Distribution and Kinematics of Neutral and Ionised Gas in Seyfert Galaxies

Carole Mundell

Research output: ThesisDoctoral Thesis


Observations of the distribution and kinematics of neutral and ionised gas in two nearby Seyfert galaxies, NGC3227 and NGC4151, are presented. These observations are the most detailed of their kind and cover several regimes in a wide range of angular and linear sizes, from the largest galactic scales ($\sim$100 kpc) to the circumnuclear regions ($\lta$10 pc). Although Seyfert galaxies lie at the low luminosity end of the Active Galactic Nuclei (AGN) phenomenon, they are the closest and most common type of AGN. As such, they represent ideal sites for the detailed study of the AGN phenomenon and its relationship to the host galaxy environment, which is unobservable in more distant and powerful types of AGN. Neutral hydrogen (HI) is a valuable tracer of galactic structure and dynamics and so detailed study of the distribution and kinematics of neutral hydrogen on all size scales is vital to determine the relationship between AGN and their host galaxies. It is shown here that, with sensitive high-resolution $\lambda$21-cm HI studies, emission on sub-kiloparsec scales may be mapped, revealing hitherto unprecedented detail in structure and kinematics. Absorption measurements have been used to probe structure on even finer scales ($\sim$10 pc in nearby Seyferts). VLA HI studies of NGC3227 show evidence of interaction (possibly with the nearby dwarf elliptical galaxy, NGC3226) in the form of extensive ($\sim$100 kpc) tidal tails. Complex motions in the galactic disk have been resolved into disk emission, which is in approximately solid body rotation, and emission from an anomalous-velocity, gas-rich cloud. The kinematics of the disk of NGC3227 appear to be remarkably undisturbed, although a bar is seen in HI emission; this appears to be a continuation of the nuclear CO bar, and may have formed as result of the interaction. No high velocity gas is detected, and fuelling via direct `dumping' of tidally disturbed gas onto the nucleus is ruled out. This suggests that the bar may play an important role in the fuelling of the AGN. Optical observations of NGC3227 reveal a 7$''$ emission-line `wedge' with relatively broad ($\sim$250 km s$^{-1}$) [OIii] emission, typical of a narrow line, rather than an extended narrow line region. This is misaligned with a radio collimation axis revealed by MERLIN. Such differences in alignment may be due to geometrical effects. However, this interpretation is only possible in the case of NGC3227 if the spiral arms are leading; this may indicate that the optical and radio collimation axes are intrinsically misaligned. The VLA HI study of NGC4151 reveals the first evidence of streaming motions in a bar, detected in neutral hydrogen. This is possibly the clearest detection of such streaming seen to date, yet this type of measurement was previously thought to be impossible using HI, because of limitations in resolution and sensitivity. A new rotation curve is presented showing a turnover at a radius of $\sim$35$''$ which was undetected in lower resolution studies. The bar pattern speed is found to be $\sim$29 km s$^{-1}$ giving one inner Lindblad resonance at a radius of 1.9 kpc, although further ILRs may be present closer to the nucleus. The neutral hydrogen absorption in the radio continuum nucleus of NGC4151 has been measured on scales $\lta$10 pc, using MERLIN. Of the 5 radio components in NGC4151, only the one which is thought to contain the active nucleus, shows significant absorption. A lower limit of 90 M$_\odot$ of neutral hydrogen is required to produce the observed absorption against the nucleus. To reconcile UV and HI column densities, the interpretation of the VLBI structure of NGC4151 is revised. A more plausible model than a small cloud in the line of sight might be a circumnuclear torus, as invoked in unified schemes; such a torus must be less that $\sim$40 pc thick and $\sim$70 pc in extent.
Original languageEnglish
Awarding Institution
  • University of Manchester
Award date1 Nov 1995
Publication statusPublished - 1995


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