Little Red Dots’ Dust Mass Limits Challenge Galaxy Evolution Models.

Observations of 60 little red dot candidates with ALMA at 1.3mm reveal no detectable dust emission. This yields a 3σ upper limit of 10 solar masses for dust mass and luminosity, an order of magnitude lower than prior limits. Results support either modest dust content or extreme Balmer breaks from dense gas surrounding a central black hole.

The pervasive dust within galaxies plays a critical role in star formation and the evolution of active galactic nuclei (AGN), yet quantifying its mass in distant, faint objects remains a significant challenge. Recent observations have focused on ‘Little Red Dots’ (LRDs) – extremely red sources potentially harbouring substantial dust reservoirs or exhibiting unusual spectral characteristics. A new analysis, utilising sensitive observations from the Atacama Large Millimeter/submillimeter Array (ALMA), places a stringent upper limit on the dust content of these enigmatic objects. This work, detailed in a paper led by Caitlin M. Casey (University of California, Santa Barbara) and involving collaborators including Hollis B. Akins, Steven L. Finkelstein, Seiji Fujimoto, Sinclaire M. Manning, and Jed McKinney (all University of Texas at Austin), alongside Maximilien Franco (Université Paris-Saclay), Daizhong Liu (Purple Mountain Observatory, Chinese Academy of Sciences), Arianna S. Long (University of Washington), Georgios Magdis & Marko Shuntov (Cosmic Dawn Center, Denmark), and Takumi S. Tanaka (University of California, Santa Barbara), is entitled ‘An upper limit of 10 M in dust from ALMA observations in 60 Little Red Dots’. The team’s observations suggest a total dust mass of no more than 10 solar masses across a sample of 60 LRDs, significantly refining previous estimates and offering insights into the nature of these distant sources.

Constraints on Dust Content in Early James Webb Space Telescope ‘Little Red Dots’

Recent observations utilising the Atacama Large Millimeter/submillimeter Array (ALMA) place stringent limits on the amount of dust surrounding a population of faint red sources – dubbed ‘Little Red Dots’ (LRDs) – identified in early James Webb Space Telescope (JWST) data. Analysis of 60 LRDs, and stacked data combining signals from multiple sources, reveals a consistent non-detection of millimetre-wave emission.

This work establishes a 3σ upper limit of 10 solar masses on the total dust mass within these objects, and a luminosity limit of 10²² W. These limits represent an order of magnitude improvement over previous submillimetre constraints on LRDs. The findings support two primary interpretations of their origin. Either LRDs harbour compact, yet modest, dust reservoirs, or the observed redness arises not from dust extinction – the absorption and scattering of light by dust – but from extreme Balmer breaks within the spectra of dense gas surrounding a central black hole. Balmer breaks are sudden drops in intensity at specific wavelengths caused by the absorption of light by hydrogen atoms.

The LRD sample was initially identified by Akins et al. (2024), building upon theoretical predictions outlined in Casey et al. (2024).

These new constraints provide a crucial test for models attempting to explain the origin of LRDs. Future research should prioritise spectroscopic observations to detect or rule out specific emission lines within the surrounding gas, allowing differentiation between the proposed mechanisms. Additionally, higher-resolution imaging at multiple wavelengths could reveal the spatial distribution of any dust present.