Astronomers debate the origin of High-Velocity Clouds (HVCs)—large clouds of gas moving through the Milky Way’s halo.
Viewpoint 1
HVCs are part of a 'galactic fountain.' Supernova explosions in the galactic disk heat gas and eject it upward into the halo. As this gas cools, it condenses and falls back toward the disk. Because this gas originates from the disk, it must have a high abundance of heavy elements (high metallicity) and contain dust particles typical of the disk.
Viewpoint 2
HVCs are primordial intergalactic gas clouds being accreted (pulled in) by the Milky Way's gravity. This gas is falling into the galaxy for the first time. Therefore, it should have an extremely low abundance of heavy elements (low metallicity) and contain virtually no dust, reflecting the composition of undeveloped space.
Match each hypothetical observation of a High-Velocity Cloud (HVC) to the viewpoint it supports or aligns with.
- An HVC is observed containing significant amounts of silicon and iron dust particles.Consistent with Viewpoint 1 only, because dust particles are formed in stellar environments like the galactic disk.
- An HVC is observed with a metallicity less than 10% of the solar average.Consistent with Viewpoint 2 only, because primordial accreted gas has not undergone stellar enrichment.
- An HVC is observed moving outward, directly away from the galactic disk at high speed.Consistent with Viewpoint 1 only, because the model describes disk gas being pushed outward into the halo by supernovas.