Excitation and temperature of extended gas in active galaxies. I Observations

Abstract

We have obtained long-slit spectroscopy of five active galaxies showing extended high excitation emission lines reaching distances of several kiloparsecs from the nuclei. Measurement of the fluxes of weak lines, such as [OIII]λ4363 and HeIIλ4686, requires careful subtraction of the contribution of the stellar population, and the procedure for this subtraction is discussed in detail. For each galaxy, we present emission-line ratios for the nucleus and the extended gas at the most distant locations for which the fluxes of the weaker high excitation lines can be measured with an uncertainty ≤30%. In a companion paper (Paper II), these line ratios are used in a reevaluation of photoionization models of extended gas in active galaxies.