Small-scale clumping of dark matter and the mean free path of ionizing photons at z = 6

Abstract Recently, the mean free path of ionizing photons in z = 6 intergalactic medium (IGM) was measured to be very short, presenting a challenge existing reionization models. At face value, measurement can interpreted as evidence that IGM clumps on scales M ≲ 10 8 ⊙ , key but largely untested prediction cold dark matter (CDM) paradigm. Motivated by this possibility, we study role underlying cosmology plays setting > 5 path. We use two classes models contrast against standard CDM prediction: (1) thermal relic warm (WDM), representing with suppressed small-scale power; (2) an ultralight axion exhibiting white noise-like power enhancement. Differences between WDM and are subdued pressure smoothing possible contribution neutral islands opacity. For example, comparing late scenarios fixed volume-weighted fraction 20% at 6, is 19 (45)% longer model m x 3 keV. The enhanced axion-like produces better agreement short 6. However, drawing robust conclusions about hampered large uncertainties process, extragalactic background, history Universe. This work highlights some open questions opacity during reionization.