|| © CAMBRIDGE UNIVERSITY PRESS 1997
1.3.3 Ultraviolet Fluxes
Quasars are often found to have unusually blue broad-band optical colors. Johnson UBV photometry in particular shows that U - B is remarkably small (i.e., large negative values), and this is often referred to as the ``ultraviolet (UV) excess'' of quasars. Quasars occupy a region on the two-color diagram which is not heavily populated by stars, as shown in Fig. 1.6.
Figure 1.6. The locations of 788 quasars from the Hewitt and Burbidge (1993) catalog on a two-color (U - B vs. B - V) diagram. The locus of the zero-age main sequence, with spectral types indicated, is also shown. The line in the lower left shows how points will be translated due to amounts of reddening corresponding to visual extinction AV (based on the extinction curve of Cardelli, Clayton, and Mathis (1989) with RV = A (V) / E (B - V) = 3.1). Most quasars with measured UVB magnitudes are color-selected, i.e., identified as quasar candidates on the basis of their blue colors, especially U - B < 0.
It must always be kept in mind that statements such as ``quasars are very blue objects'' or ``quasars show an ultraviolet excess'' are relative statements, which refer to their spectral energy distribution relative to stars. One must remember that in most stars there is relatively little flux in the U band; in cooler stars, the U band is in the Wien tail of the blackbody distribution, and in hotter stars, the Balmer continuum absorption edge occurs in the center of the band (i.e., at 3646 Å), so there is a real deficit in the number of photons in the shortward half of the band. Any AGN-type power-law spectrum shows a higher ratio of U flux to B flux than does an A star, even though there is really less energy per unit frequency at shorter wavelengths than at longer. Thus, the fundamental reason that quasars appear to have an ultraviolet excess is that quasar spectra are flatter than A-star spectra through the U and B bands.