An Upper Limit of 106 M ⊙ in Dust from ALMA Observations in 60 Little Red Dots

Abstract

By virtue of their red color, the dust in little red dots (LRDs) has been thought to be of appreciable influence, whether that dust is distributed in a torus around a compact active galactic nucleus or diffuse in the interstellar medium of nascent galaxies. In C. M. Casey et al. we predicted that, based on the compact sizes of LRDs (unresolved in JWST NIRCam imaging), detection of an appreciable dust mass would be unlikely. Here we present follow-up Atacama Large Millimeter/submillimeter Array 1.3 mm continuum observations of a sample of 60 LRDs drawn from H. B. Akins et al. None of the 60 LRDs are detected in imaging that reaches an average depth of σrms = 22 μJy. A stack of the 60 LRDs also results in a nondetection, with an inverse-variance weighted flux density measurement of S1.3 mm = 2.1 ± 2.9 μJy. This observed limit translates to a 3σ upper limit of 106M⊙ in LRDs’ dust mass, and ≲1011 L⊙ in total dust luminosity; both are a factor of 10× deeper than previous submillimeter stack limits for LRDs. These results are consistent with either the interpretation that LRDs are reddened due to compact but modest dust reservoirs (with AV ∼ 2–4) or, alternatively, that instead of being reddened by dust, they have extreme Balmer breaks generated by dense gas (>109 cm−3) enshrouding a central black hole.