Neutrinos as a new tool to characterise the Milky Way Centre

Abstract

The Central Molecular Zone (CMZ), a star-forming region rich in molecular clouds located within hundreds of parsecs from the centre of our Galaxy, converts gas into stars less efficiently than anticipated. A key challenge in refining star-formation models is the lack of precise mapping of these dense molecular hydrogen clouds, where traditional tracers often yield inconsistent results. We demonstrate how, in the near future, neutrinos will emerge as a robust mass tracer due to worldwide advancements in neutrino telescopes. Neutrinos are produced alongside gamma-rays when cosmic-rays interact with molecular clouds. The neutrino production rate is proportional to the gas density without dependence on the complex properties of a cloud. Neutrinos also have the advantage of negligible absorption and unambiguous production channels, making it a method with the lowest systematic uncertainties. In an optimistic case where most gamma-ray emission from the Galactic Centre region originates from pion decays, we expect several tens of muon neutrinos to be detected in about two decades. Neutrinos from the CMZ will provide indications on the biases of traditional mass tracers and thus indirectly enhance the accuracy of gas measurements in far galaxies from which a neutrino signal is not detectable.

Figures (4)

• Figure 1: Top panels: hydrogen column density in the Central Molecular Zone inferred from the CS Tsuboi99ApJS and dust Molinari11ApJL mass tracers, normalised to an integrated mass of $\sim10^7\,$M$_\odot$ in the displayed Galactic longitude and latitude ranges. Middle panels: probability of neutrino emission, assuming the column densities in the respective top panels, a cosmic-ray density that decreases as the inverse of the distance from Sgr A*, and an illustrative angular resolution of 0.1$^\circ$. The contours delineate iso-probability levels. Bottom panels: an example of how a future neutrino picture of the Galactic Centre could look like. The picture contains 100 events, which is an amount we could collect in a couple of decades by combining data from the future network of neutrino telescopes. The red and blue cross mark the location of HESS J1745$-$290 and G0.9+0.1, respectively.
• Figure 2: The Central Molecular Zone gamma-ray flux measured by HESS HESS18AA, MAGIC MAGIC20AA, and HAWC Albert2024ApJ_HAWC, i.e. the diffuse flux from the Galactic Centre ridge. The blue straight line shows the best fit from the HESS collaboration HESS18AA, while the blue shaded areas represent our reconstructed 1$\sigma$ probability intervals, obtained by sampling the central values and uncertainties of the fit parameters. The grey line shows a maximum likelihood fit of all data available assuming a power law function with a free exponential cut-off, whose best estimate is approximately 80 TeV.
• Figure 3: Expected number of detected muon neutrino events from the CMZ (blue line and band), HESS J1745$-$290 (dotted green line), G0.9$+$0.1 (dotted magenta line), and background (black dashed line).
• Figure 4: Discrimination significance between gas distribution models built from the CS and dust mass tracers. The dotted, solid, and dash-dotted lines correspond to different assumptions on the angular resolution. The blue line and band correspond to calculations assuming the best-fit HESS flux or its 1$\sigma$ uncertainty envelope as input.