Silencing the Giant: Evidence of AGN Feedback and Quenching in a Little Red Dot at z = 4.13

TL;DR

This study investigates how AGN activity and feedback relate to rapid quenching in the early universe by analyzing JWST/NIRSpec spectra of a red, compact galaxy at $z=4.13$ in the JADES GOODS-N field. Using rest-optical spectroscopy, it detects an extremely broad Hα line and high-ionization emission lines from an actively accreting SMBH embedded in a predominantly evolved stellar population ($log10(M_*/M_sun) ≈ 10.6$; $log10[sSFR/yr^-1] < -11$; $log10(M_BH/M_sun) ≈ 7.3$), yielding $M_BH/M_* ≈ 0.0004$. The rest-UV features, including Lyα and broad MgII, corroborate the AGN interpretation, with GN-72127 representing a clear link between high-z reddened AGN and mature hosts. The BH-to-host mass ratio aligns with local relations, supporting an evolutionary connection between high-z reddened AGN, early quiescent galaxies, and local ellipticals, and illustrating a pathway where AGN feedback contributes to rapid quenching in the early universe. By situating LRDs within this evolutionary framework, the work connects z~4 objects to the growth of quiescent systems observed later and to local ellipticals.

Abstract

The James Webb Space Telescope (JWST) has uncovered a ubiquitous population of dust-obscured compact sources at $z\gtrsim 4$. Many of these objects exhibit signs of active galactic nucleus (AGN) activity, making their study crucial for understanding the formation of supermassive black holes (SMBHs) and their growth with host galaxies. In this work, we examine low and medium resolution JWST/NIRSpec spectra from the JADES GTO public data release in the GOODS-N field of a red, luminous ($M_{\rm B}\sim-22.2$ mag) and compact ($<500$ pc) source at $z=4.13$. The rest-optical ($λ_{\rm rest} > 4000$ A) continuum of this source is strongly dominated by a massive (log$_{10}$[$M_*/M_\odot] \sim 10.6$), quenched (log$_{10}$[sSFR/yr$^{-1}$] $< -11$) galaxy, as indicated by the clear presence of a Balmer break and stellar absorption lines. Star-formation history modeling reveals a starburst episode followed by rapid quenching about 200 Myr ago. The spectrum shows extremely broad (FWHM $\sim 2500$ km/s) H$α$ emission and elevated optical line ratios, indicating an actively accreting SMBH. Moreover, our work has potentially revealed clear AGN signatures in the rest-UV in LRDs for the first time, via a detection of a strong Ly$α$ emission and a broad MgII, doublet. The derived black hole mass of log$_{10}$($M_{\rm BH}/M_\odot) \sim 7.3$ results in $M_{\rm BH}/M_*\sim 0.04$ %, consistent with the local relations, unlike the elevated ratios in other high-$z$ reddened AGN. Finally, we use JWST data from AGN at $z=4-10$ to explore an evolutionary link between high-$z$ reddened AGN, early quiescent galaxies, and local ellipticals.

Figures (1)

• Figure 1: Top:JWST/NIRCam 2$.\!\!^{\prime\prime}$0 stamps and the RGB short and long wavelength color images comprised of the F115W, F150W, F200W and F277W, F356W, and F444W bands, respectively. The MSA slitlet layout (white) is overlaid on a separate LW color image, for clarity. The source has a very clear PSF-dominated morphology present in all filters. Each panel shows the total magnitude as presented in the JADES DR3 catalog deugenio24. The source is extremely bright and is detected in all bands at $>50\sigma$. Middle: 2D MSA PRISM spectrum. Bottom: Optimally extracted 1D spectrum of the galaxy in the observed frame. We show the data in black, while the uncertainty on the spectrum is in dashed red. Best-fit spline continuum and line msaexp model to the data is shown in solid red. Assuming the best-fit to the PRISM with $z_{\rm spec}$ = $4.129\pm0.035$, we show the positions and label the prominent emission with significant ($>3\sigma$) detections as solid vertical lines. Emission lines for which we only obtain an upper limit are shown with dashed lines. The position of the Balmer/4000 Å break is shown with a shaded region.