Study of Warm Dark Matter with the Lyman- α Forest

Abstract

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 th > 5.7 keV. These limits are in tension with four independent estimates of the dark matter warmness: 0.6 ≲ m 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.