Dot to dot: high-$z$ little red dots in $M_{\rm bh}$-$M_{\rm \star}$ diagrams with galaxy-morphology-specific scaling relations

Alister W. Graham, Igor V. Chilingarian, Dieu D. Nguyen, Roberto Soria, Mark Durre, Duncan A. Forbes

First listed 2025-03-13 | Last updated 2025-05-06

Abstract

The high redshift 'little red dots' (LRDs) detected with the James Webb Space Telescope are considered to be the cores of emerging galaxies that host active galactic nuclei (AGN). For the first time, we compare LRDs with local compact stellar systems and an array of galaxy-morphology-dependent stellar mass-black hole mass scaling relations in the $M_{\rm bh}$-$M_{\star}$ diagrams. When considering the 2023-2024 masses for LRDs, they are not equivalent to nuclear star clusters (NSCs), with the latter having higher $M_{\rm bh}/M_{\star}$ ratios. However, the least massive LRDs exhibit similar $M_{\rm bh}$ and $M_{\rm \star,gal}$ values as ultracompact dwarf (UCD) galaxies, believed to be the cores of stripped/threshed galaxies. We show that the LRDs span the $M_{\rm bh}$-$M_{\rm \star,gal}$ diagram from UCD galaxies to primaeval lenticular galaxies. In contrast, local spiral galaxies and the subset of major-merger-built early-type galaxies define $M_{\rm bh}$-$M_{\star,gal}$ relations that are offset to higher stellar masses. Based on the emerging 2025 masses for LRDs, they may yet have similarities with NSCs, UCD galaxies, and green peas. Irrespective of this developing situation, we additionally observe that low-redshift galaxies with AGN align with the quasi-quadratic or steeper black hole scaling relations defined by local disc galaxies with directly measured black hole masses. This highlights the benefits of considering a galaxy's morphology - which reflects its accretion and merger history - to understand the coevolution of galaxies and their black holes. Future studies of spatially-resolved galaxies with secure masses at intermediate-to-high redshift hold the promise of detecting the emergence and evolution of the galaxy-morphology-dependent $M_{\rm bh}$-$M_{\star}$ relations.

Short digest

Places high‑z little red dots onto morphology‑aware Mbh–M★ diagrams alongside compact stellar systems and local galaxy sequences. With 2023–2024 masses, LRDs are not NSCs (NSCs have higher Mbh/M★); the least‑massive LRDs overlap UCD‑like Mbh and M★, and the full LRD set spans from UCDs to primaeval S0s, while spirals and merger‑built ETGs sit at higher M★. Low‑z AGN align with the quasi‑quadratic/steeper relations defined by local disc galaxies with direct black‑hole masses, emphasizing morphology in coevolution. As 2025 mass updates arrive, placements may shift, potentially bringing LRDs closer to NSCs/UCDs/green‑peas.

Key figures to inspect

• Figure 1: Inspect where individual LRDs fall relative to the NSC and UCD‑inner tracks in Mbh–M★, noting the stated assumption that LRD total M★ is plotted as if it were a spheroid; verify that NSCs sit at higher Mbh/M★ than LRDs.
• Figure 2: Using total galaxy M★ (inner+outer UCD components), check that LRDs span from UCDs toward primaeval S0s and that spiral and merger‑built ETG relations are offset to higher M★ at fixed Mbh.
• Figure 3: With Reines & Volonteri (2015), Chilingarian et al. (2018), and Izumi et al. (2021) added, confirm that low‑z AGN populate the same steep quasi‑quadratic/cubic relations as local discs rather than forming an offset cloud.
• Figure 1 (legend overlays): Examine the plotted green‑pea and ancillary samples (cyan squares/grey AGN) to see whether they bridge the locus between LRDs and local sequences, anticipating possible re‑placement of LRDs with 2025 mass revisions.