Constraining reionization morphology and source properties with 21cm galaxy cross-correlation surveys

Cross-correlations between 21cm observations and galaxy surveys provide a powerful probe of reionization by providing robustness against foreground contamination while linking ionization morphology to galaxies. We quantified the constraining power of 21cm galaxy cross-power spectra for inferring the neutral hydrogen fraction, x_ (z), and mean overdensity, łangle 1+δ_ ̊angle (z), exploring dependence on the field of view; redshift precision, σᵦ; and minimum halo mass, M_ for likelihood-free parameter estimation. Mock observations include thermal noise for 100h of SKA-Low with foreground avoidance and realistic galaxy-survey effects. For a fiducial survey (mathrm HI HI We employed our simulation-based inference framework EoRFlow FOV =100, ², σᵦ=0. 001, M_ =10^ deg h, min 11 M _⊙), cross-power spectra yield unbiased constraints with posterior volumes (PVs) of ∼10% relative to priors. Cross-power measurements reduce the PV by 20--30% versus 21cm auto-power alone. With foreground avoidance, spectroscopic redshift precision is essential; photometric redshifts render cross-correlations uninformative. Notably, cross-power spectra constrain ionizing source properties, the escape fraction f_ _⊙) with moderate foregrounds (mathrm and the star formation efficiency f_*, which remain degenerate in auto-power (PV >60%). Tight constraints require either deep surveys detecting faint galaxies (M_ ∼ 10^ h, min 10 M PV or conservative mass limits with optimistic foreground removal (mathrm PV 21cm galaxy cross-correlations enhance morphology constraints beyond auto-power while enabling previously inaccessible source property constraints. Realizing full potential requires precise redshifts and either faint galaxy detection limits or improved 21cm foreground cleaning.