Full-sky Models of Galactic Microwave Emission and Polarization at Sub-arcminute Scales for the Python Sky Model

TL;DR

This work tackles the challenge of Galactic foregrounds in CMB polarization by developing high-fidelity, full-sky models for dust and synchrotron emission that incorporate a polarization fraction tensor to couple intensity and polarization and enable stochastic realizations of small-scale structure. Implemented in PySM3, the models leverage GNILC- and Planck-based templates and extend to sub-arcminute scales while preserving large-scale constraints, providing ensembles of skies reflecting current uncertainties. The authors validate the models against Planck/NPIPE data and BK18 constraints, demonstrating improvements over prior PySM versions, especially in dust BB power on small scales and in frequency decorrelation behavior. They present three model suites (low, medium, high complexity) spanning a range of astrophysical realism and provide public data products to facilitate forecasting and joint analyses across experiments.

Abstract

Polarized foreground emission from the Galaxy is one of the biggest challenges facing current and upcoming cosmic microwave background (CMB) polarization experiments. We develop new models of polarized Galactic dust and synchrotron emission at CMB frequencies that draw on the latest observational constraints, that employ the ``polarization fraction tensor'' framework to couple intensity and polarization in a physically motivated way, and that allow for stochastic realizations of small-scale structure at sub-arcminute angular scales currently unconstrained by full-sky data. We implement these models into the publicly available Python Sky Model (PySM) software and additionally provide PySM interfaces to select models of dust and CO emission from the literature. We characterize the behavior of each model by quantitatively comparing it to observational constraints in both maps and power spectra, demonstrating an overall improvement over previous PySM models. Finally, we synthesize models of the various Galactic foreground components into a coherent suite of three plausible microwave skies that span a range of astrophysical complexity allowed by current data.

Figures (18)

• Figure 1: An overview of the steps for building model d10. 1. We begin with GNILC-derived $I$, $Q$, $U$ maps of the polarized dust emission at 353 GHz ("amplitude parameters", purple) and maps of the GNILC-derived dust temperature and spectral index ("spectral parameters", brown). 2. The amplitude parameters are transformed via the polarization tensor fraction formalism to $i$, $q$, $u$. 3. Both sets of maps are filtered by scale: large scales are untouched, and small-scale map structure is replaced by structure synthesized from a power spectrum extrapolated from a power-law fit to the large-scale data. 4. The small-scale structure maps are modulated by maps of the large-scale structure in either total intensity or polarization: the fluctuations in $i$ and the spectral parameters are modulated by $m_i$, and the fluctuations in $q$ and $u$ are modulated by $m_p$. 5. The large- and small-scale maps are combined. 6. The $i$, $q$, $u$ maps are transformed back to $I$, $Q$, and $U$. 7. Together, the final amplitude maps at 353 GHz, and the final spectral parameter maps, which define how the sky varies with frequency, fully specify the d10 model.
• Figure 2: Model parameters for synthesizing small-scale emission
• Figure 3: Model parameters for synthesizing spectral parameter maps at small scales
• Figure 4: Patches $(16.7^\circ\times 16.7^\circ)$ of dust intensity at 353 GHz centered at $(l,b) =(180^\circ,-10^\circ)$ (left, "low latitude") and $(l,b) =(318^\circ,-61^\circ)$ (right, "high latitude" that is centered on the BICEP/Keck field) with an angular resolution of $4.94\arcmin$ for the d1, d9, and d12 dust models and Planck PR3 data.
• Figure 5: The same sky regions as Figure \ref{['fig:353_int']} shown in 353 GHz polarized dust intensity for the d1, d9, and d12 dust models and Planck PR3 data.