Dot to dot: high-z little red dots in M_bh–M_⋆ diagrams with galaxy-morphology-specific scaling relations

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_bh–M_⋆ diagrams. When considering the 2023-2024 masses for LRDs, they are not equivalent to nuclear star clusters (NSCs), with the latter having higher M_bh–M_⋆ ratios. However, the least massive LRDs exhibit similar M_bh and M_⋆,_gal values as ultracompact dwarf (UCD) galaxies, believed to be the cores of stripped/threshed galaxies. We show that the LRDs span the M_bh–M_⋆,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_bh–M_⋆,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_bh–M_⋆ relations.