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∘ view of the inner neutral CGM (<25kpc for NGC891, <30kpc for NGC4565), we perform deep stare observations with the Green Bank Telescope (GBT) along the off-axes, 45∘
Nearby in the stack
between principal axes, achieving a 5
σ
column density sensitivity of
1.1−1.2x1017cm−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.
arXiv did not return neighbours for this paper. Try again in a bit.