We present an analysis of the diffuse emission at 5 GHz in the first quadrant of the Galactic plane using two months of preliminary intensity data taken with the C-Band All Sky Survey (C-BASS) northern instrument at the Owens Valley Radio Observatory, California.Combining C-BASS maps with ancillary data to make temperature-temperature plots we find synchrotron spectral indices of β = −2.65 ± 0.05 between 0.408 GHz and 5 GHz and β = −2.72 ± 0.09 between 1.420 GHz and 5 GHz for −10 • < |b| < −4 • , 20 • < l < 40 • . Through the subtraction of a radio recombination line (RRL) free-free template we determine the synchrotron spectral index in the Galactic plane (|b| < 4 • ) to be β = −2.56 ± 0.07 between 0.408 GHz and 5 GHz, with a contribution of 53 ± 8 per cent from free-free emission at 5 GHz. These results are consistent with previous low frequency measurements in the Galactic plane.By including C-BASS data in spectral fits we demonstrate the presence of anomalous microwave emission (AME) associated with the Hii complexes W43, W44 and W47 near 30 GHz, at 4.4 σ, 3.1 σ and 2.5 σ respectively. The CORNISH VLA 5 GHz source catalogue rules out the possibility that the excess emission detected around 30 GHz may be due to ultra-compact Hii regions. Diffuse AME was also identified at a 4σ level within 30 • < l < 40 • , −2 • < b < 2 • between 5 GHz and 22.8 GHz.

The success of H i intensity mapping is largely dependent on how well 21cm foreground contamination can be controlled. In order to progress our understanding further, we present a range of simulated foreground data from two different ∼3000 deg2 sky regions, with varying effects from polarisation leakage. Combining these with cosmological H i simulations creates a range of intensity mapping test cases that require different foreground treatments. This allows us to conduct the most generalised study to date into 21cm foregrounds and their cleaning techniques for the post-reionisation era. We first provide a pedagogical review of the most commonly used blind foreground removal techniques (PCA/SVD, FASTICA, GMCA). We also trial a non-blind parametric fitting technique and discuss potential hybridization of methods. We highlight the similarities and differences in these techniques finding that the blind methods produce near equivalent results, and we explain the fundamental reasons for this. Our results demonstrate that polarised foreground residuals should be generally subdominant to H i on small scales (k ≳ 0.1 h Mpc−1). However, on larger scales, results are more case-dependent. In some cases, aggressive cleans severely damp H i power but still leave dominant foreground residuals. We find a changing polarisation fraction has little impact on results within a realistic range (0.5% - 2%), however a higher level of Faraday rotation does require more aggressive cleaning. We also demonstrate the gain from cross-correlations with optical galaxy surveys, where extreme levels of residual foregrounds can be circumvented. However, these residuals still contribute to errors and we discuss the optimal balance between over- and under-cleaning.