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

Abstract

We present detections of 21-cm emission from neutral hydrogen (HI) 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 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 minor axes. 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 component permeating the halo. We compare our GBT measurements with interferometric data from the Westerbork Synthesis Radio Telescope (WSRT). The WSRT maps the HI emission from the disk at high S/N 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${}^{\prime }$), 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 HI emission in the CGM. For reasonable assumptions, the extended diffuse HI could account for $5.2\pm 0.9$% and $2.0\pm 0.8$% of the total HI emission of NGC 891 and NGC 4565.

14 pages, 5 figures, published in ApJ

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• HALOGAS: Strong Constraints on the Neutral Gas Reservoir and Accretion Rate in Nearby Spiral Galaxies
• Revisiting FUSE O VI Emission in Galaxy Halos
• Extraplanar gas in Edge-on Galaxies traced by SOFIA observations of [C II]
• Searching for HI around MHONGOOSE Galaxies via Spectral Stacking
• The truncation of the disk of NGC 4565: Detected up to z=4 kpc, with star formation, and affected by the warp
• Gas accretion onto galaxies and Kelvin-Helmholtz turbulence
• The role of magnetic fields in ram pressure stripping of satellite galaxies in the circumgalactic medium around massive galaxies
• A Long-term Study of Ultraluminous X-ray Sources in NGC 891
• The Molecular and Atomic Hydrogen Gas Content of the Boötes Void galaxy CG 910
• Mainly on the Plane: Observing the Extended, Ionized Disks of Milky Way Analogs in IllustrisTNG