Simulating winds in the Galactic centre. II. Observational biases on HI clouds and a multi-wavelength comparison

Abstract

High-velocity neutral hydrogen (HI) clouds found up to several kiloparsecs above the Galactic centre (GC) provide some of the most direct evidence for a nuclear wind in the Milky Way, yet the properties inferred for these clouds are influenced by how they are observed. Starting from a magnetohydrodynamical simulation in which supernova feedback drives a multiphase nuclear outflow, we generate mock HI observations matched to the angular resolution and sensitivity of Green Bank Telescope (GBT) surveys of the GC and analyse them in the same way as the real data. We find that observational effects substantially reshape the cloud population: beam smearing makes clouds appear larger, rounder and more diffuse, while noise erases faint clouds and strips low-density gas from the others. These effects combined, together with confusion with foreground disk emission, prevent the detection of more than 80% of the clouds (up to 95% during more quiescent phases), while those detected appear less massive, less turbulent and less elongated, with column densities underestimated by a factor of two; only their velocities are reliably recovered. Once these effects are included, the simulated clouds broadly reproduce the properties and sky distribution of the GBT population, but lack the fastest (> 250 km/s ) and highest-latitude (b > 10° ) clouds. Mock UV absorption and soft X-ray observations support this picture, with warm-ionized gas column densities at the low end of the observed range and X-ray emission measures too low at b > 10° . Our results imply that the true GC cloud population is more numerous, massive, turbulent and elongated than observed, and that fully reproducing observations may require a stronger recent starburst or additional feedback.