Planck 2018 results. IV. Diffuse component separation

TL;DR

This paper presents improved full-sky maps of the CMB and foreground emissions from Planck 2018 data, utilizing multiple component separation methods, and provides new measurements of dust and synchrotron spectral indices with reduced systematic biases.

Contribution

The paper introduces enhanced component separation maps from Planck 2018 data and independently measures the spectral indices of polarized dust and synchrotron emissions.

Findings

Polarized thermal dust spectral index beta_d = 1.55 +/- 0.05

Polarized synchrotron spectral index beta_s = -3.1 +/- 0.1

Significantly lower large-scale polarization power compared to previous data

Abstract

We present full-sky maps of the cosmic microwave background (CMB) and polarized synchrotron and thermal dust emission, derived from the third set of Planck frequency maps. These products have significantly lower contamination from instrumental systematic effects than previous versions. The methodologies used to derive these maps follow those described in earlier papers, adopting four methods (Commander, NILC, SEVEM, and SMICA) to extract the CMB component, as well as three methods (Commander, GNILC, and SMICA) to extract astrophysical components. Our revised CMB temperature maps agree with corresponding products in the Planck 2015 delivery, whereas the polarization maps exhibit significantly lower large-scale power, reflecting the improved data processing described in companion papers; however, the noise properties of the resulting data products are complicated, and the best available end-to-end simulations exhibit relative biases with respect to the data at the few percent level. Using these maps, we are for the first time able to fit the spectral index of thermal dust independently over 3 degree regions. We derive a conservative estimate of the mean spectral index of polarized thermal dust emission of beta_d = 1.55 +/- 0.05, where the uncertainty marginalizes both over all known systematic uncertainties and different estimation techniques. For polarized synchrotron emission, we find a mean spectral index of beta_s = -3.1 +/- 0.1, consistent with previously reported measurements. We note that the current data processing does not allow for construction of unbiased single-bolometer maps, and this limits our ability to extract CO emission and correlated components. The foreground results for intensity derived in this paper therefore do not supersede corresponding Planck 2015 products. For polarization the new results supersede the corresponding 2015 products in all respects.