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

Abstract

Observations of line emission from water masers near the center of the galaxy NGC 4258 have recently provided compelling evidence for the presence 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 weak, nonaxisymmetric disturbances grow via the swing amplification effect, leading to the formation of a ragged, multiarmed 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 maser sources appear at the intersections of the spiral arms and the radial lines of longest coherent gain path (the diameter through the disk perpendicular to the line of sight) and are thus spaced apart at the characteristic crest-to-crest radial distance between the arms. This interpretation implies a disk thickness of ~0.003 (/1.6) pc at a radius of ~0.2 pc, where the local value of the stability parameter is 1.2 2. The H2 density is ~1.8 × 1010 (/1.6)−1 cm-3, assuming that H2 dominates the mass density in that region. The disk mass is 106 M☉, which is consistent with the accuracy of the Keplerian fit to the rotation curve of the maser emission sources. These results closely concur with previous estimates based on independent considerations.