Detection of non-thermal radio emission components from the Orion Nebula: stellar jets, cloud collision or feedback from stellar winds?

Abstract

The Orion Nebula is the closest high-mass star-forming region, making it an ideal laboratory to investigate physical processes in complex star-forming environments. At radio frequencies, the dominant emission mechanisms are thermal bremsstrahlung and non-thermal synchrotron. HII regions typically emit thermal radiation tracing the ionised gas; however, detecting and characterising non-thermal emission can provide insights into magnetic fields and the energy distribution of relativistic particles in star-forming regions. We have utilised the upgraded Giant Metrewave Radio Telescope (uGMRT) to study radio emission in the Extended Orion Nebula (EON) region. We present results from wide-band interferometric observations using uGMRT bands 3 and 4, probing a frequency range not covered by other sensitive radio interferometers. We produced deep continuum images with RMS noise levels of $\sim400\,μ$Jy~beam$^{-1}$ in band 3 and $\sim200\,μ$Jy~beam$^{-1}$ in band 4. We further generated in-band and broad-band spectral index maps using these images. To establish the robustness of the spectral index measurements, we conducted a detailed analysis using simulated uGMRT data. From the continuum spectral index analysis, we report the unambiguous presence of non-thermal radio emission in the EON region. To investigate its plausible origin, we correlated our results with multiwavelength observations, identifying a strong association between non-thermal emission and outflows from young stellar objects, while also exploring alternative explanations. In future, reliable broad-band radio spectral index measurements, together with dedicated multiwavelength observations, will be invaluable for resolving the origin of non-thermal emission in the Orion Nebula and other star-forming regions.

Figures (19)

• Figure 1: Continuum image centred at 400 MHz from band 3 (top),and at 685 MHz from band 4 (bottom) of uGMRT. The contour levels are at [1,3,10,20,40] $\times 3 \sigma$ for band 3 image ($\sigma_{\mathrm{B3}}=400\mu$Jy/beam), and at [1,3,10,20,40,100] $\times 3 \sigma$ for band 4 image ($\sigma_{\mathrm{B4}}=200\mu$Jy/beam). The synthesised beams (ellipse) are shown in the bottom left corner of the respective image.
• Figure 2: Broad-band spectral index map of the Orion H ii region obtained from 400 MHz and 685 MHz images of uGMRT. The overlaid labelled contours depict the uncertainty in the spectral index, ranging from $0.2<\sigma_{\alpha}<1$. The regions with negative spectral indices have been magnified.
• Figure 3: 2D Spherical shell model (left) for extended source simulation. A profile of brightness along one of the diameters is shown (right).
• Figure 4: Histogram of S/N for band 3 (blue) and band 4 (green) continuum image. A cutoff at S/N=3 is placed in the plot, denoted by a dashed-red vertical line.
• Figure 5: Two-dimensional kernel density estimate (KDE) of signal-to-noise ratio (S/N) vs. retrieved spectral index ($\alpha$) for simulated spectral index map. The fiducial (model) spectral index value for the simulation is $\alpha_{\rm{true}}=-0.7$, indicated by the dashed black line. The colour map represents normalised data point density. Solid red bars denote the 95% data spread across thirty bins of S/N, and the black bar represents the median. The plot comprises data from $\sim38000$ pixels of the spectral index map from the simulated uGMRT data. The S/N axis is displayed on a logarithmic scale.