Autoionizing Shocks along the Twisted Jets of NGC 4258

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Session 108 -- Seyfert Galaxies
Display presentation, Thursday, 12, 1995, 9:20am - 6:30pm

[108.14] Autoionizing Shocks along the Twisted Jets of NGC 4258

G. Cecil (UNC-CH), C. De Pree (UNC-CH/NRAO), J. Morse (STScI)

We correlate new HIFI Fabry-Perot (FP) and KPNO Goldcam long-slit spectra with VLA radio, BIMA CO, and ROSAT X-ray datasets to study the twisted jets in this nearby Seyfert. The multi-arcminute extent of the jet and the extensive wavelength coverage of its interaction with the multiphase ISM make this jet an important laboratory for studying the processes likely to dominate the appearance of high-$z$ radio galaxies. Our FP datacube covers the [OIII]$\lambda$5007 line at 60 km s$^{-1}$ FWHM resolution and shows that only the centermost, least twisted strand of the 3-strand H$\alpha$ jet is prominent in high-excitation gas. Within 1 kpc radius this strand points N-S, coaligning with the radio jet, and is perpendicular to the subparsec nuclear disk delineated by water masers (Greenhill et al 1994 CfA preprint). It is thus, plausibly, the most recently active strand. The long-slit spectra span $\lambda\lambda$3650--8950\AA\ at 5\AA\ resolution. Star motions mapped by the IR Ca triplet differ substantially from the mean gas velocities and line widths, arguing against virial broadening of the emission lines. In fact, emission-line ratios along the 5$^{\prime}$ length of the line-emitting jets are consistent with those expected from shock excitation. After subtracting the starlight with the galaxy NGC~3115, we obtain sensitive limits on the N$^{+}$, O$^{+}$, and O$^{++}$ gas temperatures as well as useful upper limits on the important but undetected [NeIII] and HeII fluxes. These fluxes and limits, together with the gross kinematics, will be compared to the predictions of shock models. X-ray fluxes derived from the ROSAT PSPC and HRI datasets are consistent with jet emission from a Raymond-Smith thermal plasma with $kT = 0.3$ keV, log$N_H \approx 20.0$ cm$^{-2}$, and luminosity $1.6\times10^{40}$ erg s$^{-1}$. Gas can reach this temperature in a shock of velocity $V_s\approx$500 km s$^{-1}$, a value consistent with the width and kinematic structure of the emission-line profiles. A modest disk gas density of 0.04 cm$^{-3}$ can produce the jet's x-ray flux. Hence, the shocked gas may be autoionized and entrained as the jets scrape along the molecular clouds that are known to be adjacent to the jets out to 2 kpc radius.

This research is supported by NSF and NASA grants to UNC.

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