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.
Keywords
Cite
@article{arxiv.2603.23620,
title = {SPHEREx mapping of diffuse PAH and H II emission in the Galactic plane},
author = {Giulia Murgia and Ari J. Cukierman and Brandon S. Hensley and Matthew L. N. Ashby and James J. Bock and Tzu-Ching Chang and Shuang-Shuang Chen and Yun-Ting Cheng and Yi-Kuan Chiang and Asantha Cooray and Brendan P. Crill and Olivier Doré and C. Darren Dowell and Andreas L. Faisst and Joseph L. Hora and Howard Hui and Miju Kang and Jae Hwan Kang and Phil M. Korngut and Dennis Lee and Jeong-Eun Lee and Bomee Lee and Carey M. Lisse and Daniel C. Masters and Gary J. Melnick and Mary H. Minasyan and Chi H. Nguyen and Roberta Paladini and Volker Tolls and Robin Y. Wen and Michael W. Werner and Michael Zemcov},
journal= {arXiv preprint arXiv:2603.23620},
year = {2026}
}
Comments
15 pages, 9 figures