The intermediate-line region in active galactic nuclei: A region "praeter necessitatem"?

As a consequence of the improved signal-to-noise ratio, spectral resolution , and wavelength coverage, various authors have introduced, without strong justification, new emitting regions to account for various emission-line pr ofile differences in active galactic nuclei. The so-called C IV lambda 1549 intermediate-line region (ILR) appears to be especially ill-defined. We pr esent observational evidence that suggests the ILR is statistically indisti nguishable from the classical narrow-line region (NLR). We present the resu lts of theoretical models showing that a smooth density gradient in the NLR can produce C IV and Balmer emission lines with different widths. The puta tive ILR component has often been included with the broad line profile in s tudies of C rv broad-line region (BLR) properties. Failure to account for t he composite nature of C IV emission, and for the presence of sometimes app reciable NLR C IV emission, has important consequences for our understandin g of the BLR.