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, $x_{\mathrm{H}\mathrm{I}}(z),$ and mean overdensity, $⟨1+{\delta }_{\mathrm{H}\mathrm{I}}⟩(z)$, exploring dependence on the field of view; redshift precision, ${\sigma }_z$; and minimum halo mass, $M_{\mathrm{h},\mathrm{m}\mathrm{i}\mathrm{n}}$. We employed our simulation-based inference framework EoRFlow 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 ($\mathrm{F}\mathrm{O}\mathrm{V}=100{\mathrm{d}\mathrm{e}\mathrm{g}}^2$, ${\sigma }_z=0.001$, $M_{\mathrm{h},\mathrm{m}\mathrm{i}\mathrm{n}}={10}^{11}{\mathrm{M}}_{\odot }$), cross-power spectra yield unbiased constraints with posterior volumes (PVs) of $\sim$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 $f_{\mathrm{e}\mathrm{s}\mathrm{c},}$ and the star formation efficiency $f_{\ast }$, which remain degenerate in auto-power (PV >60%). Tight constraints require either deep surveys detecting faint galaxies ($M_{\mathrm{h},\mathrm{m}\mathrm{i}\mathrm{n}}\sim {10}^{10}{\mathrm{M}}_{\odot }$) with moderate foregrounds (PV~11%) or conservative mass limits with optimistic foreground removal (PV~19%). 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.