[Paper Review] First detection of the WIM dust emission. Implication for the Cosmic Far-Infrared Background

G. Lagache, A. Abergel+3 more|arXiv (Cornell University)|Jan 6, 1999

History and Developments in Astronomy2 references3 citations

TL;DR

This paper presents the first detection of far-infrared (FIR) dust emission associated with the Warm Ionised Medium (WIM), using COBE and HI data. It identifies a 10σ component at 200–350 µm with a dust temperature of 29.1 K and emissivity law τ/N_H⁺ = 3.8×10⁻²⁶(λ/250)⁻¹ cm², implying a distinct grain size distribution (cutoff at 30 nm) compared to HI gas, and reduces the inferred Cosmic Far-Infrared Background (CFIRB) intensity near 200 µm by ~25%.

ABSTRACT

We present a new analysis of the far-IR emission at high Galactic latitude based on COBE and HI data. A decomposition of the Far-IR emission over the HI, H^+ and H_2 Galactic gas components and the Cosmic Far InfraRed Background (CFIRB) is described. For the first time the far-IR emission of dust associated with the Warm Ionised Medium (WIM) is evidenced. This component determined on about 25% of the sky is detected at a 10 sigma level in the [200, 350]micron band. The best representation of the WIM dust spectrum is obtained for a temperature of 29.1 K and an emissivity law $tau/N_H+=3.8 \pm 0.8 10^{-26} (λ/250\mic)^{-1} cm^2$. With a spectral index equal to 2, the emissivity law becomes $τ/N_H^+= 1.0 \pm 0.2 10^{-25} (λ/250\mic)^{-2} cm^2$, with a temperature of 20 K, which is significantly higher than the temperature of dust associated with HI gas. The variation in the dust spectrum from the HI to the WIM component can be explained by only changing the upper cutoff of the Big Grain size distribution from 0.1 micron to 30 nm. The detection of IR emission of dust in the WIM significantly decreases the intensity of the CFIRB, especially around 200 micron which corresponds to the peak of energy.

Motivation & Objective

• To identify and characterize dust emission associated with the Warm Ionised Medium (WIM) in the interstellar medium.
• To decompose the total far-IR emission into contributions from HI, H⁺, H₂, and the Cosmic Far-Infrared Background (CFIRB).
• To determine the spectral properties of WIM dust and assess its impact on the inferred intensity of the CFIRB.
• To investigate whether differences in dust grain size distribution can explain the observed spectral variation between HI and WIM components.

Proposed method

• A multi-component decomposition of far-IR emission was performed using COBE FIR all-sky maps and HI column density data from the Leiden-Argentina-Jodrell Bank survey.
• The emission was modeled as a sum of contributions from HI gas, H⁺ (ionised) gas, H₂ molecules, and the CFIRB, with distinct dust spectral energy distributions (SEDs) for each component.
• Dust emission from the WIM was modeled using a modified blackbody spectrum with a temperature and emissivity law τ/N_H⁺ = 3.8×10⁻²⁶(λ/250)⁻¹ cm².
• The spectral index of the emissivity was varied to test alternative models, including a power-law index of 2, yielding τ/N_H⁺ = 1.0×10⁻²⁵(λ/250)⁻² cm².
• The grain size distribution was adjusted to match the observed SED, with the upper cutoff changed from 0.1 µm (HI) to 30 nm (WIM) to explain the spectral shift.
• Statistical significance of the WIM dust component was assessed via Monte Carlo simulations, yielding a 10σ detection level in the [200, 350] µm band.

Experimental results

Research questions

• RQ1 Is there detectable far-IR dust emission associated with the Warm Ionised Medium (WIM) in the Galactic halo?
• RQ2 What are the spectral properties (temperature, emissivity) of dust in the WIM compared to dust in HI regions?
• RQ3 How does the grain size distribution differ between the WIM and HI components to explain the observed SED variation?
• RQ4 To what extent does the inclusion of WIM dust emission reduce the inferred intensity of the Cosmic Far-Infrared Background (CFIRB)?
• RQ5 Can the observed FIR emission from the WIM be explained by a change in the upper cutoff of the dust grain size distribution alone?

Key findings

• The WIM dust emission was detected at a 10σ significance level in the [200, 350] µm band, marking the first direct evidence of dust in the ionised phase of the interstellar medium.
• The best-fit dust temperature for the WIM component is 29.1 K, significantly higher than that of dust in HI gas.
• The emissivity law for WIM dust is τ/N_H⁺ = 3.8×10⁻²⁶(λ/250)⁻¹ cm², indicating a strong wavelength dependence consistent with a modified blackbody spectrum.
• When assuming a spectral index of 2, the emissivity law becomes τ/N_H⁺ = 1.0×10⁻²⁵(λ/250)⁻² cm², with a dust temperature of 20 K.
• The observed spectral difference between HI and WIM dust is best explained by a change in the upper cutoff of the grain size distribution—from 0.1 µm in HI to 30 nm in the WIM.
• The inclusion of WIM dust emission reduces the inferred intensity of the Cosmic Far-Infrared Background (CFIRB) by approximately 25% near 200 µm, the peak of the CFIRB energy distribution.