Galaxy-SMBH co-evolution in 2,435 DESI disk galaxies and merger-free BH growth in bulgeless disks

We analyse black hole scaling relations for disk-dominated galaxies, and present evidence for secularly driven galaxy-black hole co-evolution. We do this observationally by looking at ``bulgeless'' galaxies, where the lack of a stellar bulge suggests a quiet merger history. We first compose a sample of 2,435 disk-dominated galaxies hosting optical broad line active galactic nuclei (AGN) from DESI data, and find a M BH -- M* scaling relation of (M BH)= - 3.7+0.3-0.3 + 1.00+0.03-0.03 (M*). From these, we identify 546 bulgeless galaxies hosting AGN, a factor of 5 larger than pre-existing samples, and find they follow the same M BH -- M* relation as the general disk galaxy sample. Comparing the bulgeless galaxies to those with some bulge component, we find good agreement in the M BH -- M* relation (within 3 σ), but disagreement in the M BH -- M*, bulge relation (at >7 σ). This is the opposite of what is expected from merger-driven co-evolution, suggesting secular growth is significant in establishing observed M BH -- M* scaling relations. Furthermore, in a control sample of early-type galaxies we find tentative evidence of increased AGN luminosity compared to the bulgeless sample, but no statistical difference in black hole mass. This suggests the assumed merger-dominated evolutionary history of early-type galaxies does not lead to significantly increased long-term BH growth. These results support numerous recent observational and theoretical studies which suggest secular processes fuel significant supermassive black hole growth and galaxy-black hole co-evolution.