© CAMBRIDGE UNIVERSITY PRESS 1997

1.3.4 Broad Emission Lines

The UV-optical spectra of quasars are distinguished by strong, broad emission lines. The strongest observed lines are the hydrogen Balmer-series lines (H 6563, H 4861, and H 4340), hydrogen Ly 1216, and prominent lines of abundant ions (Mg II 2798, C III] 1909, and C IV 1549); these lines appear in virtually all quasar spectra, but depending on the redshift of the quasar, some may not be observable if they fall outside the spectral window of a particular detector. Typical flux ratios are given in Table 1.1, along with typical equivalent widths, defined by

(1.9)

where F₁() is the observed flux across the emission line at the wavelength , and F_c() is the continuum level underneath the emission line. Both of these quantities are specific fluxes that are conventionally measured in units of ergs s^-1 cm^-2 Å^-1, so the equivalent width is measured in ångströms. ¹ It is nearly always sufficient to approximate eq. (1.9) by

(1.10)

where F_line is the total line flux, usually in units of ergs s^-1 cm^-2.

The correct interpretation of emission-line equivalent widths is that they provide an estimate of how large a continuum range one would need to integrate over to obtain the same energy flux as is in the emission line. This is especially relevant in the context of broad-band photometry of quasars; for example, if we compare the colors of high-redshift quasars, we find that the U band is greatly enhanced for quasars with z 2, since the Ly emission line then falls in the U band and alone attributes more than ~ 12% ( W (Ly) / W (U), where W (U) 680 Å, the width of the U bandpass) of the flux in the band. This means that in a flux-limited sample, quasars with strong emission lines in a given bandpass are more likely to be detected, which can thus lead to the erroneous conclusion that there are excess quasars at particular redshifts; we will return to this topic in Chapter 10.