A New Sample of $\sim$ 100 Intermediate-mass Black Holes Reaching $z \approx 1$

Abstract

We present a systematic search for intermediate-mass black hole (IMBH) active galactic nuclei (AGNs) at $0.5 < z \lesssim 1$ using DESI DR1 spectroscopy.We identify 98 broad-line IMBH AGNs with black hole masses $M_\mathrm{BH}<10^6$ $M_{\odot}$ through quantitative spectral decomposition and broad-H$β$ selection. This sample spans $M_\mathrm{BH}=10^{5.5}-10^{6.0}$ $M_{\odot}$ and Eddington ratios from 1.2 to 9.0 extending systematic IMBH AGN searches to intermediate redshift. Compared with a consistently selected $z<0.6$ IMBH sample, the $0.5<z<1$ sources reach higher broad-H$β$ luminosities and substantially higher Eddington ratios, indicating more extreme accretion states at earlier cosmic times. They also show broader [O III] profiles and stronger blueshifted wing components, with these kinematic differences persisting after matching in black hole mass and Eddington ratio. These results reveal two distinct signatures of evolution among IMBH AGNs at $z<1$: ability of IMBHs to reach increasingly extreme accretion states toward higher redshift, and systematic changes in their ionized-gas kinematics. The former demonstrates that rapid, including super-Eddington, growth of IMBHs can persist to relatively late cosmic times, while the latter may indicate evolution in the ionized-gas environment and associated outflow activity of actively growing IMBHs. Together, these findings provide new constraints on the evolutionary pathways of IMBHs at $z<1$, and show that seed-mass black holes can continue to undergo rapid growth well after the cosmic dawn.