Photographic radial integrated-light profiles were generated for all
unsaturated galaxy images for which comparable photoelectric aperture and/or
simulated-aperture photometry measurements could be found in the literature.
These integrated-light profiles were then interpolated to radii corresponding
to the apertures used by other observers yielding Tables 11 (click here), 12 (click here)
and 13 (click here). The integrations were performed using unsmoothed
plate-scan
data. Other observers' B- and V-band measurements were transformed into the
system by means of Eq. (1). Note that no comparisons have been
made with published measurements made with apertures of radii less than 12
arcsec. This is because, at small apertures, comparisons are
particularly sensitive to differences in the angular resolutions of the
images concerned.
| IC |  |  |  | U9362 |  |
 |
| 3355 | 32.9 | 15.89 | 15.86 | 15.83 | -0.40 | -0.43 |
| 3355 | 65.6 | 15.49 | 15.82 | 15.53 | -0.29 | -0.33 |
| 3475 | 53.3 | 14.57 | - | 15.12 | -0.05 | -0.21 |
| 3475 | 65.6 | 14.38 | - | 14.94 | -0.09 | -0.30 |
| 3475 | 68.7 | 14.34 | - | 14.90 | -0.09 | -0.32 |
| 3475 | 104.0 | 14.07 | - | - | +0.00 | - |
| 3481A | 41.4 | 15.85 | - | 16.37 | -1.73 | -1.26 |
| 3483 | 24.4 | 15.26 | - | 15.45 | +0.25 | +0.61 |
|
|
Their measurements are compared with U and magnitudes
integrated radially to radii corresponding to their apertures
.
| IC/VCC |  | |
| U9362 |  |
 |
| V1377 | 20.0 | 16.81 | - | - | +0.41 | - |
| V512 | 39.5 | 16.08 | - | - | -0.20 | - |
| I3239 | 20.0 | 16.53 | - | - | -0.32 | - |
| I3355 | 30.0 | 15.97 | 15.90 | 15.92 | -0.53 | -0.63 |
| I3416 | 20.0 | 15.91 | - | 15.75 | +0.02 | -0.16 |
| I3475 | 15.0 | 16.53 | - | 16.89 | -0.15 | -0.04 |
|
|
Their measurements are compared with U and magnitudes integrated
radially to radii corresponding to their apertures
Measurements corresponding to apertures of radius <12
have not been included
because they can be expected to be particularly sensitive to
differences in seeing conditions
.
| dw/VPC | | |
| U9362 |
| |
| 1/843 | 17.8 | 15.87 | - | 16.15 | -0.42 | -0.14 |
| 3/856 | 17.8 | 16.86 | - | 17.54 | -0.20 | -0.37 |
| 6/801 | 19.5 | 15.99 | - | 16.37 | -0.14 | +0.12 |
| 8/835 | 17.8 | 16.60 | - | 16.92 | -0.28 | -0.09 |
| 11/834 | 17.8 | 16.04 | - | 16.42 | -0.29 | -0.19 |
| 12/808 | 19.5 | 17.43 | - | 17.91 | +0.00 | -0.11 |
| 27/420 | 17.8 | 17.06 | 17.31 | 17.50 | -0.29 | -0.01 |
| 28/342 | 17.8 | 16.10 | 16.07 | 16.27 | -0.34 | -0.11 |
| 31/937 | 17.8 | 16.57 | - | 16.92 | -0.09 | +0.00 |
|
|
Caldwell's measurements are compared with U and
magnitudes integrated radially to radii
corresponding to his apertures
.
Four unsaturated VPC objects have been measured by observers at McDonald
Observatory of the University of Texas (de Vaucouleurs 1959;
de Vaucouleurs et al.
1978; de Vaucouleurs et al.
1981) as listed in Table 11 (click here). In the blue,
de Vaucouleurs et al. find IC 3481A to be 
1.7 mag brighter than we do. Interestingly, in the VPC, the
adjacent galaxy, IC 3481, is also found to be 
mag brighter
than IC 3481A.
Assigning equal weights to each galaxy, the apparent mean zero-point
offsets
were found to be 
mag with a sample
standard deviation of
mag and 
(
) mag. Removing IC 3481A from the
sample, reduces the residuals to 
(
) mag and
(
) mag.
As there is a tendency for the residuals to be smaller, the larger the aperture
employed, it is quite possible that resolution differences are responsible for
many of the apparent discrepancies.
Gallagher & Hunter (1986) measured six galaxies
with sufficiently large apertures for comparisons to be
useful. These objects are listed in Table 12 (click here).
The apparent mean zero-point discrepancies were found to be
(
) mag and 
(
) mag.
However, as Gallagher & Hunter's aperture sizes are generally quite small,
a large part of
the apparent discrepancies may well be due to resolution differences.
In the case of the most discrepant galaxy, IC 3355 (which was also measured
by de Vaucouleurs et al.), there may be an additional explanation.
