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4.5.1 Comparison of Aperture Magnitudes

Our ${m_{\rm circ}}(r)$ profiles were interpolated to give ${m_{\rm circ}}$ (hereafter just m) at the radii which the given literature source had used. The interpolation was done by fitting a quadratic function to the profile. In the cases of JFK92 and JFK95a, for which we had access to their m(r) profiles, the opposite was done: their magnitudes were interpolated to two of our apertures: 6.26'' and 10.08''.

Where we had observed a given galaxy more than once, a mean of our values was calculated before calculating the difference with the given literature source.

Note, that throughout this thesis, aperture refers to the radius of the aperture, not the diameter.

We compared with the following sources:

The result of the external comparisons is given in Table [*] and Figure [*].

Table: External Comparison of Aperture Magnitudes

Gunn r

Source ${N_{\rm gal}}$ ${N_{\rm ap}}$ N mean rms  
JFK92,  6'' 3 1 3 $ 0.026\pm0.001$ 0.002  
JFK92, 10'' 3 1 3 $ 0.025\pm0.004$ 0.007  
JFK95a,  6'' 14 1 14 $ 0.010\pm0.005$ 0.018  
JFK95a, 10'' 14 1 14 $ 0.017\pm0.004$ 0.015  
Poulain & Nieto (1994) 2 5 10 $ 0.018^{\rm b}\pm0.014$ 0.045  
Poulain & Nieto (1994)$^{\rm a}$ 2 4-5 9 $ 0.007^{\rm b}\pm0.010$ 0.031  

Johnson B

Source ${N_{\rm gal}}$ ${N_{\rm ap}}$ N mean rms  
Burstein et al. (1987) 4 3 12 $-0.052\pm0.020$ 0.071  
Burstein et al. (1987)$^{\rm a}$ 4 2-3 11 $-0.067\pm0.017$ 0.052  
JFK92,  6'' 3 1 3 $-0.015\pm0.001$ 0.003  
JFK92, 10'' 3 1 3 $-0.022\pm0.008$ 0.015  
Poulain & Nieto (1994) 2 5 10 $ 0.016\pm0.014$ 0.045  
Poulain & Nieto (1994)$^{\rm a}$ 2 4-5 9 $ 0.005\pm0.009$ 0.027  

Johnson U

Source ${N_{\rm gal}}$ ${N_{\rm ap}}$ N mean rms  
Poulain & Nieto (1994) 1 4 4 $ 0.07 \pm0.05 $ 0.09  
Poulain & Nieto (1994)$^{\rm a}$ 1 3 3 $ 0.02 \pm0.01 $ 0.02  
Sandage (1975) 2 1 2 $-0.19 \pm0.03 $ 0.04  
van den Bergh (1977)$^{\rm c}$ 3 1-4 7 $-0.06 \pm0.07 $ 0.18  
Weedman (1976) 3 1 3 $ 0.01 \pm0.01 $ 0.01  


Notes: ${N_{\rm gal}}$ is the number of galaxies in common with the given source in the given filter. ${N_{\rm ap}}$ is the number of apertures pr. galaxy. N is the total number of data points. `rms' is the root mean square standard deviation. The uncertainty on the mean has been calculated as rms/$\sqrt{N}$. The differences have been calculated as ``our''-``literature''. $^{\rm a}$ Outermost aperture of R256, which may have contamination from R269, omitted. $^{\rm b}$ The expected offset between Gunn r and Kron-Cousins R of $0\hbox{$.\!\!^{\rm m}$ }354$ (Jørgensen 1994) has been subtracted. $^{\rm c}$ The one data value marked ``:'' in van den Bergh (1977) was omitted.

Regarding tablenote `a': The following support the conclusion that the magnitude for R256 for the largest aperture of Poulain & Nieto (1994) and Burstein et al. (1987) is significantly contaminated by signal from R269. Their largest aperture is approximately 45'', and the separation between R256 and R269 is approximately 101''. At $r \approx 56''$ our data give a Johnson B surface brightness of $\mu(56'') = 23.6{^{\rm m} /{\rm arcsec}^{2}}$ for R269, and $\mu(56'') = 25.8{^{\rm m} /{\rm arcsec}^{2}}$ for R256.

The rms scatter of $0\hbox{$.\!\!^{\rm m}$ }18$ for the Johnson U comparison with van den Bergh (1977) is mostly due to the R269 magnitude within the 2.5'' aperture, where the difference is $+0\hbox{$.\!\!^{\rm m}$ }34$. This can be explained by the following. The galaxy has dust in the center, which can be seen in our images (which have good seeing). The dust causes problems in finding the appropriate center to use for the aperture photometry. Even if we and van den Bergh (1977) have used the same center, a positive magnitude difference will show up if we have better seeing than van den Bergh. Besides, we had overlapping ellipses out to r=2.7'' when fitting this galaxy, and it is not clear what the impact is on ${m_{\rm circ}}(r)$.

  \begin{figure}% latex2html id marker 6874\makebox[\textwidth]{
\makebox[\thir... -- R256;
open triangle -- R269; filled triangle -- R347.

From the above comparison we conclude that the magnitude zero point is consistent with literature data within $0\hbox{$.\!\!^{\rm m}$ }02 \pm 0\hbox{$.\!\!^{\rm m}$ }01$ for Gunn r, $-0\hbox{$.\!\!^{\rm m}$ }02 \pm 0\hbox{$.\!\!^{\rm m}$ }03$ for Johnson B, and $0\hbox{$.\!\!^{\rm m}$ }00 \pm 0\hbox{$.\!\!^{\rm m}$ }05$ for Johnson U.

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Next: 4.5.2 Comparison of Surface Up: 4.5 External Comparison of Previous: 4.5 External Comparison of

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

Bo Milvang-Jensen (