ALMA Band1 observations of the rhoOphW filament I. Enhanced power from excess microwave emission at high spatial frequencies

The rhoOphW 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 rhoOphW drops abruptly above ~30GHz 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 rhoOphW 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. An ALMA Band1 mosaic yields a deep deconvolved image of the filament at 36-44GHz, which we use 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. The spectrum is a power law, with no detectable fine structure. It follows a spectral index alpha=-0.78+-0.05, in frequency, with some variations along the filament. Interestingly, the Band1 power at high spatial frequencies increases relative to that of the IR signal, with a factor of two more power in Band1 at ~20’’ than at ~100’’ (relative to IRAC3.6um). An extreme of such radio-only structures is a compact EME source, without IR counterpart. It is embedded in strong and filamentary Band1 signal, while the IRAC maps are smooth in the same region. We provide multi-frequency intensity estimates for spectral modelling.

💡 Research Summary

This paper presents high‑resolution ALMA Band 1 (36–44 GHz) observations of the ρ Ophiuchi W (ρ Oph W) photodissociation region (PDR), aiming to characterize the excess microwave emission (EME, also known as anomalous microwave emission, AME) that dominates the centimetre‑wave continuum in this source. The authors acquired a 16‑point mosaic with the 12 m array, covering four spectral windows centred at 37.2, 39.1, 41.2 and 43.1 GHz, each 1.875 GHz wide. Data were calibrated with the standard ALMA pipeline and imaged using the GPU‑uvmem package, which fits a pixel‑by‑pixel model directly to the visibilities via a maximum‑entropy regularisation. After testing, the regularisation parameter was set to zero (λ = 0) and the optimisation stopped after ten iterations to avoid bias in the spectral extraction.

Three bright point sources (SR 4, ISO‑Oph 17, and DoAr 21) were identified in the mosaic. Their positions, flux densities at a reference frequency of 40.15 GHz, and spectral indices were determined by fitting directly in the uv‑plane and subsequently subtracted from the visibilities. DoAr 21 displayed strong, highly variable non‑thermal emission, while the other two sources showed spectra consistent with the Rayleigh‑Jeans tail of sub‑mm dust.

The Band 1 image, after point‑source subtraction, was used as a spatial template to extract the continuum spectrum via cross‑correlation in the uv‑plane. The resulting spectrum follows a pure power law, Iν ∝ ν^α, with a spectral index α = ‑0.78 ± 0.05 across the full 36–44 GHz band. No fine spectral features—such as the predicted PAH rotational “comb” lines or carbon recombination lines—were detected, indicating that the current sensitivity and channel resolution are insufficient to reveal such structures.

A spatial power analysis, comparing the Band 1 data with Spitzer/IRAC 3.6 µm maps, shows that at small angular scales (~20″) the radio power is roughly twice that of the infrared, whereas at larger scales (~100″) the two are comparable. This demonstrates that the high‑frequency radio emission is more concentrated in fine filamentary structures than the infrared PAH emission, confirming a breakdown of the usual radio‑IR correlation at high resolution. Notably, the authors identify a compact radio‑only feature embedded within the filament that has no counterpart in the IRAC images, suggesting the presence of a radio‑only EME component not accounted for in standard spinning‑dust models.

The authors estimate that only about 30 % of the total flux is recovered in the deconvolved image, primarily due to missing short spacings that filter out the most extended emission. Simulations and cross‑correlations with near‑infrared data were used to quantify this flux loss and assess systematic biases.

In conclusion, ALMA Band 1 provides a substantial improvement over previous ATCA and CBI observations, delivering high‑fidelity imaging of EME without relying on infrared priors. The measured power‑law spectrum and the enhanced small‑scale radio power support the spinning‑dust hypothesis but also reveal spatial variations that may be driven by local physical conditions (e.g., grain size distribution, ion density, or radiation field). The detection of a radio‑only compact source points to additional, perhaps transient, emission mechanisms. The paper sets the stage for a companion study that will fit detailed spinning‑dust models to the multi‑frequency spectral energy distribution derived here.