Does a Luminosity-dependent Continuum Shape Cause the Baldwin Effect?

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Abstract

We combine ground- and space-based spectrophotometry to measure the luminosity dependence of the shape of the optical/UV continuum in 108 QSO and 37 Seyfert 1 nuclei. The UV continuum at 1250 Å in high-luminosity QSOs, compared with the optical continuum at 6500 Å, is over one magnitude higher than that in the less luminous Seyfert galaxies. In terms of spectral index, α = 0.37 log L_op + C. The change in continuum slope is not uniform with wavelength: (1) The UV continuum slope is generally steeper than the optical; (2) The color excess, the difference in power-law slope, between the UV and optical is small in Seyfert galaxies, peaks in intermediate-luminosity objects (-23 > M > -26), and becomes less significant in high- luminosity QSOs. The equivalent width of the C IV emission line is smaller in objects with stronger UV excess. We therefore identify the trend of continuum shape with luminosity as the main cause of the "Baldwin effect." We verify that the Baldwin effect for C IV goes away if the underlying continuum level is replaced by that measured at 6500 Å. Our interpretation of the color-luminosity relation, especially the significant color excess among intermediate-luminosity objects, is that the "Big Blue Bump" shifts from the UV to optical in more luminous QSOs. This results in their having a lower proportion of high-energy ionizing photons (i.e., above 54 eV) relative to the optical/UV continuum. This change in continuum shape causes the equivalent widths of high-ionization emission lines, such as C IV and He II, to drop more at high luminosities than the low-ionization emission lines, such as Lyα, C III], and the Balmer lines, resulting in a stronger Baldwin effect for high-ionization lines.

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