Probing the Physics of Dusty Outflows through Complex Organic Molecules in the Early Universe

TL;DR

This study reports the first high-redshift detection of aromatic and aliphatic hydrocarbon dust features in absorption within a quasar-dominated outflow, at z = 4.601 in W2246-0526, blueshifted by approximately ΔV ≈ −5250 km s$^{-1}$. By combining JWST/MIRI MRS and NIRSpec IFU data, the authors fit the hydrocarbon absorption and ionized emission lines to derive both the dusty and ionized wind properties, including mass loss rates and kinetic powers. The analysis supports radiation pressure on dust—dominated by reprocessed infrared photons—as the primary mechanism accelerating the dusty wind, with on-source coupling efficiencies around 1% and terminal velocities potentially exceeding 10,000 km s$^{-1}$. These results demonstrate that efficient quasar-driven dusty outflows and molecule-rich dust can exist in the early universe, providing a pathway for dust and metal enrichment of the circumgalactic and intergalactic media. The work also motivates multi-wavelength follow-up (e.g., [CII], CO, OH) to fully characterize the multi-phase outflow and its impact on galaxy evolution at high redshift.

Abstract

Galaxy-scale outflows are of critical importance for galaxy formation and evolution. Dust grains are the main sites for the formation of molecules needed for star formation but are also important for the acceleration of outflows that can remove the gas reservoir critical for stellar mass growth. Using the MIRI medium-resolution integral field spectrograph aboard the James Webb Space Telescope (JWST), we detect the 3.28 $μ$m aromatic and the 3.4 $μ$m aliphatic hydrocarbon dust features in absorption in a redshift 4.601 hot dust-obscured galaxy, blue-shifted by $Δ$V=$-5250^{+276}_{-339}$ kms$^{-1}$ from the systemic redshift of the galaxy. The extremely high velocity of the dust indicates that the wind was accelerated by radiation pressure from the central quasar. These results pave a novel way for probing the physics of dusty outflows in active galaxies at early cosmic time.

Figures (9)

• Figure 1: Rest-frame normalized MIRI MRS spectrum of the 3.3$\mu$m aromatic and 3.4$\mu$m aliphatic hydrocarbon complex in W2246-0526. The best-fit absorption model is shown in brown. Each Gaussian component represents a unique feature that makes up the 3.4 $\mu$m aliphatic absorption complex with an additional Gaussian fit to the 3.3$\mu$m aromatic absorption (Table \ref{['tab:best_fit']}). We measure a radial velocity offset of -5251 km s$^{-1}$ in all the absorption features relative to systemic. The dashed-dot green and blue lines show the shift from systemic (light colors) to blueshifted (solid color) for the 3.289 $\mu$m aromatic and 3.376 $\mu$m aliphatic features, respectively, with the green/blue arrows showing the shift. The bottom panel shows the residuals of our best-fit model.
• Figure 2: Line profiles of ionized emission from the outflow in the nuclear region of W2246-0526, marked with a red circle in the inserted figure showing the extended H$\alpha$ emission in the W2246-0526 system, adapted from Vayner et al 2025 Vayner25. Extremely broad and blueshifted emission is detected in [O$\,$ III], H$\alpha$ and H$\beta$, with maximum velocities of -13,400 km s$^{-1}$ (V$_{02}$), where 0 km s$^{-1}$ is systemic. The detection of [O$\,$ III] indicates that this emission originates beyond the broad-line region of the quasar. We also overlay the C$\,$ IV$\lambda \lambda$ 1548 Å, 1550Å emission line spectrum from rest-frame UV observations taken by the MUSE instrument, showing a similar profile to the optical lines. The solid black line shows the average velocity of the profile (V$_{50}$). The red line shows the velocity of the dusty outflow traced through aromatic and aliphatic hydrocarbon absorption.
• Figure 3: Schematic diagram of the inner kpc region of the W2246-0526 system. The supermassive black hole (SMBH) and the quasar accretion disk are located in the center. Surrounding the accretion disk is a dust-dominated region with a large covering solid angle around the quasar accretion disk and the broad-line region. High dust opacities, large column densities, and optically thick regions are located near the equatorial plane, with beige to orange colors representing an increase in these quantities. Past the dust sublimation radius, low to medium-latitude regions are dominated by infrared photons (red) formed through reprocessing UV photons originating from the quasar accretion disk. Light orange arrows mark the dusty outflow driven by radiation pressure from the infrared photons near the sublimation radius, and our line of sight crossing the path is marked with a dashed black line. Along the polar region, the column densities drop, allowing ionizing, UV, and optical photons to escape. The ionized outflow photo-ionized by the quasar likely escapes along this path. Part of the ionized emitting region is interior to the dusty outflow causing reddening of the emission, however not at a level to completely obscure it.
• Figure 4: We present the total MIRI MRS spectrum in the observed wavelength from 4.92-28.6$\mu$m , extracted over the point-source emission shown in the inserted 3-color MIRI imaging composite. The spectrum is dominated by hot dust emission from the central obscured quasar. The 3.3$\mu$m aromatic and 3.4$\mu$m aliphatic features are shown in dashed-dot green and blue lines at the systemic redshift of the source. The gray-shaded regions show the wavelength range used to estimate the continuum for fitting the aromatic and aliphatic absorptions. The bright galaxy in the top left of the insert is at $z=$0.092
• Figure 5: Corner plot from MCMC fitting to the normalized nuclear spectrum of W2246-0526.