Long troughs in the Lyman-$\alpha$ forest below redshift 6 due to islands of neutral hydrogen

Understand

A long (110 cMpc/$h$) and deep absorption trough in the Ly$\alpha$ forest has been observed extending down to redshift 5.5 in the spectrum of ULAS J0148+0600.

• Although no Ly$\alpha$ transmission is detected, Ly$\beta$ spikes are present which has led to claims that the gas along this trough must be ionized.
• Using high resolution cosmological radiative transfer simulations in large volumes, we show that in a scenario where reionization ends late ($z \sim 5.2$), our simulations can reproduce troughs as long as observed.
• In this model, we find that the troughs are caused by islands of neutral hydrogen.