A Sample of Active Galactic Nuclei with Intermediate-mass Black Holes Extended to z ≈ 0.6

Abstract We present a sample of 930 intermediate-mass black hole (IMBH) active galactic nuclei (AGNs) with BH masses of M BH ≤ 2 × 10 6 M ⊙ , uniformly selected from the Seventeenth Data Release of the Sloan Digital Sky Survey, based on the detection of broad H α or H β emission lines. Taking advantage of the wide wavelength coverage of BOSS/eBOSS spectroscopy, our sample extends the redshift coverage of low- z IMBH AGNs to z ≤ 0.57, significantly improving upon previous studies that were generally limited to z ≤ 0.35. This sample encompasses BH masses from 10 4.0 to 10 6.3 M ⊙ , with Eddington ratios ranging from 0.01 to 1.9. Among the z > 0.3 subset, 24 sources exhibit detectable broad Mg ii λλ 2796, 2803 emission lines, including eight confirmed by independent DESI spectra. A preliminary analysis reveals a marked decline in both the maximum accretion rate ( L bol / L Edd ) and broad H α luminosity with decreasing redshift, possibly reflecting a cosmic evolution of accretion activity at the low-mass end, akin to the “downsizing” evolutionary trend seen in high-mass AGNs.