Dust in Little Red Dots

Abstract

JWST has revealed a ubiquitous population of “little red dots” (LRDs) at z ≳ 4, selected via their red rest-frame optical emission and compact morphologies. They are thought to be reddened by dust, whether in tori of active galactic nuclei (AGNs) or the interstellar medium, though none have direct dust detections to date. Informed by the average characteristics of 675 LRDs drawn from the literature, we provide ballpark constraints on the dust characteristics of the LRD population and estimate they have average dust masses of $\langle {M}_{\mathrm{dust}}\rangle =({1.6}_{-0.9}^{+4.8})\times {10}^{4}$ M _⊙ , luminosities of $\langle {L}_{\mathrm{IR}}\rangle =({8}_{-5}^{+3})\times {10}^{10}$ L _⊙ , and temperatures of $\langle {T}_{\mathrm{dust}}\rangle ={110}_{-36}^{+21}$ K. Notably, the spectral energy distributions are thought to peak at ∼100 K (rest-frame 20–30 μ m) regardless of heating mechanism, whether AGN or star formation. LRDs’ compact sizes R _eff ∼ 100 pc are the dominant factor contributing to their low estimated dust masses. Our predictions likely mean LRDs have, on average, a submillimeter emission factor of ∼100× fainter than current Atacama Large Millimeter/submillimeter Array limits provide. The star-to-dust ratio is a factor ∼100× larger than expected from dust formation models if one assumes the rest-optical light is dominated by stars; this suggests stars do not dominate. Despite their high apparent volume density, LRDs contribute negligibly (0.1%) to the cosmic dust budget at z ≳ 4 due to their low dust masses.

Keywords

• AGN host galaxies
• Dust continuum emission
• James Webb Space Telescope