A New High-latitude H i Cloud Complex Entrained in the Northern Fermi Bubble

Abstract

We report the discovery of 11 high-velocity H i clouds at Galactic latitudes of 25°–30°, likely embedded in the Milky Way’s nuclear wind. The clouds are detected with deep Green Bank Telescope 21 cm observations of a 3 2 × 6 2 field around QSO 1H1613-097, located behind the northern Fermi Bubble. Our measurements reach 3σ limits on NHI as low as 3.1 × 1017 cm−2, more than twice as sensitive as previous H i studies of the bubbles. The clouds span −180 ≤ vLSR ≤ −90 km s−1 and are the highest-latitude 21 cm high-velocity cloud detected inside the bubbles. Eight clouds are spatially resolved, showing coherent structures with sizes of 4–28 pc, peak column densities of log(NHI/cm2) = 17.9–18.7, and H i masses up to 1470 M⊙. Several exhibit internal velocity gradients. Their presence at such high latitudes is surprising, given the short expected survival times for clouds expelled from the Galactic center. These objects may be fragments of a larger cloud disrupted by interaction with the surrounding hot gas.