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 ) aboard the Hubble Space Telescope ( HST ) and 3.6 cm radio continuum observations obtained with the Very Large Array ( VLA ) The VLA is a facility of the National Radio Astronomy Observatory , which is operated by Associated Universities , Inc. , under cooperative agreement with the National Science Foundation . .
Emission-line diagnostics were employed for nine NLR clouds , which extend 2 \farcs 5 ( 102 pc ) from the nucleus , to examine the electron density , temperature , and ionization state of the NLR gas .
The emission-line ratios are consistent with those typically found in Seyfert nuclei and indicate that within the inner near-nuclear region ( r \scalebox { 1.2 } [ 1.056 ] { \raisebox { 1.2 pt } { $ \scriptscriptstyle \lesssim$ } } 1 \arcsec ) the ionization decreases with increasing radius .
Upper-limits to the [ O iii ] electron temperature ( T _ { e } \scalebox { 1.2 } [ 1.056 ] { \raisebox { 1.2 pt } { $ \scriptscriptstyle \lesssim$ } } % 11 , 000 K ) for the inner NLR clouds indicate that photoionization is the dominant ionization mechanism close to the nucleus .
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 .
Shock+precursor models , however , provide a better fit to the observed fluxes of an NLR cloud \sim 2 \farcs 5 south of the nucleus that is identified with the extra-nuclear cloud ( XNC ) .
The large [ O iii ] electron temperature of this cloud ( T _ { e } = 24 , 000 K ) further suggests the presence of shocks .
This cloud is straddled by two radio knots and lies near the location where a weak radio jet , \sim 2 \farcs 5 ( 102 pc ) in extent , connects the near-nuclear radio emission with a diffuse lobe structure spanning \sim 4″ ( 163 pc ) .
It is plausible that this cloud represents the location where the radio jet impinges on the disk ISM .