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We present detections of 21-cm emission from neutral hydrogen (HI) in thecircumgalactic medium (CGM) of the local edge-on galaxies NGC 891 and NGC 4565using the Robert C. Byrd Green Bank Telescope (GBT). With our 5$\sigma$sensitivity of $8.2 \times 10^{16}$ cm$^{-2}$ calculated over a 20 km s$^{-1}$channel, we achieve $>5\sigma$ detections out to $90-120$ kpc along the minoraxes. The velocity width of the CGM emission is as large as that of the disk$\approx 500$ km s$^{-1}$, indicating the existence of a diffuse componentpermeating the halo. We compare our GBT measurements with interferometric datafrom the Westerbork Synthesis Radio Telescope (WSRT). The WSRT maps the HIemission from the disk at high S/N but has limited surface brightnesssensitivity at the angular scales probed with the GBT. After convolving theWSRT data to the spatial resolution of the GBT (FWHM = 9.1$'$), we find thatthe emission detected by the WSRT accounts for $48^{+15}_{-25}$%($58^{+4}_{-18}$%) of the total flux recovered by the GBT from the CGM of NGC891(NGC 4565). The existence of significant GBT-only flux suggests the presenceof a large amount of diffuse, low column density HI emission in the CGM. Forreasonable assumptions, the extended diffuse HI could account for $5.2\pm0.9$%and $2.0\pm0.8$% of the total HI emission of NGC 891 and NGC 4565.

Detection of the diffuse HI emission in the Circumgalactic Medium of NGC 891 and NGC 4565