Lit-up gas clouds hint at galaxies’ violent pasts

Voorwerpje
FOHR-vehrp-yuh n.

An intergalactic gas cloud still glowing after being blasted by ultraviolet radiation from a neighboring galaxy.

Great globs of galactic gas are a fading reminder of a more vigorous past for some galaxies. Eight of these gas blobs, known as voorwerpjes (after the Dutch word for little objects), lie next to galaxies that show evidence of recent run-ins with other galaxies, researchers report online March 2 at arXiv.org. Those interactions probably triggered a blast of radiation from a gassy disk swirling around a supermassive black hole in the center of each galaxy. Even though the radiation switched off tens of thousands of years ago, the light is still racing away from the galaxies and is now illuminating the voorwerpjes, which are bits that the galaxies shed during the collisions.

Voorwerpjes are signposts of galaxies with recently active centers, so astronomers can use them to identify galaxies to study after their cores fade. The prototype for these blobs is “Hanny’s Voorwerp,” named after Dutch school teacher Hanny van Arkel, who discovered it while perusing images from the citizen science project Galaxy Zoo in 2007 (SN: 7/19/08, p. 10).