A measurement of the Lyman-beta forest power spectrum and its cross with the Lyman-alpha forest in X-Shooter XQ-100

Abstract

The Ly-α forest is the large-scale structure probe for which we appear to have modeling control to the highest wavenumbers, which makes it of great interest for constraining the warmness/fuzziness of the dark matter and the timing of reionization processes. However, the standard statistic, the Ly-α forest power spectrum, is unable to strongly constrain the IGM temperature-density relation, and this inability further limits how well other high wavenumbersensitive parameters can be constrained. With the aim of breaking these degeneracies, we measure the power spectrum of the Ly-β forest and its cross correlation with the coeveal Ly-α forest using the one hundred spectra of z = 3.5−4.5 quasars in the VLT/X-Shooter XQ-100 Legacy Survey, motivated by the Ly-β transition’s smaller absorption cross section that makes it sensitive to somewhat higher densities relative to the Ly-α transition. Our inferences from this measurement for the IGM temperature-density relation appear to latch consistently onto the recent tight lowerredshift Ly-α forest constraints of Gaikwad et al. (2020). The z = 3.4− 4.7 trends we find using the Ly-α–Ly-β cross correlation show a flattening of the slope of the temperature-density relation with decreasing redshift. This is the trend anticipated from ongoing He ii reionization and there being sufficient time to reach the asymptotic temperaturedensity slope after hydrogen reionization completes. Furthermore, our measurements provide a consistency check on IGM models that explain the Ly-α forest, with the cross correlation being immune to systematics that are uncorrelated between the two forests, such as metal line contamination.