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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 (Halpha lambda6563, Hbeta lambda4861, and Hgamma lambda4340), hydrogen Lyalpha lambda 1216, and prominent lines of abundant ions (Mg II lambda2798, C III] lambda1909, and C IV lambda1549); 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

Equation 9 (1.9)

where F1(lambda) is the observed flux across the emission line at the wavelength lambda, and Fc(lambda) 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

Equation 10 (1.10)

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

Table 1

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 approx 2, since the Lyalpha emission line then falls in the U band and alone attributes more than ~ 12% (approx Wlambda (Lyalpha) / W (U), where W (U) approx 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.

1 X-ray fluxes are sometimes given in units like photons s-1 cm-2 keV-1, so equivalent widths of X-ray lines are given in keV. Back.

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