Possible environmental quenching in an interacting little red dot pair at $z\sim7$

Rosa M. Mérida, Gaia Gaspar, Marcin Sawicki, Yoshihisa Asada, Guillaume Desprez, Gregor Rihtaršič, Jacqueline Antwi-Danso, Roberta Tripodi, Chris J. Willott, Maruša Bradač, Gabriel B. Brammer, Kartheik G. Iyer, Nicholas S. Martis, Adam Muzzin, Gaël Noirot, Ghassan T. E. Sarrouh, Vladan Markov

First listed 2025-01-29 | Last updated 2025-07-18

Abstract

We report the discovery of a $z\sim7$ group of galaxies that contains two little red dots (LRDs) just 3.3 kpc apart, along with three potential satellite galaxies, as part of the Canadian NIRISS Unbiased Cluster Survey (CANUCS). The spectral energy distributions (SEDs) of this LRD pair show evidence of a Balmer break, consistent with a recent ($\sim 100$ Myr) quenching of star formation. In contrast, the satellites are compatible with a recent-onset ($\sim 100$ Myr), ongoing burst of star formation. LRD1's SED is consistent with a dust-free active galactic nucleus (AGN) being the source of the UV excess in the galaxy. The optical continuum would be powered by the emission from an obscured post-starburst and the AGN at a subdominant level. LRD2's SED is more ambiguous, but it could also be indicative of a dust-free AGN. In this scenario, these LRDs would be massive ($M_\star\sim10^{10}\,M_\odot$) and dusty (A(V) $>1$ mag) and the three satellites would be lower-mass objects ($M_\star\sim10^{8-9}\,M_\odot$) subject to low dust attenuations. The proximity of the two LRDs suggests that their interaction is responsible for their recent star formation histories, which can be interpreted as environmental bursting and quenching in the epoch of reionization.

Short digest

CANUCS NIRCam imaging behind Abell 370 reveals a compact z∼7 group with two little red dots only 3.3 kpc apart plus three satellites. SEDs show strong Balmer breaks implying recent (~100 Myr) quenching in the LRDs, while the satellites are consistent with a recent-onset (~100 Myr), ongoing burst. Modeling suggests a dust-free AGN produces the UV excess in LRD1 (with the optical dominated by an obscured post‑starburst and a subdominant AGN), and LRD2 is more ambiguous; in this picture the LRDs are massive (~10^10 M⊙, AV>1 mag) and the satellites are lower-mass (10^8–10^9 M⊙) with low attenuation. The tight configuration points to interaction-driven environmental bursting and quenching during reionization.

Key figures to inspect

• RGB and smoothed medium‑band cutouts: confirm the common ≈4 μm medium‑band (Hβ+[O III]) excess that anchors z∼7, the redder colors of the LRDs versus the satellites, and the deblended geometry with projected separations of 3.27 kpc (LRD–LRD) and 0.97 kpc (LRD1–SAT0) after lensing correction.
• SED fits for LRD1/LRD2 versus satellites: read off Balmer‑break amplitude, the medium‑band bump from nebular lines, and the contrast between quenched (~100 Myr) post‑starburst+AGN solutions for LRD1 and bursty, low‑dust stellar fits for the satellites; note the ambiguity for LRD2.
• Photometric‑redshift PDFs: EAzY P(z) using the Asada et al. IGM model shows all five members share consistent redshifts (Δz≈0.023), supporting a single z∼7 group.
• Local lens model and magnification map near the Abell 370 line of sight: verify that μ is small at this position and redshift, making the physical separations and mass inferences robust to lensing.
• Segmentation/centroid and deblending view: see the LRD1–SAT0 blend resolution and the SAT1a/b/c substructure, and how matched‑aperture versus point‑source photometry was applied to derive the SEDs.