Here There Be (Dusty) Monsters: High Redshift AGN are Dustier Than Their Hosts

TL;DR

This paper analyzes dust attenuation in high-redshift broad-line AGN (z>3.5) using Balmer decrements from JWST/NIRSpec spectra across 29 BLAGN in CEERS, JADES, and RUBIES. By fitting dual-component Hα and Hβ lines and stacking sources without broad Hβ, the authors find minimal attenuation in the narrow-line region but strong attenuation in the broad-line region, suggesting that red SEDs arise from dust-enshrouded AGN emission rather than host-dominated reddening. The work implies a compact, inner dust geometry near the AGN (torus or polar dust) and large-scale, unattenuated star formation in the host contributing to blue UV light; X-ray non-detections may reflect heavy absorption or intrinsically weak X-ray output. These results shed light on black hole growth and AGN–host coevolution at cosmic dawn, underscoring the need for deeper JWST IFU studies to resolve dust distribution and kinematics.

Abstract

JWST spectroscopy has discovered a population of $z \gtrsim 3.5$ galaxies with broad Balmer emission lines, and narrow forbidden lines, that are consistent with hosting active galactic nuclei (AGN). Many of these systems, now known as ``little red dots" (LRDs), are compact and have unique colors that are very red in the optical/near-infrared and blue in the ultraviolet. The relative contribution of galaxy starlight and AGN to these systems remains uncertain, especially for the galaxies with unusual blue+red spectral energy distributions. In this work, we use Balmer decrements to measure the independent dust attenuation of the broad and narrow emission-line components of a sample of 29 broad-line AGN identified from three public JWST spectroscopy surveys: CEERS, JADES, and RUBIES. Stacking the narrow components from the spectra of 25 sources with broad H$\rmα$ and no broad H$\rmβ$ results in a median narrow H$\rmα$/H$\rmβ$ = $2.47^{+0.05}_{-0.05}$ (consistent with $A_{v} = 0$) and broad H$\rmα$/H$\rmβ$ $> 8.85$ ($A_{v} > 3.63$). The narrow and broad Balmer decrements imply little-to-no attenuation of the narrow emission lines, which are consistent with being powered by star formation and located on larger physical scales. Meanwhile, the lower limit in broad H$\rmα$/H$\rmβ$ decrement, with broad H$\rmβ$ undetected in the stacked spectrum of 25 broad-H$\rmα$ AGN, implies significant dust attenuation of the broad-line emitting region that is presumably associated with the central AGN. Our results indicate that these systems, on average, are consistent with heavily dust-attenuated AGN powering the red parts of their SED while their blue UV emission is powered by unattenuated star formation in the host galaxy.

Figures (4)

• Figure 1: Broad-line component H$\alpha$ luminosity (top) and narrow-line component H$\alpha$ luminosity (bottom) as a function of redshift for our sample of 27 BLAGN, spanning $4.13 < z < 6.78$. CEERS sources are shown in blue, JADES sources are shown in purple, and RUBIES are shown in red.
• Figure 2: Observed-frame spectra (black histograms) and associated uncertainties (gray error bars) for RUBIES-EGS-47509. The top left panel shows the H$\alpha$ spectral region, the bottom left panel the H$\beta$ spectra region, and the right panel the [OIII] doublet. Best-fit Gaussians are shown in dark blue for the narrow emission lines and red for the broad components. The total (narrow + broad) best fit model is shown in purple. Broad H$\alpha$ is significantly detected in this source ($>46\sigma$) but broad H$\beta$ is not. The completed figure set (29 images) includes similar spectra for all BLAGN in our sample.
• Figure 3: Median-stacked H$\alpha$ (left) and H$\beta$ (right) emission lines for objects in our sample that exhibit no broad H$\beta$ emission (25/29 sources). Our narrow line fits are shown in blue, broad line fits shown in red, and the total dual-Guassian fit is shown in purple. After stacking, broad H$\beta$ emission is still undetected, suggesting that the mean population of AGN show significant dust attenuation in the BLR. The full stacking procedure is described in §\ref{['Stacked Spectra']}.
• Figure 4: Narrow H$\alpha$/H$\beta$ vs. Broad H$\alpha$/H$\beta$ for the sample of 29 BLAGN. Filled circles represent sources with detected broad H$\beta$ emission, while the open circles are galaxies with undetected broad H$\beta$ and are plotted using the $3\sigma$ upper limit of broad H$\beta$. The pink star represents the stack of sources with no detected broad H$\beta$ and is plotted using the $3\sigma$ upper limit of broad H$\beta$. The one-to-one line is shown by the dashed black line. An inset is included in the top left to show two extremely reddened sources, RUBIES-EGS-42406 (black circle) with detected broad H$\beta$ emission and JADES 73488 (open circle). The $A_{v}$ for the broad and narrow flux ratios, calculated with the Calzetti1997 dust law and described in section \ref{['results']}, are shown on the dual x and y axes, respectively. 12/29 sources fall firmly below the one-to-one line, indicative of broad H$\alpha$/H$\beta$$>$ narrow H$\alpha$/H$\beta$.