2021MNRAS.503.2927C

Anomalous Microwave Emission (AME) is a significant component of Galactic diffuse emission in the frequency range 10-60 GHz and a new window into the properties of sub-nanometre-sized grains in the interstellar medium. We investigate the morphology of AME in the ≃10^* diameter λ Orionis ring by combining intensity data from the QUIJOTE experiment at 11, 13, 17, and 19 GHz and the C-Band All Sky Survey (C-BASS) at 4.76 GHz, together with 19 ancillary data sets between 1.42 and 3000 GHz. Maps of physical parameters at 1^* resolution are produced through Markov chain Monte Carlo (MCMC) fits of spectral energy distributions (SEDs), approximating the AME component with a lognormal distribution. AME is detected in excess of 20 σ at degree-scales around the entirety of the ring along photodissociation regions (PDRs), with three primary bright regions containing dark clouds. A radial decrease is observed in the AME peak frequency from ~35 GHz near the free-free region to ~21 GHz in the outer regions of the ring, which is the first detection of AME spectral variations across a single region. A strong correlation between AME peak frequency, emission measure and dust temperature is an indication for the dependence of the AME peak frequency on the local radiation field. The AME amplitude normalized by the optical depth is also strongly correlated with the radiation field, giving an overall picture consistent with spinning dust where the local radiation field plays a key role.