JADES: Discovery of Large Reservoirs of Small Dust Grains in the Circumgalactic Medium of Massive Galaxies at $z\sim3.5$ through Deep JWST/NIRCam Imaging and Grism Spectroscopy

TL;DR

This study uses deep JWST/NIRCam imaging and grism spectroscopy from the JADES Origins Field to build a large spectroscopic redshift catalog and investigate dust in the CGM of massive galaxies at z around 3.5. It discovers substantial reservoirs of small CGM dust grains, inferred from high A_V and Sigma_dust and from steep extinction curves that favor silicate grains, extending out to ~30 kpc around DSFGs. Comparisons with hydrodynamical simulations and ALMA data indicate that such CGM dust is underpredicted by simulations and largely invisible to current millimeter imaging, implying a need for revised dust transport and enrichment models. The results highlight complex dust transport processes in the early universe and warn that foreground CGM extinction can bias the identification of very high-redshift galaxies, with broad implications for future surveys and interpretations of the high-z population.

Abstract

Using JWST NIRCam imaging and grism spectroscopy from the JWST Advanced Deep Extragalactic Survey (JADES) Origins Fields, we report spectroscopic redshift measurements of 1,445 emission-line galaxies at $z=0-9$. Within this sample, we identify two prominent galaxy protoclusters at $z = 3.47$ and 3.69, each anchored by massive dusty star-forming galaxies (DSFGs). In the vicinity of these systems, we discover seven background galaxies at $z=3.6 - 6$ that simultaneously exhibit strong rest-frame optical emission lines (e.g., [O III] and H$α$) and unusually reddened UV-to-optical continua. We attribute this reddening to dust extinction arising from the circumgalactic medium (CGM) of the foreground DSFGs at projected separations of 7-30 kpc. We infer a high dust column density ($\gtrsim 10^{-1}$ Msun/kpc^2), substantially exceeding those measured in low-redshift halos and those predicted by hydrodynamical simulations like IllustrisTNG and FIRE-2. The steep extinction curves, comparable to or steeper than that of the SMC, indicate a dominant population of small dust grains in the high-redshift CGM. We conclude that DSFGs at this epoch host large reservoirs of dusty CGM enriched to solar metallicity. These extended dust components are largely invisible to (sub-)millimeter interferometers such as ALMA because of their low surface brightness. We discuss the physical processes in dust transport that might be key to reproducing our observations, including galaxy mergers, cool-phase gas outflows, dust shattering, sputtering and radiation pressure. Finally, we caution that foreground CGM dust extinction may redden background galaxies at intermediate redshifts to mimic Lyman-break galaxies at $z\gtrsim10$.

Figures (14)

• Figure 1: Six dusty star forming galaxies spectroscopically confirmed within the $z=3.47$ and 3.70 protoclusters in the GOODS-S/JOF region. The middle panel shows the on-sky distribution of galaxies with grism $z_\mathrm{spec}$ across 3.4--3.8 superimposed on NIRCam F444W image, with the inset panel displaying the redshift histogram (redshift color-coded). The ultra-deep NIRCam imaging footprint in the JOF is outlined by the two 22$\times$22 magenta boxes. The six image panels on the left and right show the NIRCam F444W-F200W-F115W RGB images of the selected DSFGs. The ALMA centroids of the dust continuum emission are indicated by the skyblue ticks. Seven reddened emission-line galaxies in the background of the five DSFGs (except for ALESS9.1) are highlighted by the dashed white circles with JADES NIRCam ID and $z_\mathrm{spec}$ indicated.
• Figure 2: JWST/NIRCam images and spectra of the massive DSFG A2GS29 and two reddened galaxies in the background. We show the F322W2 and F444W grism spectra in the wavelength ranges containing [O3], H$\alpha$ and [S3] with the emission lines highlighted. The contaminated continuum is not subtracted in the 2D spectra, but is subtracted in the 1D spectra.
• Figure 3: Left: SEDs of two CGM-reddened galaxies behind A2GS29 (Figure \ref{['fig:spec']}), shown as the green diamonds connected by green solid lines. Open symbols denote the photometric bands affected by IGM attenuation or by the presence of strong lines (e.g., H$\beta$, [O3] and H$\alpha$; wavelengths highlighted as vertical silver lines) within the bandwidths, and therefore are not used for extinction curve modeling. The wavelength of the Balmer break is also indicated by a vertical silver line. The SED templates that fit other emission-line galaxies at similar redshifts and brightness but without foreground CGM extinction are shown as the skyblue lines. These templates are projected to the same redshifts of the CGM-reddened galaxies and normalized by the flux density at 5 µm. Right: inferred dust extinction curve ($A_\lambda - A_V$; see Section \ref{['ss:03d_ext']}) from the foreground CGM, shown as the filled green diamonds connected by the solid lines. $2\sigma$ and $1\sigma$ confidence intervals are shown as the light and dark green-shaded regions, respectively.
• Figure 4: Heavy dust extinction seen in JADES ID 171525 from the CGM of foreground DSFG ALESS 10.1. Top: NIRCam grism 2D and 1D spectra of foreground ALESS10.1 and background ID 171525 (similar to that of Figure \ref{['fig:spec']}). Bottom left: NIRCam image of the system (same as that in Figure \ref{['fig:z3pc']}). Bottom middle/right: SED and inferred extinction curve seen along the line-of-sight of 171525 (similar to that of Figure \ref{['fig:sed_a2gs29']}).
• Figure 5: Probability distribution of V-band optical depth of dust extinction $\log(\tau_V)$ and power-law stope $n$ of the extinction curve along the sightlines of two background ELGs (left: ID 171525 in the background of ALESS10.1 shown in Figure \ref{['fig:sed_a2gs29']}; right: ID 635159 in the background of A2GS29, shown in Figure \ref{['fig:aless10_1']}). Each circle denotes the best-fit $\log(\tau_V)$ and $n$ using each spectral templates, color-coded by the reduced $\chi^2$ of template matching. Posterior distributions of $\log(\tau_V)$ and $n$, including the 16--50--84th percentiles of the credible intervals are shown as the skyblue bands and the black solid lines.