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

Abstract

Naturally occurring water vapor maser emission at 1.35 cm wavelength provides an accurate probe for the study of accretion disks around highly compact objects, thought to be black holes, in the centers of active galaxies. Because of the exceptionally fine angular resolution, 200 microarcseconds, obtainable with very long baseline interferometry, accompanied by high spectral resolution, < 0.1 km s −1 , the dynamics and structures of these disks can be probed with exceptional clarity. The data on the galaxy NGC 4258 are discussed here in detail. The mass of the black hole binding the accretion disk is 3.9 × 10 7 M ⊙ . Although the accretion disk has a rotational period of about 800 years, the physical motions of the masers have been directly measured with VLBI over a period of a few years. These measurements also allow the distance from the earth to the black hole to be estimated to an accuracy of 4 percent. The status of the search for other maser/black hole candidates is also discussed.

A search for extragalactic H2O maser emission towards IRAS galaxies

“…One can also speculate on the possible existence of an edge-on molecular gas disk rotating around a super massive object by analogy with other megamasers (Moran et al 1999). Such a hypothesis is supported by the nature of the observed spectrum of the H 2 O maser towards NGC 6240 and its similarity to that of the so-called disk masers.…”

“…Moran et al 1999). The detection rates of new masers remain quite low, less than ∼7% (e.g., Braatz et al 1996;Braatz et al 1997;Greenhill et al 1997).…”

“…Recent maser surveys towards active galactic nuclei (AGN) have been motivated by the VLBI observations of a sub-parsec-scale H 2 O maser disk rotating around a central massive object in NGC 4258 (Miyoshi et al 1995). The detection and imaging of such H 2 O megamaser emission enables us to probe the kinematics and dynamical structures of parsec-scale circumnuclear disks (Moran et al 1999). H 2 O megamasers studied to date can be categorised into two types; disk masers residing in a (sub-)parsec scale disk around an active nucleus, and non-disk masers which are seen significant distances from the nucleus.…”

Two Epochs of Very Large Array Observations of Water Maser Emission in the Active Galaxy NGC 6240

“…It is important that the column density N H of N1, measured by X-ray observations, is at least a factor of several higher than that of N2 (Komossa et al 2003) and that the maser in N1 could be more enhanced through the thicker dust layer in our line of sight, amplifying the background radio source of N1. Also, the non-detection of the maser except for the compact continuum nucleus is consistent with the presence of a background nuclear continuum source being necessary to excite H 2 O megamasers that are associated with AGN activity (e.g., Moran et al 1999). On the other hand, the X-ray measurements of NGC 6240 revealed that both nuclei show the Compton-thick properties (Matt et al 2000), meaning each of them could host an obscured AGN.…”

Powerful Water Masers in Active Galactic Nuclei

“…Since the Braatz et al (1996Braatz et al ( , 1997 survey, eight more megamasers have been discovered 2 , bringing the current total to 24 (see the list in Moran, Greenhill, & Herrnstein 1999, plus Mrk 348 (Falcke et al 2000b) and NGC 6240 (Hagiwara, Diamond, & Miyoshi 2002;Nakai, Sato, & Yamauchi 2002)). All of the sources are in galaxies classified as either Sy 2 (14/24) or LINERs (10/24).…”