Periodic Reporting for period 1 - RadioForegroundsPlus (Unveiling the complexity of radio foregrounds for the detectability of the CMB polarization B-mode)
Période du rapport: 2024-01-01 au 2025-03-31
The cosmic microwave background (CMB) is a faint radiation originating shortly after the Big Bang, detected uniformly across the sky. It is the oldest light we can observe and provides valuable insights into the early universe and its evolution. Detecting a specific pattern in this radiation, known as the B-mode of CMB polarization, would provide strong evidence for cosmic inflation —a very brief period of extremely rapid expansion in the very early universe— and would represent a major breakthrough in physics. For this reason, a large global effort is underway to search for the primordial polarization B-mode.
However, this signal is extremely weak and is obscured by the presence of contaminant emissions – known as foregrounds – arising from our own Galaxy and other astrophysical sources. Successfully detecting the primordial signal requires separating the true CMB from these contaminants, demanding precise knowledge of foregrounds, sophisticated component separation techniques, and realistic observational simulations. Moreover, beyond their role as contaminants, these foregrounds are highly valuable on their own, offering important information about a variety of astrophysical phenomena.
The RadioForegroundsPlus project addresses the challenge of cleaning CMB observations by delivering a state-of-the art description of the complex properties of foregrounds, developing advanced data analysis techniques, and producing templates and tools for generating realistic simulations. It focuses especially, although not exclusively, on Galactic radio foregrounds —including synchrotron, anomalous microwave emission (AME), and free-free— which dominate at the lower frequency range of CMB observations. It also aims to provide updated catalogues of extragalactic point sources (emission from other galaxies), study the Galactic magnetic field, and characterise the thermal dust emission that dominates at higher frequencies.
To achieve these goals, the project combines data from the European Planck satellite with three unique low frequency, ground-based experiments: QUIJOTE, C-BASS and S-PASS, making use of advanced data analysis techniques. These efforts are very relevant for preparing and supporting current and future CMB experiments (e.g. LiteBIRD, CMB-S4, Simons Observatory) and for forecasting their capability to detect B-mode polarization.
However, this signal is extremely weak and is obscured by the presence of contaminant emissions – known as foregrounds – arising from our own Galaxy and other astrophysical sources. Successfully detecting the primordial signal requires separating the true CMB from these contaminants, demanding precise knowledge of foregrounds, sophisticated component separation techniques, and realistic observational simulations. Moreover, beyond their role as contaminants, these foregrounds are highly valuable on their own, offering important information about a variety of astrophysical phenomena.
The RadioForegroundsPlus project addresses the challenge of cleaning CMB observations by delivering a state-of-the art description of the complex properties of foregrounds, developing advanced data analysis techniques, and producing templates and tools for generating realistic simulations. It focuses especially, although not exclusively, on Galactic radio foregrounds —including synchrotron, anomalous microwave emission (AME), and free-free— which dominate at the lower frequency range of CMB observations. It also aims to provide updated catalogues of extragalactic point sources (emission from other galaxies), study the Galactic magnetic field, and characterise the thermal dust emission that dominates at higher frequencies.
To achieve these goals, the project combines data from the European Planck satellite with three unique low frequency, ground-based experiments: QUIJOTE, C-BASS and S-PASS, making use of advanced data analysis techniques. These efforts are very relevant for preparing and supporting current and future CMB experiments (e.g. LiteBIRD, CMB-S4, Simons Observatory) and for forecasting their capability to detect B-mode polarization.
We are advancing significantly towards achieving our objectives. Notably, we have progressed in the development and application of various component separation techniques aimed at the separation of Galactic emissions. In a first step, we have generated a full-sky noise-suppressed synchrotron polarised emission map from Planck data, which is also free from CMB -- essential for accurate B-mode science.
We have also reprocessed a thermal dust map to allow its integration within the PySM package, the community-standard tool for simulations. In parallel, work is ongoing to develop an optimal sky clustering strategy, partitioning the sky into regions of more uniform foreground properties. This is important for enhancing the performance of component separation techniques, which often struggle with the anisotropic nature of the sky (i.e. the strong spatial variation of foreground properties). The optimization of a machine-learning method for point source detection is also showing promising results and paving the way for enhanced catalogues.
Other efforts include the development and public release of two software tools: SpyDust, a Python package for modelling spinning dust emission (believed to be the dominant mechanism behind AME), and ForSE+, for simulating realistic Galactic foregrounds including small angular scales. Scientific analyses are also underway to characterize the properties of foregrounds as well as to better understand the structure of the Galactic Magnetic Field. Key results include maps of the synchrotron spectral index using C-BASS and ancillary data, the modelling of AME intensity with our full datasets, the tightest constraints to date on AME polarization fractions in three star-forming regions using QUIJOTE and complementary data or the study of a moment expansion formalism to characterise the spectral and spatial properties of polarized thermal dust emission and its effect on component separation.
