Oviiand OviiiAbsorption by Hot Gas in the Vicinity of the Galaxy

Abstract

We searched for evidence of soft X-ray absorption by hot gas in the vicinity of the Galaxy in a small sample of 15 type I active galactic nuclei (AGNs) observed with the high resolution X-ray gratings on board Chandra. We find that around half of the sight lines in our sample exhibit absorption due to local H- or He-like oxygen (or both) at confidence levels ranging from >90% to >3 σ. Depending on the sight line, the absorption can be identified with hot gas in particular local structures, the Local Group (LG), or the putative local hot intergalactic medium (IGM). Several sight lines in our sample coincide with sight lines in a study of O VI absorption by local gas, so an assumption of collisional ionization equilibrium (CIE) allows us to constrain the temperature of the local hot gas. In the southern Galactic hemisphere, we identify absorption along the sight line to Fairall 9 with the Magellanic Stream, and this gas has 105.75 < T < 106.35 K if CIE applies. There may be a blend of O VII absorption features along the sight line toward Mrk 509. LG clouds along this sight line may interact with a hot Galactic corona or a low-density LG medium (or the IGM). The sight line to 3C 120 does not appear to be associated with either local structure or UV-absorbing gas and may correspond to the local hot IGM. Gas in the Magellanic Stream extension along the sight line to Ark 564 has T < 106.1 K if it is in CIE. We show that hot absorbing outflows apparently detected in the spectra of NGC 4051, PDS 456, and PG 1211+143 could actually correspond to absorption by hot, local gas since the outflow velocity from each of these AGNs coincides with the respective cosmological recession velocity of the AGN. Our work implies that apparent mildly relativistic outflows from some QSOs are more likely to correspond to signatures of local, hot gas.