Tripling the Census of Dwarf AGN Candidates Using DESI Early Data

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Abstract Using early data from the Dark Energy Spectroscopic Instrument (DESI) survey, we search for active galactic nuclei (AGN) signatures in 410,757 line-emitting galaxies. By employing the BPT emission-line ratio diagnostic diagram, we identify AGNs in 75,928/296,261 (≈25.6%) high-mass ( log ( M ⋆ / M ⊙ ) > 9.5) and 2444/114,496 (≈2.1%) dwarf ( log ( M ⋆ / M ⊙ ) ≤ 9.5) galaxies. Of these AGN candidates, 4181 sources exhibit a broad H α component, allowing us to estimate their BH masses via virial techniques. This study more than triples the census of dwarf AGNs and doubles the number of intermediate-mass black hole ( M BH ≤ 10 6 M ⊙ ) candidates, spanning a broad discovery space in stellar mass (7 < log ( M ⋆ / M ⊙ ) < 12) and redshift (0.001 < z < 0.45). The observed AGN fraction in dwarf galaxies (≈2.1%) is nearly four times higher than prior estimates, primarily due to DESI’s smaller fiber size, which enables the detection of lower-luminosity dwarf AGN candidates. We also extend the M BH – M ⋆ scaling relation down to log ( M ⋆ / M ⊙ ) ≈ 8.5 and log ( M BH / M ⊙ ) ≈ 4.4, with our results aligning well with previous low-redshift studies. The large statistical sample of dwarf AGN candidates from current and future DESI releases will be invaluable for enhancing our understanding of galaxy evolution at the low-mass end of the galaxy mass function.

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