A measurement of small-scale structure in the 2.2 ≤ z ≤ 4.2 lyα forest

The amplitude of fluctuations in the Lyα forest on small spatial scales is sensitive to the temperature of the intergalactic medium (IGM) and its spatial fluctuations. The temperature of the IGM and its spatial variations contain important information about hydrogen and helium reionization. We present a new measurement of the small-scale structure in the Lyα forest from 40 high resolution, high signal-to-noise ratio, VLT spectra for absorbing gas at redshifts between 2.2 ≤ z ≤ 4.2. We convolve each Lyα forest spectrum with a suitably chosen Morlet wavelet filter, which allows us to extract the amount of small-scale structure in the forest as a function of position across each spectrum. We monitor contamination from metal line absorbers. We present a first comparison of these measurements with high-resolution hydrodynamic simulations of the Lyα forest that track more than 2 billion particles. This comparison suggests that the IGM temperature close to the cosmic mean density (T₀) peaks at a redshift near z = 3.4, at which point it is greater than 20,000 K at ≳2σ confidence. The temperature at lower redshift is consistent with the fall-off expected from adiabatic cooling (T₀ ∝ (1 + z)²), after the peak temperature is reached near z = 3.4. In our highest redshift bin, centered around z = 4.2, the results favor a temperature of T₀ = 15-20,000 K. However, owing mostly to uncertainties in the mean transmitted flux at this redshift, a cooler IGM model with T₀ = 10,000 K is only disfavored at the 2σ level here, although such cool IGM models are strongly discrepant with the z ≈ 3-3.4 measurement. We do not detect large spatial fluctuations in the IGM temperature at any redshift covered by our data set. The simplest interpretation of our measurements is that He ii reionization completes sometime near z ≈ 3.4, although statistical uncertainties are still large. Our method can be fruitfully combined with future He ii Lyα forest measurements.