Highly Ionized HVCs Towards Pks 2155–304: Galactic or Extragalactic?

Abstract

To gain insight into the nature of highly ionized high-velocity clouds (HVCs), we have investigated two such objects using HST-STIS and FUSE data for the PKS 2155−304 sight line (Collins et al. 2003). The presence of low ion stages is suggestive of a Galactic halo location for these HVCs. The highly-ionized HVCs detected in O VI and C IV absorption may represent low column density analogs to the Galactic HVCs detected in HI emission. It has also been proposed that these O VI HVCs are extragalactic and may trace low-density shock-heated remnants from the formation of the Local Group, the so-called warm-hot intergalactic medium (WHIM). We measured column densities for metal ion species ranging in ionization state from C II/III/IV and Si II/III/IV to O VI for two HVC components. One of the components in particular is seen in both low and high ions and exhibits a multi-component structure. The presence of low ions implies that the cloud density is too high to represent the diffuse WHIM. Single-parameter photoionization models cannot explain the observed ionization pattern. If one instead assumes that O VI arises from collisional ionization, one can explain the lower ions with an extragalactic (AGN) radiation field, but with low gas pressures, P/k ≈ 1−10 cm−3 K, implying an extragalactic location for the HVCs. Similarly low pressures arise in models of the PG 1259+593 sight line through HVC Complex C, when constrained by C IV and Si IV. Since Complex C is believed to lie in the Galactic halo, with nH ∼ 0.01− 0.1 cm−3, we believe these models are over-simplified and misleading. Instead, if the photoionization models are constrained by H I and the singly-ionized species, significantly larger gas pressures are found, and a Galactic halo association for the HVCs would be likely.