2020ApJ...898...15D

We present detections of 21 cm emission from neutral hydrogen (H I) in the circumgalactic medium (CGM) of the local edge-on galaxies NGC 891 and NGC 4565 using the Robert C. Byrd Green Bank Telescope (GBT). With our 5σ sensitivity of 8.2×10¹⁶ cm^–2 calculated over a 20 km s^–1 channel, we achieve > 5σ detections out to 90–120kpc along the minor axes. The velocity width of the CGM emission is as large as that of the disk ~500 km s^–1, indicating the existence of a diffuse component permeating the halo. We compare our GBT measurements with interferometric data from the Westerbork Synthesis Radio Telescope (WSRT). The WSRT maps the H I emission from the disk at high signal-to-noise ratio but has limited surface brightness sensitivity at the angular scales probed with the GBT. After convolving the WSRT data to the spatial resolution of the GBT (FWHM = 9.'1), we find that the emission detected by the WSRT accounts for 48_–25⁺¹⁵% (58_–18⁺⁴%) of the total flux recovered by the GBT from the CGM of NGC 891 (NGC 4565). The existence of significant GBT-only flux suggests the presence of a large amount of diffuse, low column density H I emission in the CGM. For reasonable assumptions, the extended diffuse H I could account for 5.2 ± 0.9% and 2.0 ± 0.8% of the total H I emission of NGC 891 and NGC 4565.