Three dark-matter–poor dwarfs line up, hinting at a violent past that shed ordinary matter
Astronomers using the Keck Observatory measured the faint galaxy NGC 1052-DF9 and found its stars move as if it lacks a dark-matter halo, joining DF2 and DF4 along a tight line in the NGC 1052 field and pointing to a past high-speed collision that stripped ordinary matter from dark matter. DF9’s velocity dispersion is about 6.5 km/s, implying a stellar mass around 100 million solar masses—far less than the halo a galaxy of its size would normally have. While this supports a collision scenario (similar in spirit to the Bullet Cluster), alternative explanations like tidal dwarfs exist, and a smoking-gun test would be detecting gas along the trail.
