Observations of neutral hydrogen (HI) absorption in the central regions of eight active galaxies are reported. These observations represent the most detailed studies of the neutral hydrogen component so far made of these sources, providing information on the HI gas distribution and kinematics on scales of 0.2 arcsec, corresponding to linear scales of tens of parsecs in the nearest galaxies observed. The sample of active galaxies presented here consists of the radio galaxy 3C293, two Seyfert galaxies NGC7674 and NGC7469, and five luminous infrared galaxies; NGC6240, IIIZw35, UGC2369, IR0335+1523 and Zw049.0. In each of these galaxies MERLIN has been used to provide the highest angular resolution observations of the neutral hydrogen content in these sources made to date. In addition to these high resolution HI studies the newly operational Giant Meterwavelength Radio Telescope has also been used to provide arcsecond resolution observations of the HI gas content in two of the MERLIN sources, 3C293 and NGC7674.Against the central few kiloparsecs of the radio galaxy 3C293 both the neutral hydrogen distribution and the radio continuum emission is imaged on linear scales of 150pc. The distribution and velocity structure of the HI gas against this radio galaxy is attributed to two gas structures. One of these gas structures is indicative of quiescent gas lying 10kpc from the galaxy centre and is physically associated with both ionised gas and dust lanes in the galaxy. The second HI velocity structure is situated within 600pc of the active galactic nucleus (AGN) and is observed to form a circumnuclear disc with a velocity gradient of 179km/s/arcsec. This circumnuclear disc of gas is shown to enclose a mass of M_dyn<600pc)leq3.9*10^9(sin^{-2}i) Msolar, where i represents the inclination of the disc. Extremely localised HI absorption is observed against the unresolved nuclei of the Seyfert galaxies NGC7674 and NGC7469. In the case of the Seyfert-2 galaxy NGC7674, it is shown that the HI is distributed within a 100pc of the AGN. Against the unresolved Seyfert-1 nucleus of NGC7469 the HI absorption is thought to either result from quiescent gas in the outer parts of this galaxy or from gas close to the nucleus. The HI absorption distribution in both of these sources is consistent with the absorption arising in circumnuclear dusty tori. Of the five luminous infrared galaxies observed, HI absorption is detected in four cases. The observations of IIIZw35, IR0335+015 and Zw49.0 all revealed unresolved 1.4GHz radio continuum structure against which HI absorption is detected. However, against the prototypical luminous infrared merger NGC6240 extensive radio continuum is resolved and imaged. It is concluded that the radio continuum structure of NGC6240 is consistent with the radio emission arising from both a starburst region and one or two AGN existing in the remnant nuclei of the merging galaxies. Extremely broad HI absorption is detected against the two dominant radio continuum components. The HI gas structure is consistent with a neutral gas disc forming between the two merging nuclei as they orbit around each other.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:606745 |
| Date | January 2002 |
| Creators | Beswick, Robert J. |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://www.manchester.ac.uk/escholar/uk-ac-man-scw:172309 |
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