Does the Lockstep Growth between Black Holes and Bulges Create Their Mass Relation?

Recent studies have revealed a strong relation between the sample-averaged black hole (BH) accretion rate (BHAR) and star formation rate (SFR) among bulge-dominated galaxies—i.e., “lockstep” BH-bulge growth—in the distant universe. This relation might be closely connected to the BH-bulge mass correlation observed in the local universe. To further understand BH-bulge coevolution, we present Atacama Large Millimeter/submillimeter Array (ALMA) CO(2-1) or CO(3-2) observations of seven star-forming bulge-dominated galaxies at z = 0.5-2.5. Using the ALMA data, we detect significant (>3σ) CO emission from four objects. For our sample of seven galaxies, we measure (or constrain with upper limits) their CO line fluxes and estimate their molecular gas masses (M _gas). We also estimate their stellar masses (M _star) and SFRs, by modeling their spectral energy distributions. Using these physical properties, we derive the gas depletion timescales (τ _dep ≡ M _gas/SFR) and compare them with the bulge/BH growth timescales (τ _grow ≡ M _star/SFR ∼ M _BH/BHAR). Our sample generally has τ _dep shorter than τ _grow by a median factor of ≳4, indicating that the cold gas will be depleted before significant bulge/BH growth takes place. This result suggests that BH-bulge lockstep growth is mainly responsible for maintaining the mass relation, not creating it. We note that our sample is small and limited to z < 2.5; JWST and ALMA will be able to probe to higher redshifts in the near future.