What is the nature of Little Red Dots and what is not, MIRI SMILES edition (2401.08782v2)

Abstract: We study little red dots (LRD) detected by JADES and covered by the SMILES MIRI survey. Our sample contains 31 sources, $\sim70$% detected in the two bluest MIRI bands, 40% in redder filters. The median/quartiles redshifts are $z=6.9_{5.9}^{7.7}$ (55% spectroscopic). We analyze the rest-frame ultraviolet through near/mid-infrared spectral energy distributions of LRDs combining NIRCam and MIRI observations, using a variety of modeling techniques that include emission from stars, dust, and (un)obscured active galactic nuclei (AGN). The NIRCam$-$MIRI colors, for $\geq10$ $\mu$m, are bluer than direct pure emission from AGN tori; the spectral slope flattens in the rest-frame near-infrared, consistent with a 1.6 $\mu$m stellar bump. Both observations imply that stellar emission makes the dominant contribution at these wavelengths, expediting a stellar mass estimation: the median/quartiles are $\log \mathrm{M_\star/M_\odot}=9.4_{9.1}^{9.7}$. The number density of LRDs is $10^{-4.0\pm0.1}$ Mpc$^{-3}$, accounting for $14\pm3$% of the global population of galaxies with similar redshifts and masses. The flat ultraviolet spectral range is dominated by young stars. The rest-frame near/mid-infrared (2-4 $\mu$m) spectral slope reveals significant amounts of dust (bolometric stellar attenuation $\sim3-4$ mag) heated by strong radiation fields arising from highly embedded compact sources. Our models imply $<0.4$ kpc heating knots, containing dust-enshrouded OB stars or an AGN producing a similar radiation field, obscured by $\mathrm{A(V)}>10$ mag. We conclude that LRDs are extremely intense and compact starburst galaxies with mass-weighted ages 5-10 Myr, very efficient in producing dust, their global energy output dominated by the direct and dust-recycled emission from OB stars, with some contribution from obscured AGN in the mid-infrared.

Analysis of Little Red Dots Using MIRI SMILES Data

The paper titled "What is the nature of Little Red Dots and what is not, MIRI SMILES edition" explores the analysis of certain astronomical phenomena referred to as "Little Red Dots" (LRDs). Utilizing data from the JWST Advanced Deep Extragalactic Survey (JADES) and the MIRI SMILES program, the researchers aim to characterize the nature of LRDs with a focus on the spectral energy distributions (SEDs) of these entities observed through NIRCam and MIRI filters.

Overview of Methodology

The paper analyzes 31 LRDs, detected in the NIRCam bands and investigated using the MIRI mid-infrared bands. These observations probe the rest-frame near- and mid-infrared where both stellar emission and obscured Active Galactic Nuclei (AGN) emission may peak. The analysis applies four different spectral modeling codes—synthesizer-AGN, prospector-SF, prospector-AGN, and prospector-AGN$+$—each incorporating distinct assumptions regarding stellar populations and AGN contributions to determine the nature of these LRDs.

Key Findings

1. Spectral Energy Distribution (SED) Characteristics:
   • The paper finds that the NIRCam observations show LRDs with distinctive blue characteristics in the UV and red in the optical, suggesting substantial dust presence.
   • MIRI mid-infrared data reveals a flattening at rest wavelengths between 1 and 2μm, challenging the assumption that these are AGN-dominated spectra.
2. Stellar and AGN Contributions:
   • The data analysis using synthesizer-AGN and prospector-AGN$+$ suggests that, for the majority of LRDs, stars are the primary contributors to the emission from UV-to-NIR wavelengths.
   • Prospector-AGN, which adopts an AGN-dominant perspective for the optical range, results in lower stellar masses, indicating that the interpretation of these data significantly affects inferred stellar properties.
3. Implications of Dust Emission Models:
   • Printed distinctions emerge between the heating mechanisms in LRDs: either from embedded young OB stars or AGN activity within the torus.
   • Generating accurate stellar mass estimates for LRDs relies on accounting for various parameters governing dust emissions, thus influencing the resultant mass-to-light ratios.
4. Demographics and Physical Properties:
   • A notable fraction of the LRDs fails to fit the broad-line characteristics of AGNs based on their spectral profiles, suggesting a blend or predominance of extreme starburst activity for some galaxies.
   • Combinations of independent extinction parameters and energy contribution models help in resolving the unusual SED shapes recorded.

Speculative Future Developments

The implications of these findings extend into refining models that differentiate stellar population broadening and dust effects from AGN characteristics in early galaxies, which are not fully constrained by traditional models. A possible future direction involves deeper exploration into more nuanced AGN torus models or composite emission scenarios that accommodate a wider variety of intrinsic dust properties and radiative transfer.

Conclusion

The paper emphasizes the intricate nature of LRDs and specifies the variability in their characteristics due to a mix of physical conditions, from massive dust-enshrouded starbursts to potential AGN contributors. This work underscores the complexity of modeling SEDs with current assumptions and highlights the need for advanced models to more accurately depict these enigmatic celestial entities.