Study of Warm Dark Matter with the Lyman-α Forest

Quasar light becomes absorbed by the Lyman-α line of neutral hydrogen in “clouds” along the line-of-sight in the highly ionized intergalactic medium after re-ionization. The received flux spectrum is known as the “Lyman-α forest”. Limits on the dark matter “standard thermal relic mass” have been obtained in the literature from the power spectrum of the Lyman-α forest, e.g. $m_{\text{th}}>5.7$ keV. These limits are in tension with four independent estimates of the dark matter warmness: $0.6？m_{\text{th}}？1.6$ keV. To try to understand this tension we attempt to measure the dark matter warmness by two methods: the power spectrum of the Lyman-α forest, and the width of individual absorption lines, e.g. “Lyman-α trees”. We obtain a measurement of the dark matter warmness with the Lyman-α forest power spectrum, in agreement with the four independent estimates. In conclusion, the warm dark matter free-streaming is clearly observed as a cut-off of the Lyman-α forest power spectrum.