What Is the Nature of Little Red Dots and what Is Not, MIRI SMILES Edition

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 z = 6.9 5.9 7.7 (55% spectroscopic). The spectral slopes flatten in the rest-frame near-infrared, consistent with a 1.6 μm 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 log M ⋆ / M ⊙ = 9.4 9.1 9.7 . 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 μm) 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).

A NIRCam-dark Galaxy Detected with the MIRI/F1000W Filter in the MIDIS/JADES Hubble Ultra Deep Field

“…The SED of Cerberus is, in fact, very similar to the LRDs recently presented and characterized in a number of papers (e.g., Labbé et al 2023bLabbé et al , 2023aMatthee et al 2024;Barro et al 2024;Williams et al 2024;Pérez-González et al 2024;Kokorev et al 2024), but shifted to longer wavelengths. Based on detections of Cerberus in stacked images, we can infer that the F277W − F444W > 1 mag general selection criterion of LRDs and, more specifically, the 3 mag color of the example source 203749 in Pérez-González et al (2024) compare well, if we multiply the observed wavelengths by a factor of ∼2, with F444W − F1000W ∼ 3.5 mag, or, using the F560W 5σ upper limit, F560W − F1000W  2 mag.…”

“…The reduction of the MIRI data followed the methodology described in Pérez-González et al (2024), based on the official JWST pipeline (v1.13.4, jwst_1202.pmap) and offline bespoke procedures. Briefly, a superbackground strategy is used to build background maps for each single image using all the other exposures (since they were taken during the same campaign), which results in a very homogeneous background in terms of level and noise.…”

“…Quantitatively for the data used in this letter, the standard deviation of the background pixels for each cal.fits file obtained with the standard official pipeline is reduced by a factor of ∼3.5 by using our superbackground method, and the depth in the final mosaic improves by 0.8 mag. We refer the reader to Appendix A in Pérez-González et al (2024) for more details. Known sources are masked to avoid biasing the determination of the very local background in the superbackground image.…”

“…Remarkably, with the advent of JWST, both industries, searching for red objects and for those with higher and higher redshifts, have been merged at a profound level, with the possibility of finding double-break (Lyman and Balmer) red galaxies (Labbé et al 2023b), now known as little red dots (LRDs; Furtak et al 2023;Übler et al 2023;Matthee et al 2024;Barro et al 2024;Greene et al 2024;Williams et al 2024;Killi et al 2023;Pérez-González et al 2024;Kokorev et al 2024), or high-redshift galaxies with very high equivalent width emission lines, first discovered with Spitzer (Smit et al 2014) and now appearing in many JWST works (Pérez-González et al 2023a;Rinaldi et al 2023). Interestingly, some of these sources have been confirmed to present broad emission line components possibly linked to an active galactic nucleus (AGN; Kocevski et al 2023;Greene et al 2024).…”

“…The MIRI instrument on board JWST (Rieke et al 2015;Wright et al 2023) now offers the possibility to search for red galaxies with longer color baselines, beyond the wavelengths first probed by Spitzer/IRAC and now easily accessible with impressive depths (∼30 mag and beyond) using JWST/NIRCam. Even though MIRI cannot reach NIRCam depths, very deep imaging at 5.6-12.8 μm reaching magnitudes around 27-29 is already available and opens a whole new window for the search for and characterization of high-redshift galaxies (Papovich et al 2023;Barro et al 2024;Rinaldi et al 2023;Pérez-González et al 2024).…”