The Search for Million Degree Gas Through The NVII Hyperfine Line

TL;DR

This study searches for million-degree gas in the universe using the NVII hyperfine line with the Green Bank Telescope, finding no definitive emission or absorption but discussing the potential for future observations to detect hot intergalactic gas.

Contribution

It provides the first observational constraints on NVII hyperfine line emission and absorption in hot gas, and evaluates the feasibility of future detections with longer observations.

Findings

No significant emission or absorption detected above 5 sigma.

The strongest emission candidate is inconsistent with X-ray limits.

The strongest absorption candidate exceeds model predictions, suggesting future potential.

Abstract

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 NVII ion is found in this material and it 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 NVII transmission. For absorption studies, 3C273, 3C 279, 3C 345, and 4C+39.25 were observed but no feature were seen above the 5 sigma 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 sigma. We examine whether the strongest emission feature, in Abell 2390 (3.7 sigma), and the strongest absorption feature, toward 4C+39.25 (3.8 sigma), might be expected from theoretical models. The emission feature would require ~1E10 Msolar of 1E6 K gas, which is inconsistent with X-ray limits for the O VII Kalpha line, so it is unlikely to be real. The NVII absorption feature requires a NVII column of 6E16 cm^-2, 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 NVII absorption line observations can begin to be useful in in the search for hot intergalactic gas.