ALMA Band 1 observations of the

Context. The ρ Oph W photo-dissociation region (PDR) is an example source of bright excess microwave emission (EME), over synchrotron, free-free, and the Rayleigh-Jeans tail of the sub-millimetre (sub-mm) dust continuum. Its filamentary morphology follows roughly that of the IR poly-cyclic aromatic hydrocarbon (PAHs) bands. The EME signal in ρ Oph W drops abruptly above ~30GHz and its spectrum can be interpreted in terms of electric-dipole radiation from spinning dust grains (or ‘spinning dust’). Aims. Deep and high-fidelity imaging and spectroscopy of ρ Oph W may reveal the detailed morphology of the EME signal, free from imaging priors, while also enabling a search for fine structure in its spectrum. The same observations may constrain the spectral index of the high-frequency drop. Methods. An ALMA Band 1 mosaic yields a deep deconvolved image of the filament at 36-44 GHz, which we used as template for the extraction of a spectrum via cross-correlation in the uv plane. Simulations and cross-correlations on near-infrared ancillary data yield estimates of flux loss and biases. Results. The spectrum is a power law, with no detectable fine structure. It follows a spectral index α = −0.78 ± 0.05, in frequency, with some variations along the filament. Interestingly, the Band 1 power at high spatial frequencies increases relative to that of the IR signal, with a factor of two more power in Band 1 at ~20" than at ~100 " (relative to IRAC 3.6 μm). An extreme example of such radio-only structures is a compact EME source, without an IR counterpart. It is embedded in strong and filamentary Band 1 signal, while the IRAC maps are smooth in the same region. We provide multi-frequency intensity estimates for spectral modelling.