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2.4. Turnover Frequency versus Largest Linear Size

The relationship between the turnover frequency and the projected linear size of the radio source constrains (1) the mechanism for the turnover - e.g., free-free or synchrotron self absorption - and (2) models for the source evolution. Sources should follow trajectories on the num-l plane just as they do on the power-size plane (see section 6).

Fanti et al. (1990b) found an anticorrelation between linear size and turnover frequency for the CSS sources. This result is even stronger in the combined sample of CSS and GPS sources discussed by O'Dea & Baum (Fig. 5). This figure shows the following: (1) The properties of the GPS sources (upper left) and the CSS sources (lower right) are not bimodal but contiguous. There is a continuous distribution of sources across the num-l plane. This suggests that the GPS and CSS sources are simply scaled versions of each other. (2) The correlation is the same for the galaxies and quasars. (3) There is a simple linear relationship on the log-log plot. O'Dea & Baum (1997) find

Equation 4 (4)

or num propto l-0.65. The fact that a simple physical relationship exists suggests that the physical properties of the GPS and CSS sources are related and that the mechanism for the turnover depends simply on the source size.

Figure 5

Figure 5. The intrinsic turnover frequency vs. linear size for the Fanti et al. CSS sample and the Stanghellini et al. GPS sample. The quasars are represented by crosses, and the galaxies by solid squares. Adapted from O'Dea & Baum (1997).

Note, however, that if the GPS sources evolve in luminosity as they age (section 12), the sources in the upper left part of the plane may dim sufficiently that they leave the current flux density-selected samples before they reach the lower right part of the plane. Thus, Figure 5 does not necessarily imply that sources evolve along the locus of points of the observed correlation. O'Dea & Baum show that assuming the turnover is due to synchrotron self-absorption, the evolution model of Begelman (1996) produces plausible evolution on the num-l plane. Bicknell et al. (1997) show that the assumption of free-free absorption can also reproduce similar evolutionary tracks on the num-l plane.

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