2024MNRAS.52710358D

We probe the neutral circumgalactic medium (CGM) along the major axes of NGC 891 and NGC 4565 in 21-cm emission out to ≳100 kpc using the Green Bank Telescope (GBT), extending our previous minor axes observations. We achieve an unprecedented 5σ sensitivity of 6.1 × 10¹⁶ cm^–2 per 20 km s^–1 velocity channel. We detect H I with diverse spectral shapes, velocity widths, and column densities. We compare our detections to the interferometric maps from the Westerbork Synthesis Radio Telescope (WSRT) obtained as part of the HALOGAS survey. At small impact parameters, $\gt 31\!-\!43~{{\ \rm per\ cent}}$ of the emission detected by the GBT cannot be explained by emission seen in the WSRT maps, and it increases to $\gt 64\!-\!73~{{\ \rm per\ cent}}$ at large impact parameters. This implies the presence of diffuse circumgalactic H I. The mass ratio between H I in the CGM and H I in the disc is an order of magnitude larger than previous estimates based on shallow GBT mapping. The diffuse H I along the major axes pointings is corotating with the H I disc. The velocity along the minor axes pointings is consistent with an inflow and/or fountain in NGC 891 and an inflow/outflow in NGC 4565. Including the circumgalactic H I, the depletion time and the accretion rate of NGC 4565 are sufficient to sustain its star formation. In NGC 891, most of the required accreting material is still missing.