UC Berkeley

Using the first spectroscopic data release from the Dark Energy Spectroscopic Instrument (DESI DR1), we conduct the largest systematic search for active galactic nuclei (AGN) signatures in ≈1.7 million low-redshift (0.001 ≤ z ≤ 0.45) line-emitting galaxies. Based on the [N ii]/Hα versus [O iii]/Hβ emission-line ratio diagnostic, we identify AGN in 314,245/1,211,573 (25.9%) high-mass ( log(M⋆/M⊙)>9.5 ) and 9648/467,214 (2.1%) dwarf ( log(M⋆/M⊙)≤9.5 ) galaxies. The AGN fraction in line-emitting galaxies increases monotonically with stellar mass, from ∼1.4% at the low-mass end to ∼93.3% at the high-mass end. Among these AGN, 17,949 exhibit broad Hα emission (BL-AGN), which we use to estimate their black hole (BH) masses via single-epoch virial methods. Leveraging this large BL-AGN sample, we extend the MBH − M⋆ scaling relation down to log(M⋆/M⊙)≈7.8 and log(MBH/M⊙)≈4.4 . We find a bimodal distribution of galaxies in this space, characterized by two distinct empirical relations. The relation for galaxies with high stellar and BH masses extrapolates to overlap with the high-redshift overmassive BH candidates. Our results suggest at least two broad evolutionary pathways for these overmassive BHs and their host galaxies across cosmic time. With this paper, we release the EmFit value-added catalog, containing emission-line flux and width measurements for ∼7.4 million galaxies, the largest catalog with emission-line decomposition into narrow, broad, and outflow components to date. This work significantly expands upon the early DESI results and provides a statistical sample for probing the galaxy−BH connection in the low-mass galaxy regime.