Interstellar and Intergalactic Medium

The warm-hot intergalactic medium (WHIM) is the hottest portion of the intergalactic medium; its temperature 105 K < T < 107 K is the result of shock heating as gas flows along the filaments of the cosmic web. Numerical simulations indicate that the WHIM is only now overtaking the cooler DIM as the more massive component of intergalactic gas. The WHIM is difficult to detect – to the point where astronomers long complained of a “missing baryon problem.” However, the cooler portions of the WHIM can be detected by looking for absorption lines of O vi along lines of sight to bright quasars. The portion of the WHIM at T ∼ 106 K can be detected from absorption lines of O vii. The very hottest portion of the WHIM, it is hoped, will be detected from absorption lines of iron, which still clings to its innermost electrons at T ∼ 107 K.