Spiral Structure In The Circum-nuclear Disk At The Center Of The Galaxy NGC 4258

Abstract

Observations of line emission from water masers near the center of the galaxy NGC 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 - a stability regime where weak non-axisymmetric disturbances tend to grow via the swing amplification effect, forming a ragged, multi-armed spiral pattern similar to that observed in Sc galaxies. This suggests a natural explanation for the apparent clustering of the high-velocity emission sources into several distinct clumps, and for the observed regularity in the distance intervals between them. The clumps of sources appear at the intersections of the spiral arms and the radial line of longest coherent gain path, and are thus spaced apart at the characteristic crest-to-crest radial distance between the arms. This interpretation implies an H_2 density and disk thickness of about 1.8x10^{10} (Q/1.6)^{-1} cm^{-3}, and 0.003(Q/1.6) pc at a radius of 0.2 pc, respectively, where the local value of the stability parameter is 1.2 < Q < 2.