Disks, tori, and cocoons: emission and absorption diagnostics of AGN environments

Abstract

One of the most important problems in the study of active galaxies is understanding the detailed geometry, physics, and evolution of the central engines and their environments. The leading models involve an accretion disk and torus structure around a central dense object, thought to be a supermassive black hole. Gas found in the environment of AGN is associated with different structures: molecular accretion disks, larger scale atomic tori, ionized and neutral "cocoons" in which the nuclear regions can be embedded. All of them can be studied at radio wavelengths by various means. Here, we summarize the work that has been done to date in the radio band to characterize these structures. Much has been learned about the central few parsecs of AGN in the last few decades with contemporary instruments but the picture remains incomplete. In order to be able to define a more accurate model of this region, significant advances in sensitivity, spectral and angular resolution, and bandpass stability are required. The necessary advances will only be provided by the Square Kilometer Array and we discuss the possibilities that these dramatic improvements will open for the study of the gas in the central region of AGN.

Radio properties of Compact Steep Spectrum and GHz‐Peaked Spectrum radio sources

“…The presence of significant amount of gas in young radio galaxies is supported by a larger incidence of HI absorption in these objects , Pihlström, Conway, & Vermeulen 2003, Gupta et al 2006 compared to what is typically found in old and larger radio sources (Morganti et al 2001). This dense medium is likely the result of the merger that triggered the radio source (Morganti et al 2004a). The knowledge of the distribution of the gas, either settled in a circumnuclear structure like a disk or a torus, or inhomogeneously distributed in clouds, provides important information on the environment in the innermost region of AGN and its role in the radio jet evolution.…”

Extragalactic H2O masers and X-ray absorbing column densities

“…Being traditionally known to Based on observations with the 100-m telescope of the MPIfR (Max-Planck-Institut für Radioastronomie) at Effelsberg. trace oxygen-rich red giant stars and to pinpoint sites of (massive) star formation, it has more recently become a major tool to determine geometric distances and three dimensional motions of nearby galaxies and to elucidate the nuclear environment of active galaxies, allowing us to map accretion disks and to determine masses of nuclear engines (for recent reviews, see Greenhill 2002Greenhill , 2004Maloney 2002;Morganti et al 2004;Henkel et al 2005a;Lo 2005; for 3-D motions, see Brunthaler et al 2005).…”

New H$_\mathsf{2}$O masers in Seyfert and FIR bright galaxies

“…This includes the determination of geometric distances and 3-dimensional velocity vectors of galaxies, masses of nuclear engines, maps of accretion disks and physics of nuclear jet-molecular cloud interaction (for recent reviews, see Falcke et al (2000); Mrk 1157: Braatz et al (2004); NGC 1068: Claussen et al (1984); Mrk 1066: this paper, for a more recent independent detection see Braatz et al (2004); Mrk 3: Braatz et al (2004); Mrk 34: this paper; NGC 4151: Braatz et al (2004). Greenhill 2002Greenhill , 2004Maloney 2002;Morganti et al 2004;Henkel et al 2005). So far, almost 1000 active galaxies have been surveyed.…”