Diffuse HI emission in the circumgalactic medium of NGC891 and NGC4565 - III: azimuthal profiles

Abstract

Diffuse HI emission in the circumgalactic medium (CGM) of NGC891 and NGC4565 has been previously shown to trace an inflow along minor axes pointings and to co-rotate with the HI disk along major axes pointings out to ~100 kpc (Das2020b,Das2024a). To obtain a 360$^\circ$ view of the inner neutral CGM ($\rm < 25 kpc$ for NGC891, $\rm < 30 kpc$ for NGC4565), we perform deep stare observations with the Green Bank Telescope (GBT) along the off-axes, 45$^\circ$ between principal axes, achieving a 5$\sigma$ column density sensitivity of $1.1-1.2 x 10^{17} \rm cm^{-2}$ over a 20 kms$^{-1}$ velocity width. While detecting HI emission in the inner CGM with single-dish telescopes is common, separating the true CGM emission from disk contamination is extremely challenging and has so far been largely unsuccessful. To achieve that, we compare our single-dish detections to deep interferometric maps from the Westerbork Synthesis Radio Telescope (WSRT) HALOGAS survey, and improve upon our previous methods by incorporating velocity offset corrections and channel-wise brightness-temperature scaling. We find that $30-38$ % and $18-28$ % of the emission detected by the GBT cannot be explained by WSRT in NGC891 and NGC4565, respectively, implying a true CGM detection. There is $4-6$ ($3-7$) times more HI along the off-axes than major (minor) axes, nullifying the common assumption of azimuthal symmetry of the neutral CGM. The velocity profile of the diffuse inner CGM suggests a lagged co-rotation with the HI disk in both galaxies. This exercise illustrates the power of deep observation and careful cross-instrument comparisons to characterize the diffuse HI in the CGM.