Dots in a murky sky.

A familiar joke in the astronomical world is that you can sleep through somebody's seminar or conference talk and still make the speaker sweat by nodding awake long enough to ask, "Have you considered the effects of dust?" The speaker likely will mutter, "Well, we realize of course that dust may well be important, but for now we have assumed that its effect on our observations is negligible." Ignoring dust, or the complexities of its effects, is a mistake astronomers have made many times. The field may be going through another cycle as it attempts to understand active galactic nuclei (AGN)-bright regions at the centers of galaxies that are thought to be powered by accretion of gas onto a supermassive black hole.

A luminous quasar at a redshift of z = 7.085.

The intergalactic medium was not completely reionized until approximately a billion years after the Big Bang, as revealed by observations of quasars with redshifts of less than 6.5. It has been difficult to probe to higher redshifts, however, because quasars have historically been identified in optical surveys, which are insensitive to sources at redshifts exceeding 6.

The characteristic blue spectra of accretion disks in quasars as uncovered in the infrared.

Quasars are thought to be powered by supermassive black holes accreting surrounding gas. Central to this picture is a putative accretion disk which is believed to be the source of the majority of the radiative output. It is well known, however, that the most extensively studied disk model-an optically thick disk which is heated locally by the dissipation of gravitational binding energy-is apparently contradicted by observations in a few major respects.