Detection of diffuse HI emission in the circumgalactic medium of NGC891 and NGC4565 -- II

Abstract

We probe the neutral circumgalactic medium (CGM) along the major axes of NGC891 and NGC4565 in 21-cm emission out to $\gtrsim 100$kpc using the Green Bank Telescope (GBT), extending our previous minor axes observations. We achieve an unprecedented $5\ddot{I}ƒ$ sensitivity of $6.1\times 10^{16}$ cm${}^{-2}$ per 20 km s${}^{-1}$ velocity channel. We detect HI with diverse spectral shapes, velocity widths, and column densities. We compare our detections to the interferometric maps from the Westerbork Synthesis Radio Telescope (WSRT) obtained as part of the HALOGAS survey. At small impact parameters, $>31-43\%$ of the emission detected by the GBT cannot be explained by emission seen in the WSRT maps, and it increases to $>64-73\%$ at large impact parameters. This implies the presence of diffuse circumgalactic HI. The mass ratio between HI in the CGM and HI in the disk is an order of magnitude larger than previous estimates based on shallow GBT mapping. The diffuse HI along the major axes pointings is corotating with the HI disk. The velocity along the minor axes pointings is consistent with an inflow and/or fountain in NGC891 and an inflow/outflow in NGC4565. Including the circumgalactic HI, the depletion time and the accretion rate of NGC4565 are sufficient to sustain its star formation. In NGC891, most of the required accreting material is still missing.

18 pages, 12 figures, accepted for publication in MNRAS