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:

• JFK92: CCD photometry in Gunn r and Johnson B. Magnitudes cleaned for foreground stars and other galaxies. 3 galaxies in common: R218, R234, and R269. Two (synthetic) apertures: 6.26'' and 10.08''.

• JFK95a: CCD photometry in Gunn r. Magnitudes cleaned for foreground stars and other galaxies. 14 galaxies in common: R112, R216, R218, R234, R237, R243, R256, R266, R269, R283, R288, R338, R343, and R347. Two (synthetic) apertures: 6.26'' and 10.08''.

• Poulain & Nieto (1994): Photoelectric photometry in Kron-Cousins $R_{\rm C}$, Johnson B, and Johnson U. The expected offset between Gunn r and Kron-Cousins $R_{\rm C}$ is $(r - R_{\rm C}) = 0\hbox{$.\!\!^{\rm m}$ }354$ (Jørgensen 1994). Magnitudes cleaned for foreground stars^4.1. Magnitudes probably not cleaned for other galaxies. 2 galaxies in common: R256 and R347. 5 apertures: 11.4'', 15.7'', 21.7'', 30.7'', and 43.4''. For Johnson U, R347 and the 15.7'' aperture are not available.

• Burstein et al. (1987): Photoelectric photometry in Johnson B. Magnitudes cleaned for foreground stars^4.2. Magnitudes probably not cleaned for other galaxies. 4 galaxies in common: R234, R256, R269, and R347. 3 apertures: 16.9'', 30.7'', and 45.4''.

• Sandage (1975): Photoelectric photometry in Johnson U. Magnitudes cleaned for foreground stars^4.3. Magnitudes probably not cleaned for other galaxies. 2 galaxies in common: R234 and R269. 1 aperture: 27.2''.

• van den Bergh (1977): Photoelectric photometry (although this is not explicitly stated) in Johnson U. Magnitudes probably not cleaned cleaned for foreground stars and other galaxies (no information is given about this). 3 galaxies in common: R234, R256, and R269. 1-4 apertures: 2.5'', 5.0'', 7.5'', and 10.0''.

• Weedman (1976): Photoelectric photometry in Johnson U. Magnitudes probably not cleaned for foreground stars and other galaxies, but the used aperture is also quite small. 3 galaxies in common: R234, R256, and R269. 1 aperture: 8.25''.

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 ${m_{\rm circ}}(r)$.

 

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.