Red Quasars Not so Dusty

Webster et al (1995) claimed that up to 80% of QSOs may be obscured by dust. They inferred the presence of this dust from the remarkably broad range of B?K optical-infrared colours of a sample of at-spectrum PKS radio QSOs. If such dust is typical of QSOs, it will have rendered invisible most of those which would otherwise been have detected by optical surveys. We used the William Herschel Telescope to image 54 B3 radio QSOs in K, and we nd that although several have very red optical-infrared colours, most of these can be attributed to an excess of light in K rather than a dust-induced deecit in B. We present evidence that the infrared excess comes from the light of stars in the host galaxy, or from synchrotron radiation associated with at-spectrum radio sources. Thus, either the Webster et al QSOs are atypical in having large extinctions, or their reddening is not due to dust. Either way, there is no evidence that a large fraction of QSOs has been missed by optical surveys. This work is reported in more detail in Benn et al (1997).