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

TL;DR

This study presents ALMA Band 1 imaging of the rho Oph W filament to probe enhanced microwave emission (EME) at high spatial frequencies, testing the spinning-dust hypothesis. The authors extract a robust Band 1 spectrum via uv-plane cross-correlation with a deconvolved filament model, finding a power-law spectrum with a spatially varying slope $I_\nu \propto \nu^{\alpha}$ and a global index $\alpha = -0.78 \pm 0.05$, while revealing a notable increase in high-frequency power relative to IR morphologies. They detect a compact EME source without an infrared counterpart and place upper limits on carbon recombination lines and PAH combs, highlighting the limitations of infrared templates for detailed spectral indexing at Band 1 resolutions. Flux-loss corrections based on IRAC templates are essential for constructing multi-frequency SEDs, which will feed spinning-dust modelling in a companion paper and help constrain the carriers and physical conditions driving EME in PDRs.

Abstract

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.

Figures (14)

• Figure 1: ALMA Band 1 continuum imaging of the $\rho$ Oph W filament. Each image is shown in linear stretch, and $x-$ and $y-$ axis are offset along R.A. and Dec. from J2000 16:25:57.0 $-$24:20:48.0. a) Restored image, in natural weighting, after point-source subtraction, with a beam of 8696$\times$6436/--86deg (see text), and a noise of 5.55 $\mu$Jy beam$^{-1}$. b) Same as panel a, but before point-source subtraction. The three bright point sources are the young stellar objects SR 4 near the centre, ISO-Oph-17 to the east, and DoAr 21 to the south. The cyan circles are centred on the phase directions of each field and their radii correspond to half the radius of the first Airy null (corresponding to a primary beam attenuation of $\sim$0.34). c) Deconvolved model image, $I^m$, after point-source subtraction and with 1$^2$ pixels.
• Figure 2: Point source fits to DoAr 21, over individual interferometer scans, each $\sim$10 min long. The top and bottom rows respectively record spectral index, $\alpha$, and flux density, $F_\nu$, at the reference frequency of 40.15 GHz. The $x-$axis gives the Julian date, expressed in days from the start of the observations.
• Figure 3: Flux densities for DoAr 21, on 11-Mar-2024, extracted over 120-channel averages. The black line is the best fit point-source model from Table \ref{['table:PSs']}. Spectral windows are plotted in alternating colours.
• Figure 4: Flux densities for SR 4, extracted over 120-channel averages. The black line is the best fit point-source model from Table \ref{['table:PSs']}. Spectral windows are plotted in alternating colours.
• Figure 5: Spectrum of the diffuse emission from $\rho$ Oph W, as estimated with the cross-correlation slope, $a_\nu$. The black line is the best-fit power-law spectrum, with spectral index indicated in the legends. Plots a) to c) correspond to spectra extracted over different parts of the mosaic, as indicated in each plot. Field IDs follow from Fig. \ref{['fig:mosaic']}. Spectral windows are plotted in alternating colours.