Papers › On the general nature of 21cm-Lyman-α emitters cross-correlations during reionisation
On the general nature of 21cm-Lyman-α emitters cross-correlations during reionisation
Anne Hutter, Caroline Heneka, Pratika Dayal, Stefan Gottlöber, Andrei Mesinger, Maxime Trebitsch, Gustavo Yepes
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We explore how the characteristics of the cross-correlation functions between the 21cm emission from the spin-flip transition of neutral hydrogen (HI) and early Lyman-α (Lyα) radiation emitting galaxies (Lyα emitters, LAEs) depend on the reionisation history and topology and the simulated volume. For this purpose, we develop an analytic expression for the 21cm-LAE cross-correlation function and compare it to results derived from different Astraeus and 21cmFAST reionisation simulations covering a physically plausible range of scenarios where either low-mass (<10^(9.5)M_⊙) or massive (>10^(9.5)M_⊙) galaxies drive reionisation. Our key findings are: (i) the negative small-scale (<2 cMpc) cross-correlation amplitude scales with the intergalactic medium's (IGM) average HI fraction (⟨χ_(HI)⟩) and spin-temperature weighted overdensity in neutral regions (⟨1+δ⟩_(HI)); (ii) the inversion point of the cross-correlation function traces the peak of the size distribution of ionised regions around LAEs; (iii) the cross-correlation amplitude at small scales is sensitive to the reionisation topology, with its anti-correlation or correlation decreasing the stronger the ionising emissivity of the underlying galaxy population is correlated to the cosmic web gas distribution (i.e. the more low-mass galaxies drive reionisation); (iv) the required simulation volume to not underpredict the 21cm-LAE anti-correlation amplitude when the cross-correlation is derived via the cross-power spectrum rises as the size of ionised regions and their variance increases. Our analytic expression can serve two purposes: to test whether simulation volumes are sufficiently large, and to act as a fitting function when cross-correlating future 21cm signal Square Kilometre Array and LAE galaxy observations.
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