The broadening of Lyman-α forest absorption lines

We provide an analytical description of the line broadening of H I absorbers in the Lyman- forest resulting from Doppler broadening and Jeans smoothing. We demonstrate that our relation captures the dependence of the line width on column density for narrow lines in z 3 mock spectra remarkably well. Broad lines at a given column density arise when the underlying density structure is more complex, and such clustering is not captured by our model. Our understanding of the line broadening opens the way to a new method to characterize the thermal state of the intergalactic medium and to determine the sizes of the absorbing structures.