The C-Band All Sky Survey: New Insights into the Microwave Sky

  • Roke Cepeda-Arroita

Student thesis: Phd


Observational work on Galactic foregrounds with the C-Band All Sky Survey (C-BASS) at 4.76 GHz is presented in intensity and polarisation, with a focus on Anomalous Microwave Emission (AME). Observations with the QUIJOTE experiment, the 100 m Green Bank Telescope, and the CO Mapping Array Pathfinder are also used, together with multiwavelength data in the range 408 MHz - 0.7 micrometers. In addition, C-BASS data is analysed, quantifying the effectiveness of the removal of systematics through simulations and setting an optimal effective frequency. C-BASS provides the most sensitive map of polarised synchrotron emission at 1-degree scales, uncovering many large-scale Galactic features for the first time. This map will be crucial in the search for cosmological B-modes. An absolute measurement of polarisation fractions is presented, which shows a maximum of 27 +/- 2% near the North Polar Spur, implying that the magnetic fields in the NPS are significantly tangled at sub-parsec scales. A method of quantifying the coherence of polarisation angles is presented, and the physical nature of new sources is discussed, finding that spurs extend much further than the NPS, favouring the possibility of multiple local supernovae shells imprinting these large-scale loops. Spectral variations of AME across a single region are detected for the first time, with a radial decrease in the peak frequency across the Lambda Orionis PDR from 35 GHz in the inner region to 21 GHz. This suggests that the local radiation field plays a key role in determining the peak frequency by changing the size distribution of spinning dust carriers. A strong spatial correlation of AME with thermal dust emission and PAHs is seen at 1-degree scales, supporting the spinning dust hypothesis. This study also confirms PDRs as favourable regions for strong AME. The need for multiple datasets in the range 1-20 GHz to improve the Commander component separation is highlighted, since the present-day reduction suffers from significant degeneracies between AME and free-free emission. A follow-up study of the region at arcminute scales with the GBT and COMAP reveals that AME does not originate from compact dust cores; being largely diffuse, while the free-free emission is well traced by H-alpha emission. This breakdown of the correlation between dust and AME at smaller scales suggests that the spinning dust grains can coagulate in dense molecular clouds, leading to a reduced emissivity: a mechanism that spinning dust models do not account for. A preliminary analysis of C-BASS data using Commander reveals that AME is ubiquitous in the sky, uncovering a large filament of AME tracing a dust structure on the outside of the NPS; the largest AME source discovered to-date.
Date of Award31 Dec 2022
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorPaddy Leahy (Supervisor) & Clive Dickinson (Supervisor)


  • cosmological B-modes
  • B-modes
  • Astrophysics
  • CO Mapping Array Pathfinder
  • Green Bank Telescope
  • polarised synchrotron emission
  • Cosmology
  • spectral variations
  • polarisation fraction
  • peak frequency
  • photodissociation region
  • spinning dust carrier
  • Commander
  • polarisation angles
  • North Polar Spur
  • thermal dust emission
  • Galactic foregrounds
  • synchrotron emission
  • C-Band All Sky Survey
  • diffuse radiation
  • Anomalous Microwave Emission
  • radio astronomy
  • surveys
  • radio continuum
  • Cosmic Microwave Background
  • free-free emission
  • radiation mechanism: non-thermal
  • radiation mechanism: thermal
  • microwave emission
  • C-BASS
  • AME
  • component separation
  • spinning dust
  • interstellar medium

Cite this