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B.M. Sabra, J.C. Shields (Ohio U.), A.V. Filippenko (UC-Berkeley)
Nebular filaments are seen at optical wavelengths around NGC1275 and M87, the central galaxies in the Perseus and Virgo clusters, respectively. The optical line ratios in the two filamentary systems are different. The excitation mechanisms of both sets of nebulae are still unknown. This paper presents the results of a study aimed at identifying those mechanisms by measuring line ratio properties from an extensive spectroscopic data set acquired at the Lick observatory 3-meter telescope. Line ratio diagrams were used to study the ionization state of the nebular gas. Excitation and kinematic properties as a function of radius were also measured. For NGC1275, results show that the filaments are in an extremely low ionization state. The observed line ratios are not well matched by predictions of shock models, and a lack of correlation between excitation and linewidth also argues against shocks. A photoionization model where the ionizing is attenuated through an ionized screen to introduce a helium edge at 54 eV was found to reproduce the observed line ratios. The M87 filaments are in a higher ionization state compared to those of NGC1275. Implications for the underlying energy source in the M87 system will be also discussed.