O VII Absorption Columns as a Baryon Scale: Hydrogen Columns and the Warm-Hot Share Implied by Metal-Line Absorbers

Highly ionised oxygen absorption against bright background sources is the most direct spectroscopic probe of the warm-hot intergalactic medium, and also the most assumption-laden: an oxygen column becomes a baryon column only after division by the oxygen abundance, the metallicity, and the fraction of oxygen in the observed ion. This paper works that conversion for a stated system — an O VII column of 5.00e+15 per square centimetre, metallicity one fifth solar, and an O VII ion fraction of one half — obtaining a hydrogen column of 1.02e+20 per square centimetre and a baryon surface density of 1.08e-04 solar masses per square parsec. Combining the column with an adopted incidence of eight such systems per unit redshift gives a warm-hot density parameter of 0.01065, or 0.216 of all baryons. The sensitivities are the story: at metallicity one tenth solar the share doubles to 0.432, and halving the ion fraction has the same effect, so the absorption census currently brackets the warm-hot share between roughly one seventh and two fifths of the baryons from these two factors alone.