Planck intermediate results

Abstract

The dust-H i correlation is used to characterize the emission properties of dust in the diffuse interstellar medium (ISM) from far infrared wavelengths to microwave frequencies. The field of this investigation encompasses the part of the southern sky best suited to study the cosmic infrared and microwave backgrounds. We cross-correlate sky maps from Planck, the Wilkinson Microwave Anisotropy Probe (WMAP), and the diffuse infrared background experiment (DIRBE), at 17 frequencies from 23 to 3000 GHz, with the Parkes survey of the 21 cm line emission of neutral atomic hydrogen, over a contiguous area of 7500 deg 2 centred on the southern Galactic pole. We present a general methodology to study the dust-H i correlation over the sky, including simulations to quantify uncertainties. Our analysis yields four specific results. (1) We map the temperature, submillimetre emissivity, and opacity of the dust per H-atom. The dust temperature is observed to be anti-correlated with the dust emissivity and opacity. We interpret this result as evidence of dust evolution within the diffuse ISM. The mean dust opacity is measured to be (7.1 ± 0.6) × 10 −27 cm 2 H −1 × (ν/353 GHz) 1.53 ± 0.03 for 100 ≤ ν ≤ 353 GHz. This is a reference value to estimate hydrogen column densities from dust emission at submillimetre and millimetre wavelengths. (2) We map the spectral index β mm of dust emission at millimetre wavelengths (defined here as ν ≤ 353 GHz), and find it to be remarkably constant at β mm = 1.51 ± 0.13. We compare it with the far infrared spectral index β FIR derived from greybody fits at higher frequencies, and find a systematic difference, β mm − β FIR = −0.15, which suggests that the dust spectral energy distribution (SED) flattens at ν ≤ 353 GHz. (3) We present spectral fits of the microwave emission correlated with H i from 23 to 353 GHz, which separate dust and anomalous microwave emission (AME). We show that the flattening of the dust SED can be accounted for with an additional component with a blackbody spectrum. This additional component, which accounts for (26 ± 6)% of the dust emission at 100 GHz, could represent magnetic dipole emission. Alternatively, it could account for an increasing contribution of carbon dust, or a flattening of the emissivity of amorphous silicates, at millimetre wavelengths. These interpretations make different predictions for the dust polarization SED. (4) We analyse the residuals of the dust-H i correlation. We identify a Galactic contribution to these residuals, which we model with variations of the dust emissivity on angular scales smaller than that of our correlation analysis. This model of the residuals is used to quantify uncertainties of the CIB power spectrum in a companion Planck paper.

Optically Thick H I Dominant in the Local Interstellar Medium: An Alternative Interpretation to “Dark Gas”

“…This trend is similar to what is found in the case of high-latitude clouds (Paper I). Recent studies by the Planck Collaboration have also found that T d increases with decreasing gas column density (Planck Collaboration et al 2014a, 2014b. By following the same argument as in Paper I, we expect that the H i emission is moderately optically thick in general and that the saturation due to the optical depth effect causes weaker W H i for lower T d and higher τ 353 .…”

Joint Analysis of BICEP2/Keck ArrayandPlanckData

“…We thus tested our assumption by measuring the β d of the dust total intensity in the BICEP2/Keck field using the template fitting analysis described in Ref. [44], and find the same value.…”

Development of the Model of Galactic Interstellar Emission for Standard Point-Source Analysis of Fermi Large Area Telescope Data

“…In the local ISM, Grenier et al (2005b) have shown that the use of H I, CO, and a dust residual map improves the fits to the interstellar γ-ray emission compared to using the sole dust map for the total gas. While the hydrogen is likely to have the same γ-ray emissivity per atom in the atomic, DNM, and molecular components of the ISM due to the good penetration of CRs at the GeV-TeV energies relevant for the LAT (Skilling & Strong 1976), dust opacity (optical depth per gas nucleon) is known to increase from the diffuse H I to dense H I and H 2 gas (Stepnik et al 2003;Abergel et al 2013Abergel et al , 2014Ade et al 2014b). Restricting the use of dust to tracing the DNM alleviates the impact of dust opacity gradients because the DNM spans a moderate range of gas volume densities and is rather diffuse in space.…”