The rise and fall of Little Red Dots could be driven by the environment

TL;DR

This work probes how environment shapes the rise and fall of Little Red Dots (LRDs) by analyzing The Stingray, a compact, interacting trio at high redshift identified in CANUCS with JWST. Using spectro-photometric modeling (Bagpipes and Dense Basis) and emission-line diagnostics from NIRSpec, the authors derive star-formation histories, AGN properties, and mass growth, showing interaction-driven bursts that push stellar and black hole growth beyond secular trends. They identify a transitional LRD (tLRD) that exhibits LRD-like UV/line features yet does not fully satisfy optical LRD criteria, suggesting a phase in which LRD emission emerges or fades under environmental influence. The findings support a boosted hierarchical assembly scenario in the early universe and highlight environmental triggering as a key driver of LRD evolution, underscoring the need for a larger census of transitional objects to map the LRD lifecycle.

Abstract

The Little Red Dot (LRD) paradigm comprises three main unknowns that are intrinsically connected: (1) What is the nature of these sources? (2) How do they form? (3) How do they evolve? Larger spectroscopic samples and high-resolution data are needed to delve deeper into the mechanisms ruling these sources. Understanding their formation and evolution requires identifying the rise and fall of the key features that characterize these systems, such as their compactness and ``V''-shaped spectral energy distributions. In this work, we present a galaxy system nicknamed The Stingray that was identified in the Canadian NIRISS Unbiased Cluster Survey (CANUCS). This group contains three sources at $z_{\mathrm{spec}} = 5.12$, including an active galactic nucleus (AGN), a Balmer break galaxy, and a star-forming satellite. The latter resembles a Building Block System in which interactions boost stellar mass and black hole mass growth beyond what is expected from secular processes alone. The AGN in this system exhibits features indicative of a transitional object, bridging a normal AGN and an LRD phase. These are a blue rest-frame ultraviolet slope, compact size, and a broad H$α$ line (all of which are characteristic of LRDs), but a flatter rest-frame optical slope compared to that observed in LRDs. The features in this source point to the emergence or fading of an LRD, potentially triggered by environmental effects.

Figures (13)

• Figure 1: Cutouts of the galaxy group in different bands. Top: (1) 7.7$\times 7.1$ arcsec$^2$ cutouts of two RGB images based on $F200W$, $F410M$, and $F444W$ (left) and $F200W$, $F360M$, and $F444W$ (right) NIRCam imaging, not convolved to $F444W$ resolution. The yellow square encloses the region that contains the part of the Stingray studied in this work; its dimensions are 2.0$\times 1.4$ arcsec$^2$. We include a view in a less saturated scale to spotlight the difference in color between the different galaxies. These images highlight the $F410M$ excess in the tLRD (see panel 2), due to H$\alpha$ emission at this $z$, with respect to the continuum probed by $F360M$. The pink arrow in the left panel points to a $z\sim1.5$ galaxy group $\sim1.3$ away from the tLRD that would resemble the tail of the Stingray. We include a cutout showing the position of the MSA slits. (2) Cutouts of $F300M - F335M$ (top) and $F410M - F430M$ (bottom) showing the H$\beta$+OIII and H$\alpha$ emission, respectively. Bottom: Postage stamps of these sources in different NIRCam bands, not convolved to $F444W$ resolution. They are plotted following the same scale.
• Figure 2: G395M MSA spectra (blue) and photometry (open red circles) of the tLRD. The spectrum was rebinned in wavelength to improve S/N and readability. Lighter circles correspond to the HST photometry, which was not considered in our spectro-photometric analysis. In orange and pink, we show the best-fitting models from Bagpipes (spectra+photometry) and Dense Basis (photometry only), respectively. Dense Basis was allowed to explore a broader range of $z$, which includes the photo-$z$, to ensure convergence. This explains why there is a slight offset between the lines in both models. The photometric points associated with the Bagpipes model are shown as orange circles. We include the ACS and WFC3/HST (dashed) and NIRCam/JWST (solid) filter transmission curves at the top of the panel. Vertical dashed lines highlight the position of the main emission lines at this $z$. We show the Bagpipes residuals underneath. For the spectrum, the residuals are the ratio between the spectrum and the model. The grey shaded region corresponds to one standard deviation. For the photometry, we display the difference between the photometry and the model weighted by the photometry. Uncertainties are also depicted, representing the inverse of the S/N. The spectrum dominates the fitting in the optical, resulting in larger residuals in the photometry.
• Figure 3: G395M MSA spectra (blue) and photometry (open red circles) of the BBG. We include an inset with the photometry expressed in $\mu$Jy where it is easier to spot the presence of the Balmer break. See Fig. \ref{['fig:AGN_spec']} for a complete description of the markers and color codes used in this plot.
• Figure 4: G395M MSA spectra (blue) and photometry (open red circles) of SAT1. See Fig. \ref{['fig:AGN_spec']} fo a complete description of the markers and color codes used in this plot.
• Figure 5: H$\alpha$ fit (black line) for the tLRD compared to the data (blue). The spectrum was rebinned in wavelength to improve S/N and readability. The solid (dashed) red line shows the fit to the broad (narrow) component. The normed residuals (spectrum $-$ model, normalized by the error) are displayed underneath. The shaded gray region corresponds to $\pm$1$\sigma$.