EXCITATION AND TEMPERATURE OF EXTENDED GAS IN ACTIVE GALAXIES .1. OBSERVATIONS

We have obtained long-slit spectroscopy of five active galaxies showin g extended high excitation emission lines reaching distances of severa l kiloparsecs from the nuclei. Measurement of the fluxes of weak lines , such as [OIII]lambda 4363 and HeII lambda 4686, requires careful sub traction of the contribution of the stellar population, and the proced ure for this subtraction is discussed in detail. For each galaxy, we p resent emission-line ratios for the nucleus and the extended gas at th e most distant locations for which the fluxes of the weaker high excit ation lines can be measured with an uncertainty less than or equal to 30%. In a companion paper (Paper II), these line ratios are used in a reevaluation of photoionization models of extended gas in active galax ies.