The galaxy-electron cross-spectrum with ACT, SPT-3G and DESILS LRGs

The galaxy--electron cross-power spectrum P ge(k) measures the small-scale distribution of gas around galaxies, the quantity reshaped by feedback. We measure P ge from the kinetic Sunyaev--Zel'dovich (kSZ) signal of the photometric DESI Legacy Imaging Surveys luminous red galaxies, cross-correlating a velocity-weighted galaxy momentum template with the CMB temperature from ACT DR6 and, for the first time, SPT-3G. The absolute normalization of the cross-spectrum is fixed by a surrogate-field Monte Carlo that propagates the survey mask, lightcone evolution, and photometric-redshift weights, converting the measured pseudo-spectrum into P ge(k) in physical units. We detect P ge at 11.7σ with ACT and 8.4σ with SPT-3G, and at 12.3σ combining SPT-3G with the ACT northern cap, which it does not overlap. The two surveys have independent optics, beams, filtering and noise and share about two thirds of the SPT-3G galaxies; their amplitudes agree channel by channel, the largest difference being 0.7σ. All 26 null tests on the adopted night maps are consistent with zero signal, among them the six ACT frequency differences, the three SPT-3G frequency differences, and the four ACT cap differences of the main selection. Turning the amplitude into a gas fraction requires the host halo masses of the sample, which the cross-spectrum does not itself measure; taking them from its measured number density and from a published CMB-lensing calibration of the same galaxies, the gas within the virial radius is 25 -- 33% of the cosmic baryon budget of the halo.