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

Abstract

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 $z\simeq0.1 - 0.9$ 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 $10^9\,M_\odot$, the CO(2-1) detectability strongly depends on the assumed CO-to-H$_2$ conversion ($α_{\rm CO}$). With a large $α_{\rm CO} \simeq 315 - 725\,M_\odot\,({\rm K\,km\,s}^{-1}\,{\rm pc}^2)^{-1}$ 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 $α_{\rm CO}$ with a 50-hour deep integration, we produce mock position-velocity diagrams that may distinguish a $10^7\,M_\odot$ 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.