Dynamic interactions between the two Magellanic Clouds have flung large quantities of gas into the halo of the Milky Way .
The result is a spectacular arrangement of gaseous structures including the Magellanic Stream , the Magellanic Bridge , and the Leading Arm ( collectively referred to as the Magellanic System ) .
In this third paper of a series studying the Magellanic gas in absorption , we analyze the gas ionization level using a sample of 69 Hubble Space Telescope /Cosmic Origins Spectrograph sightlines that pass through or within 30° of the 21 cm-emitting regions .
We find that 81 % ( 56/69 ) of the sightlines show UV absorption at Magellanic velocities , indicating that the total cross section of the Magellanic System is \approx 11 000 square degrees , or around a quarter of the entire sky .
Using observations of the Si iii/Si ii ratio together with Cloudy photoionization modeling , we calculate the total gas mass ( atomic plus ionized ) of the Magellanic System to be \approx 2.0 \times 10 ^ { 9 } M _ { \odot } ( d /55 kpc ) ^ { 2 } , with the ionized gas contributing around three times as much mass as the atomic gas .
This is larger than the current-day interstellar H i mass of both Magellanic Clouds combined , indicating that they have lost most of their initial gas mass .
If the gas in the Magellanic System survives to reach the Galactic disk over its inflow time of \sim 0.5–1.0 Gyr , it will represent an average inflow rate of \sim 3.7–6.7 M _ { \odot } yr ^ { -1 } , potentially raising the Galactic star formation rate .
However , multiple signs of an evaporative interaction with the hot Galactic corona indicate that the Magellanic gas may not survive its journey to the disk fully intact , and will instead add material to ( and cool ) the corona .