Physical conditions in the narrow-line regions of M51 and NGC 4151

Details Physical conditions in the narrow-line regions of M51 and NGC 4151 Physical conditions in the narrow-line regions of M51 and NGC 4151

Apr 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004phdt........18b&link_type=abstract

Thesis (PhD). THE JOHNS HOPKINS UNIVERSITY, Source DAI-B 64/10, p. 4983, Apr 2004, 162 pages.

Physics

Scientific paper

The relative proximity and luminosity of Seyfert galaxies allows for detailed studies of their spatially extended narrow-line regions (NLR), which are the largest structures in these active galactic nuclei (AGN) powered by the active nucleus. In this dissertation, I have used the high spatial resolution of the Hubble Space Telescope ( HST) and Very Large Array (VLA) to examine the physical conditions producing the kinematic and ionization structure observed in the NLR of two Seyfert galaxies, namely NGC 4151 and M51. The physical conditions in the NLR of NGC 4151 were investigated using medium spectral resolution HST/STIS slitless spectra and HST/Wide Field and Planetary Camera 2 (WFPC2) images. The slitless data allowed us to spatially map the velocity field of the complete inner NLR of NGC 4151. The observations show a biconical distribution of emission- line clouds with blueshifted radial velocities to the southwest of the nucleus and redshifted clouds to the northeast of the nucleus. The NLR clouds are distributed in at least two kinematic components, including a population of low-velocity (|v| < 400 km s-1), low-velocity dispersion (Δv < 130 km s-1 ) clouds and high-velocity (400 < |v| < 1700 km s-1), high-velocity dispersion (Δv ≥ 130 km s-1) clouds. Our results suggest that a wind-driven outflow is responsible for the acceleration of the NLR clouds. Within 3.'' 2 (˜200 pc) of the nucleus, the [O III] λ5007/Hβ emission-line ratio decreases approximately as r-0.3 . Because the ionization parameter is proportional to r -2 n-1, it appears that the density, n, of these NLR clouds falls off approximately as r-1.7. The physical conditions in the NLR of M51 were explored using long-slit spectra obtained with the Space Telescope Imaging Spectrograph (STIS) aboard HST and 8.4 GHz (3.6 cm) radio continuum observations obtained with the VLA. Emission-line diagnostics were employed for nine NLR clouds, which extend 2.'' 5 (102 pc) from the nucleus, to examine the electron density, temperature, and ionization state of the NLR gas. To test the hypothesis of central source ionization, I generated detailed photoionization models for each NLR cloud using the photoionization code Cloudy. The emission-line fluxes for most of the NLR clouds can be reproduced reasonably well by simple photoionization models using a central power-law continuum source and supersolar nitrogen abundances. The optimized photoionization models reveal the presence of a nitrogen abundance gradient within the inner narrow-line region of M51. The nitrogen abundance appears to be larger closer to the nucleus (˜4.5 N&sun; ) and decreases with increasing radius, reaching ˜3 N&sun; for the outer clouds. I have also compared our observed line ratios with the MAPPINGS II shock models to determine if shocks play a significant role in the excitation of the NLR gas. Shock+precursor models provide a better fit than photoionization models to the observed fluxes of an NLR cloud ˜ 2.'' 5 south of the nucleus that is identified with the extra-nuclear cloud (XNC) detected in earlier studies. In our 3.6 cm data, this cloud is straddled by two radio knots and lies near the location where a weak radio jet, ˜ 2.'' 5 (102 pc) in extent, connects the near-nuclear radio emission with a diffuse (lobe) structure spanning ˜ 4'' (163 pc).

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