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