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

TL;DR

SPHEREx's near-infrared spectral survey provides the first large-scale maps of diffuse PAH and ionized hydrogen emission in the Galactic plane, revealing their spatial relationship and PAH depletion in ionized regions.

Contribution

This study presents the first large-scale, all-sky maps of PAH and ionized hydrogen emission in the Galactic plane using SPHEREx data, highlighting their spatial correlation and PAH depletion in ionized regions.

Findings

Strong correlation between PAH emission and dust radiance.

Identification of PAH depletion within ionized gas regions.

Demonstration of ionizing radiation as a key factor in PAH variation.

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.