IC 3355 lies only
15 arcmin from the direction of the giant elliptical, NGC 4406, and
the prominent galaxies NGCs 4435, 4438 and 4402 are even closer at hand.
As IC 3355's direction is so central within the cluster, photometry of this
galaxy may therefore be very sensitive to the background procedures
adopted, due to the extended haloes of the giant galaxies in the vicinity
and/or any intra-cluster light that may be present.
There are nine VPC objects in common with Caldwell's (1983) sample of
dwarfs
as listed in Table 13 (click here). The apparent mean
zero-point shifts were found to be 
(
) mag and
(
) mag.
These comparisons may however, be even more susceptible to differences in
image resolution, as Caldwell's apertures tend to be smaller than those of de
Vaucouleurs et al. and Gallagher & Hunter.
The integrated-light profiles of IC 3475 (in ) and NGC 4468 (in U) are
depicted in Figs. 19 (click here) and 20 (click here) respectively. NGC 4468 has
been measured by several other observers, whose measurements tend to be about
0.2 mag brighter interior to a radial distance from the image centre of
50 arcsec. The outermost measurement however, deviates from the
photographic profile by a negligible amount (<0.01 mag).
Figure 19: Comparisons with previous B-band aperture-photometry measurements of
IC 3475, converted to by means of Eq. (1), with (B-V)=0.82.
All of the data points correspond to values tabulated previously. The curve
represents the radially integrated profile from the unsmoothed
image-scan of Plate J9229
Figure 20: Comparison with the U-band simulated-aperture CCD photometry of
Bower et al. (1992) and the aperture photometry of
Michard (1982) and
Sandage & Visvanathan (1978) for NGC 4468. The curve represents the
radially integrated profile from the unsmoothed image-scan of
Plate U9362
Figure 21: Comparison between the CCD surface photometry of
Vigroux et al.
(1986) for IC 3475 and the smoothed photographic
surface photometry on which the VPC is based.
Vigroux et al.'s error bars are depicted
CCD surface photometry has already been published by Vigroux et al. (1986)
for IC 3475. Vigroux et al.'s profile of IC 3475 was directly comparable
with our own smoothed elliptical surface-brightness profile.
In both cases, elliptical
isophotes were fitted to the images and specified by the same
reduced-radius parameter as well as by a position angle and an ellipticity.
Conversion of 
to 
was by means of Eq. (1) with a
(B-V) colour of 0.82.
As can be seen from Fig. 21 (click here), there is a difference in profile
slope, but the agreement between the photographic and CCD photometry is
generally good, and well within Vigroux et al.'s quoted errors.
The difference in profile slope is almost certainly due to the heavy
smoothing applied to the VPC plate-scan data, as extrapolation of the
two profiles to 
yields 
for Vigroux et al.'s
profile (using n=1) and 
for the VPC profile (for which
n=1.2).
Figure 22: Comparisons of saturated [and smoothed]
photographic profiles generated during the
compilation of the VPC with other
observers' CCD surface-photometry. In most cases, the extent of the saturated
core within the photographic image is clearly discernable. The dashed lines
represent the bright isophote limits listed in Table 5 (click here)
Additional comparisons with CCD surface photometry were only possible in the cases of a few saturated galaxy images. This was because the published CCD surface photometry is confined to the very brightest galaxies in the Virgo Cluster, most of which were resolved into multiple images by the image-segmentation software. Those saturated galaxy images that did not suffer from image fragmentation, and for which relevant surface photometry could be found in the literature, are compared with these other sources in Fig. 22 (click here). Note that the VPC surface-brightness profiles plotted are based on the heavily smoothed plate-scan data.
As (B-V) colour-gradient information was not available for NGC 4458 or
NGC 4468, and Peletier et al. (1990) found no evidence for any colour gradient
in the case of NGC 4551, mean (B-V) colours from the RC3 were used for
the transformations
from the system to the B system. These colours were
0.90 for NGC 4458, 0.86 for NGC 4468 and 0.96 for NGC 4551. Note that
Peletier et al.'s 
-band profile of NGC 4551 is likely to be more reliable than
Jedrejewski's (1987) even though published aperture photometry was used for
calibration purposes in both cases, as the former profile was the weighted mean
of four different observations. It should also be noted that the exceptionally
good agreement between Peletier et al.'s -band profile of NGC 4551
and the corresponding photographic profile, cannot be attributed wholly or even
largely to the coincidence that the VPC's -band zero point was partially
based on one of Peletier et al.'s four -band frames of that galaxy.
This is because, not only did we reduce the [one] common CCD frame
independently from Peletier et al., but our absolute calibration was based on
on observations of standard stars (as described in
Sect. 3.4 (click here)) rather than on existing aperture-photometry measurements.
