SPHEREx mapping of diffuse PAH and H II emission in the Galactic plane

Abstract

We present preliminary SPHEREx maps of diffuse Galactic emission tracing polycyclic aromatic hydrocarbons (PAHs) and ionized hydrogen gas, and we study their relationship across the Galactic plane. Since its launch in early 2025, the SPHEREx space telescope has been conducting an all-sky near-infrared spectral survey from 0.75 to 5.0 microns. We produce a large-scale map of the 3.3-micron PAH emission feature, which is bright and detectable throughout the Galactic plane, and find a strong correlation with the thermal dust radiance measured by Planck. We also trace ionized hydrogen gas by producing a map of Brackett-alpha emission at 4.05 microns. By combining the two maps, we identify extended shells of PAH emission associated with photodissociation regions surrounding ionized gas. We construct a PAH abundance map and find a significant anticorrelation between PAH abundance and ionized hydrogen, indicating systematic PAH depletion within ionized gas regions across the Galactic plane and demonstrating that ionizing radiation is a dominant driver of PAH abundance variations. These early SPHEREx results provide a large-scale view of PAHs and ionized hydrogen and preview the capability of the mission to map diffuse emission in the interstellar medium.

Figures (9)

• Figure 1: Example Band-4 image illustrating the processing steps used to extract diffuse emission. Left: L2 calibrated spectral image dominated by stars and galaxies. Wavelength varies continuously from $2.42~\mu\mathrm{m}$ at the top to $3.82~\mu\mathrm{m}$ at the bottom due to the LVF response. The bright, nearly horizontal band corresponds to enhanced emission in the wavelength range containing the $3.3$-$\mu\mathrm{m}$ PAH feature. Middle: the same exposure after source masking, sigma clipping, flagging, and smoothing, revealing diffuse emission. Right: an extracted wavelength range centered on the $3.3$-$\mu\mathrm{m}$ PAH feature ($\lambda_{\rm min} = 3.26~\mu\mathrm{m}$ to $\lambda_{\rm max} = 3.36~\mu\mathrm{m}$), used to construct the PAH emission map. Different wavelength ranges can be extracted to produce maps of other spectral features.
• Figure 2: Full-sky maps for three representative wavelengths, shown in Mollweide projection in Galactic coordinates. Top row: full-sky maps at $0.76~\mu\mathrm{m}$, $3.3~\mu\mathrm{m}$ near the PAH feature, and $4.05~\mu\mathrm{m}$ near Br$\alpha$, produced by directly extracting and reprojecting individual spectral channels without source masking. In addition to stellar emission, a band of zodiacal light running from the lower left to the upper right of each map is visible, especially at $0.76~\mu\mathrm{m}$. It shows discrete discontinuities arising from time variability of the observed zodiacal light. Middle row: corresponding hit-count maps indicating the number of contributing exposures per pixel. Bottom row: simulated zodiacal-light full-sky maps processed through the same pipeline. The similarity between the simulated and observed patterns highlights the contribution of zodiacal emission to the real maps.
• Figure 3: Maps of diffuse emission along the Galactic plane after continuum subtraction and application of the Planck 70% Galactic mask. The gap near the Galactic center reflects exposures excluded due to large astrometric uncertainties. Improved astrometric solutions in future data releases are expected to make this region accessible for this type of analysis. Top: Map of $3.3$-$\mu\mathrm{m}$ PAH emission. Bottom: Map of $4.05$-$\mu\mathrm{m}$ Br$\alpha$ emission tracing ionized gas. Some stripes are visible as artifacts from the scanning pattern and time-variable foregrounds.
• Figure 4: Composite RGB map of the Galactic plane. Red shows $3.3$-$\mu\mathrm{m}$ PAH emission, and blue shows $4.05$-$\mu\mathrm{m}$ Br$\alpha$. These are overlaid on a map representing continuum emission formed from three channels spanning the SPHEREx wavelength range: 0.8 (blue), 2.5 (green), and $4.2~\mu\mathrm{m}$ (red). This rendering highlights the spatial relationship between the diffuse components with PAH emission outlining shells around regions of ionized gas.
• Figure 5: Composite RGB map of the Orion Nebula using the same visualization scheme as Figure \ref{['fig:rgb']}. Red shows $3.3$-$\mu\mathrm{m}$ PAH emission, and blue shows $4.05$-$\mu\mathrm{m}$ Br$\alpha$. These are overlaid on a map representing continuum emission formed from three channels spanning the SPHEREx wavelength range: 0.8 (blue), 2.5 (green), and $4.2~\mu\mathrm{m}$ (red). The image illustrates the spatial separation between PAH emission and ionized gas at high resolution, consistent with the large-scale PAH depletion observed in the Galactic plane maps.