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# 5.4 Internal Comparison of Global Photometric Parameters

We performed an internal comparison of the global photometric parameters. The result is shown in Table . The comparisons are also shown in Fig. -. The data symbols are as follows: boxes - Gunn r; triangles - Johnson B; crosses - Johnson U. Each figure has two panels. In the left panel we plot the difference versus the first observation value. In the right panel we plot the difference versus the seeing difference, in order to check for seeing dependence. , , and are correlated with , see the right panels of Fig. , , and . Our r1/4 fits did take the seeing into account, so perhaps the real PSF of our data has larger wings than the model PSF that we used. As a result, the individual error estimates for , , and are too low. This can be seen in the left panels of Fig. , , and - the scatter is larger than what the individual error bars can account for.

The errors in and are highly correlated, cf. Fig. , with a linear correlation coefficient of r = 0.99. This is because is the mean surface brightness within , so if due to random errors is determined too large, will correspondingly become too faint. This has the effect that the combination of and that enters the fundamental plane, approximately , has a much lower uncertainty than and individually, cf. Table  and Fig. . The rms scatter for this combination is 0.012 in Gunn r, implying a typical internal uncertainty of only 0.008.

c4 is also seeing dependent, in the expected sense that larger seeing makes |c4| smaller. See Fig. . Recall that |c4| is the amplitude of the observed extremum in the c4(r)-profile. We found and not to be seeing dependent.

Table: Internal Comparison of Global Photometric Parameters

 Gunn r Parameter N mean rms 53 0.051 53 0.170 FP'' 53 0.012 53 0.095 53 0.047 53 0.025 c4 47 0.008 |c4| 47 0.007 53 0.006 53 0.004 Johnson B Parameter N mean rms 42 0.061 42 0.214 FP'' 42 0.014 42 0.102 42 0.050

Notes: FP'' = .

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Properties of E and S0 Galaxies in the Clusters HydraI and Coma
Master's Thesis, University of Copenhagen, July 1997

Bo Milvang-Jensen (milvang@astro.ku.dk)