2007ApJ...666..139B

Gas in the million degree range occurs in a variety of astronomical environments, and it may be the main component of the elusive missing baryons at low redshift. The N VII ion is found in this material and has a hyperfine spin-flip transition with a rest frequency of 53.042 GHz, which can be observed for z>0.1, when it is shifted into a suitably transparent radio band. We used the 42-48 GHz spectrometer on the Green Bank Telescope to search for both emission and absorption from this N VII transmission. For absorption studies, 3C 273, 3C 279, 3C 345, and 4C +39.25 were observed, but no features were seen above the 5 σ level. For emission line studies, we observed Abell 1835, Abell 2390, and the star-forming galaxy PKS 1345+12, but no features were seen exceeding 5 σ. We examine whether the strongest emission feature, in Abell 2390 (3.7 σ), and the strongest absorption feature, toward 4C +39.25 (3.8 σ), might be expected from theoretical models. The emission feature would require ∼10¹⁰ M_☉ of 10⁶ K gas, which is inconsistent with X-ray limits for the O VII Kα line, so it is unlikely to be real. The N VII absorption feature requires a N VII column of 6x10¹⁶/cm2, higher than model predictions by at least an order of magnitude, which makes it inconsistent with model expectations. The individual observations were less than 1 hr in length, so for lengthy observations, we show that N VII absorption line observations can begin to be useful in the search for hot intergalactic gas.