paper

Molecular Gas in NUclei of GAlaxies (NUGA): VI. Detection of a molecular gas disk/torus via HCN in the Seyfert2 galaxy NGC6951?

arXiv:astro-ph/0701403 · doi:10.1051/0004-6361:20066785

Abstract

Several studies of nearby active galaxies indicate significantly higher HCN-to-CO intensity ratios in AGN than in starburst (SB) environments. HCN enhancement can be caused by many different effects, such as higher gas densities/temperatures, UV/X-ray radiation, and non-collisional excitation. As active galaxies often exhibit intense circumnuclear SB, high angular resolution/sensitivity observations are of paramount importance to disentangling the influence of SB from that of nuclear activity on the chemistry of the surrounding molecular gas. The tight relation of HCN enhancement and nuclear activity may qualify HCN as an ideal tracer of molecular gas close to the AGN, providing complementary and additional information to that gained via CO. NGC6951 houses nuclear and SB activity, making it an ideal testbed in which to study the effects of different excitation conditions on the molecular gas. We used the new ABCD configurations of the IRAM PdBI to observe HCN(1-0) in NGC6951 at high angular resolution (1''). We detect very compact (<50pc) HCN emission in its nucleus, supporting previous hints of nuclear gas structure. Our observations also reveal HCN emission in the SB ring and resolve it into several peaks, leading to a higher coincidence between the HCN and CO distributions than previously reported. We find a significantly higher HCN-to-CO intensity ratio (>0.4) in the nucleus than in the SB ring (0.02-0.05). As for NGC1068, this might result from a higher HCN abundance in the centre due to an X-ray dominated gas chemistry, but a higher gas density/temperature or additional non-collisional excitation of HCN cannot be entirely ruled out, based on these observations. The compact HCN emission is associated with rotating gas in a circumnuclear disk/torus.

arXiv commentLetter accepted for publication in a special issue of A&A presenting results of the new extended configuration at the IRAM PdBI, to appear most likely around June 2007. Letter has 4 pages and 2 figures. Abstract has been shortened to meet the line limits of astrop-ph

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