Radio and Gamma-ray Evidence for the Supernova Origin of High Velocity Cloud Complex M

Abstract

Using $λ$-21-cm galactic neutral atomic hydrogen data from the HI4PI survey of Bekhti et al. (2016) and 0.75-30 MeV $γ$-ray emission from the Imaging Compton Telescope, we have searched for the origin event that accelerated high velocity cloud Complex M. Radio plots of $l-b$, $l-v$, and $b-v$ show a cavity centered at ($l$, $b$) $\sim$ (150$^{\circ}$, 50.$^{\circ}$) and extending about $\pm$33$^{\circ}$. The best view of the cavity is at a velocity of -25 km s$^{-1}$, which shows a circular cross section on the back (receding) face. Complex M, at -85 km s$^{-1}$, is on the front (approaching) face. The $γ$-ray emission reveals several minima, the largest centered at ($l$, $b$) $\sim$ (150$^{\circ}$, 50.$^{\circ}$) and coincident with the position and extent of the cavity seen in the radio data. Using the know distance to Complex M and assuming that the cavity is spherical, we can bootstrap the distance to the original, explosive source of the cavity, D = 307 pc, calculate the radius of the cavity, R = 166 pc, and approximate the expansion velocity, V$_E$ $\approx$ 40 km $s^{-1}$, of the cavity. The total energy of the expanding cavity is 3.0 $\pm$ 1.0 ${\times}$ 10$^{50}$ ergs, well within the range of a single supernova. These results indicate that this explosion took place about four million years ago. As the blast wave from this supernova propagated outwards, it began to sweep up interstellar gas and carved out the Local Chimney, a low-density extension of the Local Bubble that reaches into the galactic halo.