CLAS+ finds over 3,500 new changing-look galaxy candidates using only images, not expensive spectra.

Supermassive black holes at the centers of galaxies can feed on gas and shine brightly as active galactic nuclei. Sometimes, these objects switch their look rapidly over months or years. These are called changing-look active galactic nuclei. Finding them is tough because astronomers usually need multiple spectra, which split light into detailed rainbows, taken years apart. Spectra are hard and slow to collect. To solve this, researchers compared older spectra from past surveys against newer camera images taken through specialized narrow-band filters. Narrow filters capture light in small color slices, acting like a quick snapshot of a spectrum. If an object changed brightness between the older spectrum and the newer filter picture, it likely changed its state. The team discovered thousands of strong candidates, with most being completely new. Many candidates brightened over time, especially at higher redshifts, meaning further away in space.

Why it matters

Astronomers who study how supermassive black holes grow can now access a much larger list of targets without waiting for costly telescope time. This makes it easier to test theories about why these galaxies suddenly brighten or dim, which challenges existing models of galaxy evolution.

Questions answered