Constraining the Origin of a Very High Velocity Cloud Toward M33

An ionized Very-High Velocity Cloud (VHVC, vLSR ∼ −350 km s−1 was recently discovered in multiple ion absorption lines (e.g., O i, C ii, Si ii, Si iii) in the sightline of M33 (Zheng et al. 2017). However, the VHVC’s origin is still unknown due to the lack of distance information and the poorly constrained HI content. Three possible origins are suggested by Zheng et al: (1) part of the Milky Way’s halo gas, (2) associated with a nearby HI high-velocity cloud (HVC) – so called the Wright’s Cloud (Wright 1974), or (3) M33’s extended HI northern warp. Currently available HI observations (e.g., AGES and GALFA-HI) are not sensitive enough to discount any HI leading to association with the suggested origin possibilities. We used the GBT to better constrain the VHVC’s origin by obtaining a high-sensitivity HI datacube reaching to a 5σ column density of 5.5 × 1015 cm−2 (or 1σ noise level at 3.5 mK/km s−1 ). With the GBT observation, we analyzed the diffuse HI context of the VHVC, and compared it with the nearby HI structures in both position and velocity, better constraining the origin. Using the resources provided by the Green Bank Observatory, a region of 1◦ by 2.5◦ was mapped, spanning 22◦ < RA < 23◦ and 28.5◦ < DEC < 31◦. Two previously undetected HI HVC were detected, with very similar velocities to both the VHVC and Wright’s cloud, leading the origin of the VHVC closely towards a connection between those structures.