than or similar to 10(6) M., which is consistent with the accuracy of the Keplerian fit to the rotation curve of the ma ser emission sources. These results closely concur with previous estim ates based on independent considerations.