[Previous] | [Session 48] | [Next]
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.
[Previous] | [Session 48] | [Next]
Bulletin of the American Astronomical Society, 34, #4
© 2002. The American Astronomical Soceity.