Abstract
The Python Sky Model (PySM) is a Python package used by Cosmic Microwave Background (CMB) experiments to simulate maps, in HEALPix pixelization, of the various diffuse astrophysical components of Galactic emission relevant at CMB frequencies (i.e. dust, synchrotron, free-free and Anomalous Microwave Emission), as well as the CMB itself. These maps may be integrated over a given instrument bandpass and smoothed with a given instrument beam. PySM 2, released in 2016, has become the de-facto standard for simulating Galactic emission, for example it is used by CMB-S4, Simons Observatory, LiteBird, PICO, CLASS, POLARBEAR and other CMB experiments, as shown by the 80+ citations of the PySM 2 publication. As the resolution of upcoming experiments increases, the PySM 2 software has started to show some limitations, the solution to these issues was to reimplement PySM from scratch focusing on these features: reimplement all the models with the numba Just-In-Time compiler for Python to reduce memory overhead and optimize performance; use MPI through mpi4py to coordinate execution of PySM 3 across multiple nodes and rely on libsharp, for distributed spherical harmonic transforms; employ the data utilities infrastructure provided by astropy to download the input templates and cache them when requested. At this stage we strive to maintain full compatibility with PySM 2, therefore we implement the exact same astrophysical emission models with the same naming scheme. In the extensive test suite we compare the output of each PySM 3 model with the results obtained by PySM 2.
Highlights
Cosmologists need to create synthetic maps of the Cosmic Microwave Background (CMB) and of the galactic emission based on available data and on physical models that extrapolate observations to different frequencies
The Python Sky Model (PySM) is a Python package used by Cosmic Microwave Background (CMB) experiments to simulate maps, in HEALPix (Górski et al, 2005; Zonca et al, 2019) pixelization, of the various diffuse astrophysical components of Galactic emission relevant at CMB frequencies, as well as the CMB itself
These maps may be integrated over a given instrument bandpass and smoothed with a given instrument beam
Summary
The Cosmic Microwave Background (CMB) radiation, emitted just 370 thousand years after the Big Bang, is a pristine probe of the Early Universe. After being emitted at high temperatures, the CMB was redshifted by the subsequent 13.8 billion years of cosmic expansion, such that it is brightest at microwave frequencies today. Our own Milky Way galaxy emits in the microwave portion of the spectrum, obscuring our view of the CMB. Examples of this emission are thermal radiation by interstellar dust grains and synchrotron emission by relativistic electrons spiraling in magnetic fields. Cosmologists need to create synthetic maps of the CMB and of the galactic emission based on available data and on physical models that extrapolate observations to different frequencies. The resulting maps are useful to test data reduction algorithms, to understand residual systematics, to forecast maps produced by future instruments, to run Monte Carlo analysis for noise estimation, and more
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