A Vision for Little Red Dots in the Low-Redshift Universe with ALMA2040

The discovery of Little Red Dots (LRDs) at high redshifts with JWST has revealed a previously unknown phase of black-hole growth. However, their black-hole (BH) masses, BH-to-stellar mass ratios, cold-gas reservoirs, and depletion times remain uncertain. The recent discovery of low-redshift LRDs at z 0.1 - 0.9 creates a decisive opportunity to access their low- J CO lines for measuring molecular gas reservoirs, and the nearest systems can in principle be spatially resolved. We use J1025+1402 ("The Egg") at z=0.1007, the best-studied local LRD, as a benchmark to explore the scientific opportunities with ALMA2040. Assuming a molecular gas mass of 10 9,M, the CO(2-1) detectability strongly depends on the assumed CO-to-H 2 conversion (α CO). With a large α CO 315 - 725,M,(K,km,s-1, pc 2)-1 at a low gas-phase metallicity, the CO line would remain very challenging for the current ALMA to detect, while ALMA2040 will likely deliver a robust CO detection and thus a gas-reservoir constraint for The Egg. Furthermore, with the best angular resolution at its CO(2-1) frequency (14.6 mas, or 27 pc), ALMA2040 may spatially resolve the CO emission from The Egg. Assuming an optimistic, SMC-like α CO with a 50-hour deep integration, we produce mock position-velocity diagrams that may distinguish a 10 7,M black hole from the no-BH model through its steeper central velocity structure. We conclude that ALMA2040 has the potential to directly measure the gas masses, depletion times, and excitation of local LRDs, while the CO-bright subset may ultimately yield dynamical constraints on the mass of their central black holes.