Molecular absorption in the cores of AGN

Abstract

<p>One of the fundamental concepts in the unified scheme of AGN is that both Seyfert 1 and Seyfert 2 galaxies harbour supermassive black holes that act as nuclear engines, but in Seyfert 2 galaxies the nucleus is blocked from direct view by an optically and geometrically thick molecular torus. However, whilst the unified scheme has proven very successful, little is known about the physical properties of the torus itself. Searches for absorption lines of common molecules like CO and OH have mostly yielded non-detections. Before concluding that tori are not molecular, radiative excitation effects, which uppress the opacity in the lowest transitions due to the proximity to the high brightness temperature radio radiation from the nucleus causing excitation into higher states, must be investigated.<br /> To explore these effects, we conducted a survey searching for excited OH from a sample of 31 Seyfert 2 galaxies. Here, I present the results of Effelsberg observations at 6.0 GHz yielding five new detections, and at 4.7 GHz yielding no detections. I also present spectral-line VLBI observations carried out at 13.4 GHz, which detected excited OH towards the cores of Cygnus A and NGC 1052. These observations confirm the presence of a molecular torus in these sources. A subsample of the 31 Seyfert 2 galaxies was further searched for methanol at 6.7 GHz, yielding the first ever extragalactic detection of methanol towards NGC 3079. It is, however, unclear if the methanol is associated with the active nucleus or the starburst around the source or possibly connected to the superbubble in this source. A blue-shifted feature in the spectrum also indicates that methanol is present in the large-scale molecular outflows.<br /> Finally, I present the exciting new discovery of a water maser at redshift 2.64, which is the most distant source where water has ever been found. Luminous extragalactic water masers are associated with the nuclear activity of their host galaxies. The maser emission originates from within a few parsecs of the central engine, being associated nuclear accretion disks or through interactions between the jet emerging from the nucleus and ambient molecular clouds, or nuclear outflows. The transition observed requires gas temperatures in excess of 300 K and particle densities n(H) > 10e7 cm^(-3). It is the first time that such a dense interstellar gas component has been observed in the early Universe. This discovery has been possible only due to the magnification provided by a foreground galaxy; it acts as a cosmic telescope reducing the integration time required for a detection by a factor of ~ 1000.</p>