Cosmoglobe DR2. V. Spatial correlations between thermal dust and ionized carbon emission in Planck HFI and COBE-DIRBE

TL;DR

This study analyzes the spatial correlations between thermal dust and ionized carbon emission across multiple frequencies, revealing the dominance of ionized carbon at high frequencies and demonstrating that a simple model with isotropic components captures most of the observed signal.

Contribution

It introduces a multi-tracer fitting approach to map physical environments of dust and ionized carbon emission, highlighting the dominance of C ii at high frequencies and the effectiveness of a simple isotropic SED model.

Findings

C ii dominates emission above 1 THz, accounting for nearly all signal above 10 THz.

A simple model with five isotropic components explains 98% of the signal below 1 THz.

C ii has an effective temperature of about 25 K, the hottest among the components.

Abstract

We fit five tracers of thermal dust emission to ten Planck HFI and COBE-DIRBE frequency maps between 353 GHz and 25 THz, aiming to map the relative importance of each physical host environment as a function of frequency and position on the sky. Four of these correspond to classic thermal dust tracers, namely H i (HI4PI), CO (Dame et al. 2001a), H{\alpha} (WHAM, Haffner et al. (2003a, 2016)), and dust extinction (Gaia; Edenhofer et al. 2024), while the fifth is ionized carbon (C ii) emission as observed by COBE- FIRAS. We jointly fit these five templates to each frequency channel through standard multi-variate linear regression. At frequencies higher than 1 THz, we find that the dominant tracer is in fact C ii, and above 10 THz this component accounts for almost the entire fitted signal; at frequencies below 1 THz, its importance is second only to H i. We further find that all five components are well described by a modified blackbody spectral energy density (SED) up to some component-dependent maximum frequency ranging between 1 and 5 THz. In this interpretation, the C ii-correlated component is the hottest among all five, with an effective temperature of about 25 K. The H{\alpha} component has a temperature of 18 K, and, unlike the other four, is observed in absorption rather than emission. Despite the simplicity of this model, which relies only on external templates coupled to spatially isotropic SEDs, we find that it captures 98 % of the full signal root mean squared (RMS) below 1 THz. This high efficiency suggests that spatial variations in the thermal dust SED, as for instance reported by Planck and other experiments, may be more economically modelled on large angular scales in terms of a spatial mixing of individually isotropic physical components.