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 10.003(Q/1.6) pc at a radius of 10.2 pc, where the local value of the stability parameter is 1.2 = Q = 2. The H 2 density is 11.8 ϫ 10 10 (Q/1.6) Ϫ1 cm Ϫ3 , assuming that H 2 dominates the mass density in that region. The disk mass is =10 6 M J , 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.

Periastron shifts of stellar orbits near the Galactic Center

“…The Galactic Center is the closest and, besides NGC 4258 Myoshi et al 1995), the most convincing candidate for a massive nuclear BH (Maoz 1998).…”

“…The two cases which are most convincing are the Galactic Center Genzel et al 1997Genzel et al , 2000Ghez et al 1998Ghez et al , 2000 and the nucleus of NGC 4258 Myoshi et al 1995). Theoretical estimates of the stability of these nuclear masses at their measured densities indicate that they cannot be stable for more than a few 10 7 years before they collapse to a massive BH (Maoz 1998). With a distance of only 8 kpc, the Galactic Center is the closest nucleus.…”

Observational evidence for massive black holes in the centers of active galaxies

“…It is unlikely that this mass is in the form of a dense star cluster (Maoz 1995). The average density of stars in the solar neighborhood is about 1 M ⊙ pc −3 , and the density of the densest known star cluster is about 10 5 M ⊙ pc −3 .…”

Far-infrared emission from intracluster dust in Abell clusters

“…Historically, this has been the first attempt to find evidence for the existence of ICD (Zwicky 1957(Zwicky , 1962. Despite quite some effort to search for the visual extinction of background quasars and galaxies seen through foreground galaxy clusters, no common consent has been achieved whether properties of background sources are actually affected by the presence of dust in foreground clusters (Girardi et al 1992;Ferguson 1993;Maoz 1995, and references therein). Aguirre (1999a,b) and Simonsen & Hannestad (1999) have drawn attention to the possibility that the observed dimming of distant type Ia supernovae might be caused by intervening dust rather than resulting from a non-zero cosmological constant.…”