SMBHs in dwarf galaxies: JWST at z=0.7

Supermassive Black Holes in Dwarf Galaxies: JWST Discoveries at z≈0.7

The James Webb Space Telescope has uncovered supermassive black holes (SMBHs) in the dwarf galaxies Pelias and Neleus at redshifts of z~0.71 and z~0.75, with masses reaching up to 60% of the galaxies' stellar mass. This challenges the local M_BH/M_gal ratios of ~0.1–0.5%, typical for nearby galaxies. NIRISS/NIRSpec spectral data reveal young stellar populations with low dust content, but MIRI detects an excess in the mid-infrared range, indicating a dusty active galactic nucleus (AGN).

Spectral Features and Contradictions

The galaxies exhibit a blue UV-optical spectrum in the rest frame, characteristic of low-mass stellar systems. However, MIRI photometry shows a steep rise in the near- and mid-infrared, unexplained by starlight or dust heating from star formation. The authors attribute this to hot dust around an active nucleus, absorbing UV/optical light and re-emitting in the infrared.

The absence of X-ray emission suggests strong absorption or weak accretion. The SMBH masses are relatively low, consistent with exceeding the Eddington limit—a rapid growth mode typical of early stages in low-mass galaxies.

Comparison with Local Scales

| Parameter | Local Galaxies | Pelias/Neleus |

|-----------|----------------|---------------|

| M_BH/M_gal | 0.1–0.5% | up to 60% (upper limit) |

| Stellar Mass | >10^9 M_⊙ | ~10^7 M_⊙ |

| Redshift | z<0.1 | z~0.7 |

The stellar masses of Pelias and Neleus place them among the least massive AGN hosts. This suggests SMBH formation occurred before significant star formation.

Connection to "Little Red Dots"

The spectral energy distributions (SEDs) of these objects resemble little red dots (LRDs)—compact sources in the early universe. LRDs are interpreted as dusty AGNs with massive SMBHs. Pelias and Neleus may be LRD analogs at lower z, where accretion dominates against a backdrop of forming galaxies.

• Key Similarities: blue UV-optical slope + infrared excess;
• Differences: lower distance, intermediate z;
• Implications: extending the SMBH growth model to dwarf systems.

Black Hole Growth Mechanisms

Exceeding the Eddington limit explains rapid growth: L > L_Edd, where accretion is supercritical. The Eddington limit formula:

L_Edd = 1.25 × 10^{38} (M_BH / M_⊙) erg/s

Systematic errors in extrapolating local M_BH-scaling relations to dwarfs could mimic this excess. Direct mass estimates via dynamics or reverberation are needed.

Observational Prospects

Multiwavelength data are required for confirmation:

• X-ray: Chandra, Athena to search for faint emission;
• Infrared: JWST MIRI for detailed dust torus analysis;
• Radio: ALMA for molecular gas and jets;
• Future: Nancy Roman, ELT for systematic searches.

Such objects will test whether dusty super-Eddington accretion is a standard evolutionary phase for dwarf galaxies.

Key Takeaways

• JWST detected AGNs in galaxies with M_* ~10^7 M_⊙, previously inaccessible;
• M_BH/M_gal up to 60% contradicts local correlations at z~0.7;
• SEDs similar to LRDs, extending them to intermediate z;
• Exceeding L_Edd is key to rapid SMBH growth in small systems;
• X-ray/radio observations are needed for verification.

— Editorial Team