ALMA Band 1 observations of the rho Oph W filament. I. Enhanced power from excess microwave emission at high spatial frequencies

The ̊ho,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 ̊ho,Oph,W drops abruptly above ∼30,GHz and its spectrum can be interpreted in terms of electric-dipole radiation from spinning dust grains (or `spinning dust'). Deep and high-fidelity imaging and spectroscopy of ̊ho,Oph,W could 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 could constrain the spectral index of the high-frequency drop. 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 yielded estimates of flux loss and biases. The spectrum is a power law, with no detectable fine structure. It follows a spectral index α = -0.78 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. Finally, we provide multi-frequency intensity estimates for spectral modelling.