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

Abstract

We present a sample of 930 intermediate-mass black hole (IMBH) active galactic nuclei (AGNs) with black hole masses of $M_{\mathrm{BH}}⩽2\times 10^6$ M${}_{\odot }$, uniformly selected from the Seventeenth Data Release of the Sloan Digital Sky Survey, based on the detection of broad H$\alpha$ or H$\beta$ 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 where generally limited to $z⩽0.35$. This sample encompasses black hole masses from $10^{4.0}$ to $10^{6.3}$ M${}_{\odot }$, with Eddington ratios ranging from 0.01 to 1.9. Among the $z>0.3$ subset, 24 sources exhibit detectable broad Mg II $\lambda \lambda 2796,2803$ emission lines, including eight confirmed by independent DESI spectra. A preliminary analysis reveals a marked decline in both the maximum accretion rate ($L/L_{\mathrm{Edd}}$) and broad H$\alpha$ 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.

Accepted for publication in ApJS