Highly Ionized Gas in the Zone of Avoidance: Using the CIViL ^⋆ Survey to Pinpoint Physical Conditions of Gaseous Halos

Abstract This paper investigates the physical conditions of the circumgalactic medium of L ⋆ galaxies through explorations of observed ion tracer gas kinematics and comparisons of observations to different ionization models. For this analysis, we utilize C iv observations from the CIViL ⋆ survey (∼0.14 ≤ z gal ≤ 0.25) and directly compare them to observations of matched lines of sight from the Cosmic Origins Spectrograph-Halos survey. We find that the kinematic parameters for C iv and O vi are likely (>95%) drawn from the same parent distribution, suggesting that these two ions are kinematically coincident and potentially originate under the same physical conditions. We find that the measured C iv /O vi and N v /O vi ratios are inconsistent with single-phase equilibrium models. For 70% of the objects in our sample, regions allowed by the column density ratios in the density-temperature space do not overlap, creating a “zone of avoidance.” We also investigate the origins of C iv , N v , and O vi by exploring a cooling flow model under collisional ionization. We find that both N v and O vi are consistent with the predictions of the model, but the column densities of C iv are ∼2.5 times higher than the predictions. As C iv has a lower ionization energy than N v and O vi , it is possible that C iv has contributions from both the warm/hot and cool photoionized phase.