The apparent mean zero-point offsets for the B profiles were
(
) mag,
(
) mag,
() mag.
These mean residuals were computed for those parts of the profiles for which
the reduced radius exceeded 20 arcsec, and N in these cases
refers to the number of data points on the relevant profile(s) compared (rather
than the number of galaxies compared). Discrepancies between the photographic
profiles and Peletier et al.'s U and profiles were negligible,
though Jedrejewski's profile was significantly brighter than both the
photographic profile and Peletier et al.'s profile.
From the comparisons made in this section, any major error in the zero points
of the VPC's magnitude scales would seem unlikely, though it would appear
possible that the true zero points could be slightly brighter than those
adopted by as much as 
mag, particularly in the case of the
-band. One possibility is that Eq. (1) may not be as sound as it
had been hoped and that its degree of applicability may vary significantly with
galaxy type. In judging the -band comparisons made in this section, it
must be remembered that the galaxy samples of other observers do not tend to
cover all morphological types, but are often confined to a single type of
object. Minor systematic discrepancies between the
different mean residuals computed for different observers' photometry can
therefore be expected.
There is still some disagreement as to the optimum relationship describing
transformations between the UKST system (IIIa-J, GG395) and the Johnson
B and V systems. Equation (1) is identical to that used by Buta & Corwin
(1986). Blair & Gilmore (1982) derived a colour term of 
empirically, by plotting the colours of a large stellar sample, whilst
Warren et al. (1991) derived a colour term of -0.35 by a similar method.
The latter authors noted also that their empirical value disagreed with their
predicted value of -0.26, which was based on models of the manufacturer's
filter transmission data and emulsion sensitivity functions.
More recently, Metcalfe et al. (1995) have demonstrated the case for the
colour term being at least as negative as -0.35 based on a galaxy sample.
It should also be remembered that even the unsmoothed plate-scan data suffered
from relatively coarse sampling (2.1 pixels) as well as relatively poor
seeing. When other observers' photoelectric observations were made
under good seeing conditions, slightly negative 
residuals would be expected for small apertures. Although
this bias would make the VPC zero points look slightly fainter in these
comparisons, it is not applicable to the VPC's -total magnitude
scale, nor does it have more than second-order consequences for the
VPC's isophotal magnitudes (as is evident from Table 8 (click here)).
In conclusion, the exact zero-point errors in the VPC's magnitude scales
cannot be estimated with confidence on the basis of the existing
photoelectric measurements for Virgo galaxies in the literature, which exhibit
some mutual disagreements. However, on the basis of all of the comparisons
made in this section, our best estimates of the observed
U-band and -band zero-point discrepancies are -0.10 and -0.15
magnitudes respectively. It cannot be overemphasized that a simple weighted
mean of the zero-point discrepancies found with respect to existing
photoelectric aperture and CCD photometry (which would give most weight to
Caldwell's 1983 measurements) would be unlikely to yield a meaningful result
because most observers did not document the FWHM of their
seeing discs and because of the variation in the aperture sizes used (with
Caldwell's apertures being amongst the smallest). It is probably fair to assume
that most observers' photometry had the twin advantages of better seeing and
sampling than the VPC's (even before smoothing) as discussed in the previous
paragraph. The actual zero-point errors are therefore almost without
any doubt smaller than the observed discrepancies.
The overall very provisional zero-point error estimates are therefore
mag(other work)-mag(VPC)=0.0+0.0-0.05 and 0.0+0.0-0.1
for the U and bands respectively.
In the case of the -band magnitude scale, there is very little
photometry in the literature, and meaningful comparisons with other sources
were not possible until Durrell's (1997) observations of several dwarfs
that are also [unsaturated] VPC objects. Durrell (1997) has already
noted the excellent agreement he found between his total magnitudes (which
were derived from CCD observations) and those of Young & Currie (1995),
hereunder YC3, which he based on three dwarf galaxies in common between the two
samples (
mag). As described in
Sects. 4.2 (click here) and 4.3 (click here), we have improved our total-magnitude
extrapolation procedure since YC3. We now find an even
smaller zero-point offset of 
0.06
mag based on the same three galaxies.
Acknowledgements
We thank the UKST Unit for loan of the plate material; the ROE's Image & Data Processing Unit for scanning the plates with COSMOS; Starlink, Oxford University Computing Service, the QSO & Observational Cosmology Group at BAO and Wu Xiang-ping for use of their computing facilities; Tony Lynas-Gray for maintaining the Oxford astrophysics VAX cluster beyond its period of support by Starlink; Jon Godwin for his software environment and in particular for his image-segmentation software; Reynier Peletier for several CCD frames; Michael Drinkwater for spotting a galaxy that we had overlooked and Bruno Binggeli for a machine-readable version of the VCC. CKY gratefully acknowledges a SERC studentship for the early part of this work and a PDRF from the National Postdoctoral Fellowship Office of China for the later part.