Structure of the broad-line region of 3C 273

The number of discrete emitters in broad-line region cloud models of active galactic nuclei is still unknown. A finite number of clouds will cause intrinsic fluctuations in the emission line profiles. We observed the quasar 3C 273 with high spectral resolution (R similar or equal to 10kms(-1)) and high signal to noise ratio to estimate the number of individual emitters in the broad-line region. The residua of the emission-line wings of the low ionization lines Ha and HP were cross-correlated. We compared the results of the cross-correlation analysis with models of different cloud distributions. The smoothness of the line profiles implies lower limits to the number of individual clouds and to the width of the line profiles of these clouds. For emitters with a line profile width of FWHM similar or equal to 15kms(-1) corresponding to a temperature of T-e, similar or equal to 2 10(4) K, we found that the number of clouds is larger than N-cl, = 10(8). For clouds of significantly larger individual profile width (FWHM similar or equal to 100 km s(-1)) it is found that the number of clouds can be of the order of N-cl,1 approximate to 10(8). Since the number of individual emitters is several orders of magnitude larger than the number of stars involved in bloated star models these models can be ruled out as dominant source for broad-line region clouds. However, the non-detection of a cross-correlation signal is consistent with BLR models which assume a continuous hydrodynamical flow as source of the line emitting material.