Oxygen absorption in M87: Evidence for a warm plus hot interstellar medium

We present a reanalysis of the ROSAT PSPC data within the central 100 kpc o f M87 to search for intrinsic oxygen absorption similar to that recently me asured in several galaxies and groups. Since M87 is the brightest nearby ga laxy or cluster possessing an average temperature (similar to 2 keV) within the PSPC bandpass, it is the ideal target for this study. Using a spatial- spectral deprojection analysis we find the strongest evidence to date for i ntrinsic oxygen absorption in the hot gas of a galaxy, group, or cluster. S ingle-phase plasma models modified by intervening Galactic absorption canno t fit the 0.2-2.2 keV PSPC data as they underpredict the 0.2-0.4 keV region and overpredict the 0.5-0.8 keV region where the emission and absorption r esiduals are obvious upon visual inspection of the spectral fits. These abs orption and emission features are significant out to the largest radii inve stigated. Since the excess emission between 0.2-0.4 keV rules out intrinsic absorption from cold gas or dust, the most reasonable model for the excess emission and absorption features is warm, collisionally ionized gas with a temperature of similar to 10(6) K. Simple multiphase models (cooling flow, two phases) modified by both intervening Galactic absorption and by a sing le oxygen edge provide good fits and yield temperatures and Fe abundances o f the hot gas that agree with previous determinations by ASCA and SAX. The multiphase models of M87 inferred from the PSPC can account for the excess EUV emission observed with EUVE and the excess X-ray absorption inferred fr om Einstein and ASCA data above 0.5 keV. This evidence for a multiphase war m + hot interstellar medium in M87 essentially confirms the original detect ion by Canizares et al. within the central similar to 2' using the Einstein FPCS. Although the total mass of the warm gas implied by the oxygen absorp tion is consistent with the matter deposited by a cooling flow, the suppres sion of the mass deposition rate and the distortion of the X-ray isophotes in the region where the radio emission is most pronounced suggest some feed back effect from the active galactic nucleus on the cooling gas.