The project is also supporting the work of different CMB collaborations, with important contributions to activities such as science forecasts for the Simons Observatory (ground-based CMB experiment located in Chile) or the study of the gain calibration requirements for LiteBIRD (a CMB space mission led by JAXA but with important contributions from Europe and North America).
We have also reprocessed a thermal dust map to allow its integration within the PySM package, the community-standard tool for simulations. In parallel, work is ongoing to develop an optimal sky clustering strategy, partitioning the sky into regions of more uniform foreground properties. This is important for enhancing the performance of component separation techniques, which often struggle with the anisotropic nature of the sky (i.e. the strong spatial variation of foreground properties). The optimization of a machine-learning method for point source detection is also showing promising results and paving the way for enhanced catalogues.
Other efforts include the development and public release of two software tools: SpyDust, a Python package for modelling spinning dust emission (believed to be the dominant mechanism behind AME), and ForSE+, for simulating realistic Galactic foregrounds including small angular scales. Scientific analyses are also underway to characterize the properties of foregrounds as well as to better understand the structure of the Galactic Magnetic Field. Key results include maps of the synchrotron spectral index using C-BASS and ancillary data, the modelling of AME intensity with our full datasets, the tightest constraints to date on AME polarization fractions in three star-forming regions using QUIJOTE and complementary data or the study of a moment expansion formalism to characterise the spectral and spatial properties of polarized thermal dust emission and its effect on component separation.
The project is also supporting the work of different CMB collaborations, with important contributions to activities such as science forecasts for the Simons Observatory (ground-based CMB experiment located in Chile) or the study of the gain calibration requirements for LiteBIRD (a CMB space mission led by JAXA but with important contributions from Europe and North America).
RadioForegroundsPlus is advancing beyond the state of the art in several aspects. First, our goals can only be achieved by combining the ESA’s Planck satellite maps with the unique low frequency surveys used in this project (QUIJOTE, C-BASS and S-PASS). By having within the team experts on the previous data sets, we are in a privileged position to perform the required analyses and to provide a state-of-the-art description of radio foregrounds.
A major achievement so far is our work on AME. We are producing the most comprehensive catalogue of compact AME sources in intensity to date, while also imposing the tightest constraints yet on AME polarization in specific Galactic regions. We remark that the properties of this emission are still uncertain and, in particular, it has not been confirmed whether it is polarised. Even a low polarization fraction at the 1% level could significantly bias the weak signature of the B-mode of polarization. Therefore, the results obtained by RadioForegroundsPlus are essential for minimizing the impact of this foreground on future CMB experiments and ensuring the reliability of their cosmological conclusions.
The project is also advancing the field through the development and application of cutting-edge component separation methods. These include, the production for the first time of synchrotron polarized emission maps using the GNILC technique, the generation of optimal sky partitioning strategies to deal with foreground anisotropies, and the development of a machine-learning approach for the detection of point sources. Moreover, all derived data products will be released to the community and, whenever possible, integrated within widely-used repositories or software packages.
The data products and models generated by RadioForegroundsPlus are expected to provide significant added value to the CMB community, improving forecasts and strengthening Europe's role in global initiatives to characterize and mitigate foregrounds in current and future CMB experiments.
A major achievement so far is our work on AME. We are producing the most comprehensive catalogue of compact AME sources in intensity to date, while also imposing the tightest constraints yet on AME polarization in specific Galactic regions. We remark that the properties of this emission are still uncertain and, in particular, it has not been confirmed whether it is polarised. Even a low polarization fraction at the 1% level could significantly bias the weak signature of the B-mode of polarization. Therefore, the results obtained by RadioForegroundsPlus are essential for minimizing the impact of this foreground on future CMB experiments and ensuring the reliability of their cosmological conclusions.
The project is also advancing the field through the development and application of cutting-edge component separation methods. These include, the production for the first time of synchrotron polarized emission maps using the GNILC technique, the generation of optimal sky partitioning strategies to deal with foreground anisotropies, and the development of a machine-learning approach for the detection of point sources. Moreover, all derived data products will be released to the community and, whenever possible, integrated within widely-used repositories or software packages.
The data products and models generated by RadioForegroundsPlus are expected to provide significant added value to the CMB community, improving forecasts and strengthening Europe's role in global initiatives to characterize and mitigate foregrounds in current and future CMB experiments.