Mapping water ice with infrared broadband photometry

University of Vienna, Department of Astrophysics, Türkenschanzstraße 17, 1180 Wien, Austria

^★ Corresponding author: stefan.meingast@univie.ac.at

Received: 16 May 2025
Accepted: 24 July 2025

Abstract

Interstellar ices play a fundamental role in the physical and chemical evolution of molecular clouds and star-forming regions, yet their large-scale distribution and abundance remain challenging to map. In this work, I present the ice color excess method (ICE), which parametrizes the peak optical depth (τ_3.0^max) of the prominent 3 μm absorption feature, which is predominantly caused by the presence of solid H₂O. The method builds on well-established near-infrared color excess techniques and uses widely available infrared broadband photometry. Through detailed evaluation of passband combinations and a comprehensive error analysis, I constructed the ICE color excess metric Λ(W₁ − I₁). This parameter emerges as the optimal choice that minimizes systematic errors while leveraging high-quality, widely available photometry from Spitzer and WISE data archives. To calibrate the method, I compiled from the literature a sample of stars located in the background of nearby molecular clouds for which spectroscopically measured optical depths are available. The empirical calibration yielded a remarkably tight correlation between τ_3.0^max and Λ(W₁ − I₁). This photometric technique opens a new avenue for tracing the icy component of the interstellar medium on Galactic scales, providing a powerful complement to spectroscopic surveys, and enables new insights into the environmental dependence of the formation and evolution of icy dust grains.