Light from a distant lantern reflects off the surrounding fog, and from the delay of the reflection you can infer the distance. If the fog is uneven, the reflection arrives earlier or later, and the distance comes out wrong. That's roughly how astronomers estimate the masses of supermassive black holes at the centers of galaxies: they watch how quickly gas clouds (the broad emission line region) respond to flares from the bright core — the quasar.
For four years, a team led by scientists observed the quasar E1821+643—the brightest one for which such a measurement has been made. The gas was expected to respond with a delay of about 460 days, but the actual delay was only 83 days—5.6 times shorter. It's as if an echo in the mountains came back in one second instead of five.
The solution came when the light from the gas was spread out by velocity. It turned out that the gas consists of two parts: a slow core that behaves "by the rules," and a fast cloud flying toward us. This fast cloud barely delays its response, which sharply shortens the overall delay. The same pattern, judging by the collected sample, appears in other bright quasars, so errors in mass estimates may be systematic.
This discovery matters not only for counting black holes. Quasars are used as "standard candles" to measure distances in the universe and test cosmological models. If their true brightness differs from the calculated one, the map of cosmic expansion could be distorted. Now astronomers will have to recalibrate, and perhaps find new ways to peer into the very heart of a quasar.
The delay of the gas glow around quasar E1821+643 turned out to be 5.6 times shorter than expected, like an echo in the mountains returning in one second instead of five.