Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997aas...191.7812w&link_type=abstract
American Astronomical Society, 191st AAS Meeting, #78.12; Bulletin of the American Astronomical Society, Vol. 29, p.1335
Astronomy and Astrophysics
Astronomy
Scientific paper
Millimeter wave molecular absorption lines seen towards background AGNs offer a unique possibility to probe the small scale structure of the molecular gas in galaxies at large distances. The reason for this is the very limited angular extent of the continuum sources at millimeter wavelengths. We have found indirect indications of very small scale structure in the obscuring molecular gas in two galaxies at intermediate redshift. (1) The relative depth of two absorption features in the CO(1-0) absorption profile seen towards PKS1413+135 at z=0.25 show significant changes on a time scale of a few months. The change is correlated with variations of the background flux and the observed changes in the absorption profile is interpreted as being caused by structural changes in the continuum source during an outburst. (2) The second case involves the lensing galaxy to the background radio--loud AGN B0218+357, situated at a redshift z=0.68. High angular resolution interferometry shows that the absorption only occurs in front of one of the two lensed images. The opacities inferred from the molecular absorption are very high. From saturated isotopic transitions of CO we derive tau_CO ga 1500, corresponding to A_V>500 mag. Despite the high opacity, the obscured AGN shows up on images obtained with the HST. No difference in the extinction can be found between the nucleus of the lensing galaxy and the background AGN. A comparison of the radio--to--optical flux ratio for B0218+357 with that of similar flat--spectrum radio--loud AGNs shows, however, that B0218+357 is optically underluminous by a large factor. This implies that part of the emission region is completely obscured while a small part remains unobscured. This implies very small scale structures in the obscuring molecular gas, with high density contrasts.
Combes François
Wiklind Thomas
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