Galactic foregrounds​
​
​Galactic foregrounds are diffuse emissions originating from the interstellar medium of our own Galaxy (the Milky Way) that contaminate observations of the CMB and other extragalactic signals (e.g., Dickinson et al., 2016). They arise from a variety of complex astrophysical processes, including thermal emission from interstellar dust grains heated by starlight, synchrotron radiation from relativistic electrons spiralling in Galactic magnetic fields, free-free emission produced when free electrons are scattered by ions in ionised gas (thermal bremsstrahlung), anomalous microwave emission likely linked to spinning dust grains, and CO line emission from rotational transitions of carbon monoxide molecules in molecular clouds. Each of these components has a distinct spectral energy distribution (SED) and spatial morphology, but they often overlap in frequency with CMB and extragalactic signals, making accurate separation challenging. Among these, thermal dust emission dominates at frequencies above ~100 GHz, while synchrotron radiation dominates at low frequencies. Aside from astrophysical interest, careful characterization and removal of Galactic foregrounds is essential for precise CMB analysis, cosmological parameter estimation, and studies of large-scale structure.
​​​​
(Click on paper's title to access the corresponding publication)
​
An improved source-subtracted and destriped 408-MHz all-sky map
Remazeilles, Dickinson, Banday, Bigot-Sazy, Ghosh, MNRAS 451, 4311(2015)
​​
Synchrotron radiation arises when energetic cosmic-ray electrons, primarily from supernova explosions, are accelerated in the magnetic fields of our Galaxy. It is the dominant Galactic foreground at low frequencies in CMB observations, and also the main contaminant at radio frequencies for future line-intensity mapping experiments such as the Square Kilometre Array Observatory (SKAO), with brightness temperature exceeding the cosmological 21-cm line signal by several orders of magnitude. In this paper, we constructed a substantially improved full-sky template of Galactic synchrotron emission at 408 MHz by reprocessing the original survey of Haslam et al., A&AS (1982). Compared to earlier versions, we achieved significant improvements in (i) subtracting extragalactic radio sources, (ii) correcting for instrumental systematics (correlated 1/f noise), and (iii) characterizing the effective beam resolution. The resulting product, known as the "Reprocessed Haslam 408-MHz map", is now the reference map of the 408-MHz sky. It has been publicly released on the NASA’s LAMBDA archive, and remains the most widely used synchrotron template in radio astronomy.​
​
Planck Collaboration (corresponding author: Remazeilles; 150+ co-authors), A&A 596, A109 (2016)​​​​​​​​​​​
​
The thermal emission from interstellar dust grains in our Galaxy is a major Galactic foreground at high frequencies in CMB observations. Improving our understanding of the spectral and morphological properties of Galactic thermal dust emission is essential for the accurate recovery of CMB signals. A long-standing challenge has been to disentangle Galactic dust from extragalactic dust emission, the latter arising from the cumulative contribution of dusty, star-forming galaxies across cosmic time, known as the cosmic infrared background (CIB), which shares the same modified-blackbody spectral signature as Galactic dust. In this Planck Collaboration paper, for which I am the lead author, we developed a novel approach to separate Galactic thermal dust and CIB in Planck data. To overcome the spectral degeneracy between the two components, we applied the GNILC component-separation method (Remazeilles et al., MNRAS 2011), which exploits spatial and statistical information (power spectra) to disentangle components when their spectral information is degenerate. This method enabled us to disentangle Galactic dust emission from the CIB in Planck observations and to produce new full-sky maps of Galactic dust, along with templates of dust temperature and spectral index, with significantly reduced CIB contamination. The Planck GNILC dust maps have been publicly released through the Planck Legacy Archive, and one of them was even featured on the front cover of A&A (volume 596).
​
The Planck GNILC dust maps also serve as the Galactic dust templates for the d9, d10, and d11 models in the latest PySM3 simulation release, as described in the publication by The Pan-Experiment Galactic Science Group (2025).
​
Planck CO revisited: Improved CO line-emission maps from Planck space-mission observations
​Ghosh, Remazeilles, Delabrouille, A&A 688, A54 (2024)​
​
The Planck space mission observed the first three rotational transitions of Galactic carbon monoxide (CO), producing full-sky maps of CO line emission (Planck Collaboration XIII, A&A 2014). However, these initial maps suffer from significant noise and contamination by other astrophysical emissions. To overcome these limitations, we developed a dedicated post-processing pipeline based on an enhanced version of the GNILC technique (Remazeilles et al., MNRAS 2011), specifically adapted to CO extraction. This method yields well-characterised, full-sky CO maps at uniform 10′ angular resolution with strongly reduced noise and foreground contamination. The new maps, publicly released at NERSC, provide cleaner templates of Galactic CO line emission and are particularly valuable for realistic sky simulations in preparation for upcoming CMB experiments. Our maps were also featured on the front cover of A&A (volume 688). By enhancing the fidelity of CO mapping, this work supports both Galactic astrophysics and precision cosmology through more accurate foreground modelling.
​