Overmassive Black holes live in compact galaxies in the early Universe

Yuxuan Wu, Tao Wang, Daizhong Liu, Qinghua Tan, Luis C. Ho, Zhiyu Zhang, Yong Shi, Ke Xu, Kotaro Kohno, Ran Wang, Takuma Izumi, Zhaozhou Li

First listed 2025-06-17 | Last updated 2025-06-25

Abstract

A significant population of quasars have been found to exist within the first Gyr of cosmic time. Most of them have high black hole (BH) masses ($M_{\rm BH} \sim 10^{8-10} M_{\odot}$) with an elevated BH-to-stellar mass ratio compared to typical local galaxies, posing challenges to our understanding of the formation of supermassive BHs and their coevolution with host galaxies. Here, based on size measurements of [CII] 158$μ$m emission for a statistical sample of $z \sim 6$ quasars, we find that their host galaxies are systematically more compact (with half-light radius $R_{\rm e} \sim 1.6$ kpc) than typical star-forming galaxies at the same redshifts. Specifically, the sizes of the most compact quasar hosts, which also tend to contain less cold gas than their more extended counterparts, are comparable to that of massive quiescent galaxies at $z \sim 4-5$. These findings reveal an intimate connection between the formation of massive BHs and compactness of their host galaxies in the early universe. These compact quasar hosts are promising progenitors of the first population of quiescent galaxies.

Short digest

Uniform ALMA [CII] 158 μm size measurements for a curated sample of 22 z≈6 quasars show their hosts are systematically compact, with a median Re≈1.58 kpc versus 2.26 kpc for main-sequence SFGs measured in the same line. The hosts also have elevated MBH/M* and, for the most compact systems, reduced cold-gas content and sizes akin to massive quiescent galaxies at z≈4–5. Simple evolution over a gas-depletion timescale moves them toward the local MBH–M* relation as stellar mass builds. The compact, gas-poor subset likely marks the late stage of rapid BH growth and are prime progenitors of the first quiescent galaxies.

Key figures to inspect

• Figure 1 (MBH–M*): Check how far individual quasars sit above constant MBH/M* tracks and identify objects with M* upper limits (grey arrows), confirming the prevalence of overmassive BHs in the sample.
• Figure 2 (compactness comparisons): In the Re–M* plane, quantify the offset below the ALPINE SFG size relation; in the Σ*–MBH/M* panel, note the overlap with local galaxies hosting overmassive BHs and the position relative to the feedback-ineffective threshold line.
• Figure 3 (evolution over tdep): Left panel arrows show predicted motion in MBH/M* after one depletion timescale—inspect which sources exhibit galaxy- versus BH-dominated growth and the overall drift toward the local relation. Right panel contrasts starburstiness and sizes with quiescent galaxies at z≈4–5; verify that…
• Figure 4 (schematic pathway): Follow the proposed route from compact, overmassive-BH hosts to quiescence and compare the marked region for Little Red Dots to assess whether similarly compact, overmassive systems would track the same evolutionary path.