What Is the Nature of Little Red Dots and what Is Not, MIRI SMILES Edition
Pablo G. Pérez‐González, Guillermo Barro, G. H. Rieke, Jianwei Lyu, Marcia Rieke, Stacey Alberts, Christina C. Williams, Kevin Hainline, Fengwu Sun, Dávid Puskás, Marianna Annunziatella, William Baker, Andrew J. Bunker, Eiichi Egami, Zhiyuan Ji, Benjamin D. Johnson, Brant Robertson, Bruno Rodríguez Del Pino, W. Rujopakarn, Irene Shivaei, Sandro Tacchella, Christopher N. A. Willmer, Chris J. Willott
Abstract
Abstract We study 31 little red dots (LRD) detected by JADES/NIRCam and covered by the SMILES/MIRI survey, of which ∼70% are detected in the two bluest MIRI bands and 40% in redder MIRI filters. The median/quartiles redshifts are <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>z</mml:mi> <mml:mo>=</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>6.9</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>5.9</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>7.7</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> (55% spectroscopic). The spectral slopes flatten in the rest-frame near-infrared, consistent with a 1.6 μ m stellar bump but bluer than direct pure emission from active galactic nuclei (AGN) tori. The apparent dominance of stellar emission at these wavelengths for many LRDs expedites stellar mass estimation: the median/quartiles are <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⋆</mml:mo> </mml:mrow> </mml:msub> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>9.4</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>9.1</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>9.7</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> . The number density of LRDs is 10 −4.0±0.1 Mpc −3 , accounting for 14% ± 3% of the global population of galaxies with similar redshifts and masses. The rest-frame near-/mid-infrared (2–4 μ m) spectral slope reveals significant amounts of warm dust (bolometric attenuation ∼3–4 mag). Our spectral energy distribution modeling implies the presence of <0.4 kpc diameter knots, heated by either dust-enshrouded OB stars or an AGN producing a similar radiation field, obscured by A ( V ) > 10 mag. We find a wide variety in the nature of LRDs. However, the best-fitting models for many of them correspond to extremely intense and compact starburst galaxies with mass-weighted ages 5–10 Myr, very efficient in producing dust, with their global energy output dominated by the direct (in the flat rest-frame ultraviolet and optical spectral range) and dust-recycled emission from OB stars with some contribution from an obscured AGN (in the infrared).