AAS 201st Meeting, January, 2003
Session 48. Blazars and AGNs
Poster, Tuesday, January 7, 2003, 9:20am-6:30pm, Exhibit Hall AB

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[48.09] Ionization Structure in the Narrow Line Region of M51

L.D. Bradley II, M.E. Kaiser (JHU), W.A. Baan (ASTRON)

We have investigated the physical conditions in the narrow line region (NLR) of M51 using long-slit spectra obtained with the Space Telescope Imaging Spectrograph (STIS) and 8.4~GHz radio continuum observations obtained with the {\em Very Large Array (VLA)}. Emission-line diagnostics as a function of projected radial distance are employed for nine emission-line clouds which extend 2.5\arcsec\ (102~pc) from the nucleus to examine the reddening, density, temperature, and ionization state of the NLR gas. Using the observed H\alpha/H\beta Balmer line ratio as a measure of the extinction in the NLR, we observe that the emission-line clouds closest to the obscured nucleus are generally more reddened than those at larger radii. The ionization state of the gas as probed by the [OII] 3727/[OIII] 5007 line ratio indicates that within the inner near-nuclear region (r \la 1\arcsec) the ionization decreases with increasing radius.

We generated photoionization models to compare with the dereddened fluxes of each NLR cloud. 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 super-solar (~3x) nitrogen abundances. The photoionization models provide a poorer match to the observed fluxes for an NLR cloud ~2.5\arcsec\ south of the nucleus which is identified with the extra-nuclear cloud (XNC) detected in earlier studies (e.g. Ford et al. 1985) and may indicate the presence of additional ionization via shocks. This cloud lies near the location where a weak radio jet, ~2.5\arcsec in extent, connects the near-nuclear emission with a diffuse (lobe) structure (Crane & van der Hulst 1992) spanning ~4\arcsec\ (163\,pc). We also detect weak radio emission extending ~1.2\arcsec\ north of the nucleus which encompasses the [OIII] emission and is roughly opposite the southern jet.

Support for these observations was provided by contract NAS5-30403.

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