Tripling the Census of Dwarf AGN Candidates Using DESI Early Data

Abstract

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 Hα 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 ≤ 106 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.