HVC Background

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In the mid-1900’s gas clouds were discovered surrounding our galaxy; composed primarily of neutral hydrogen and moving at velocities inconsistent with those in the Milky Way disk. These clouds can be up to around 10 million times the mass of the Sun and 10,000 Ly across. 

The velocity of the Sun’s orbit about the Galactic Center is called the Local Standard of Rest (LSR) which is 220 km/s; we measure the hydrogen clouds’ velocity as a differential between the measured velocity of the cloud and the Local Standard of Rest (VLSR). 

These clouds are initially classified based on their VLSR ; High Velocity Clouds (HVC) have |VLSR|>90 km/s  while Intermediate Velocity Clouds (IVC) have velocities 30 km/s≤|VLSR|≤90 km/s .

Milkyway galaxy
Image of Milky Way galaxy from the side. It is surrounded by High Velocity Clouds of various shapes and sizes.