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6. Scope of the catalogue

Previous catalogues of Virgo Cluster galaxies have been limited in several respects. First, the vast majority of their galaxy magnitudes were estimated by eye without independent zero-point calibrations, secondly, the galaxy samples themselves were selected by eye and thirdly, they were confined to a single pass band. The VPC does not suffer from any of these limitations. Also, the VPC does not discriminate against background galaxies (unlike the VCC which excludes non-Zwicky objects that were deemed to be in the background).

  figure685
Figure 9: Map of the areas of the sky surveyed by the VPC (vertically shaded area) and VCC (heavily shaded area tex2html_wrap_inline3353 VPC survey area)

The VPC however, covers a region of sky smaller in angular extent than those covered by de Vaucouleurs & Pence (1979) or by the VCC. The VPC survey area, which is centred on Cluster A, is compared with the VCC's in Fig. 9 (click here).

6.1. Galaxy magnitudes

Most VPC-galaxy magnitudes have been derived by the numerical integration of segmented plate-scan data and have been calibrated with several CCD frames and/or several hundred aperture-photometry measurements. We therefore expect these magnitudes to be a significant improvement on those found in previous catalogues. For the highest surface-brightness VPC galaxies however, the plate-scan data suffered from saturation effects, and alternative measurements had to be sought from the literature. As can be seen from Fig. 10 (click here), saturation was a problem in the tex2html_wrap_inline3309 band for most galaxies brighter than tex2html_wrap_inline3799, though fewer galaxies were saturated in the U band. For a very small minority of saturated galaxies, no previous measurements could be found in the literature and eye estimates were necessary.

  figure696
Figure 10: Frequency distribution of VPC galaxies as a function of tex2html_wrap_inline3571. The heavily shaded areas represent objects saturated in tex2html_wrap_inline3309

6.2. The galaxy sample

There has been much discussion concerning the completeness of galaxy surveys in the literature, and particularly of those conducted by eye; see e.g. Phillipps et al. (1988). As the VPC galaxy sample was selected primarily by means of an automated process, it was possible to quantify the observational limits to the sample with reasonable precision. These limits were:

(1) tex2html_wrap_inline3811 for the whole survey area
and
(2) profile-slope parameter (see Sect. 3.6 (click here)) tex2html_wrap_inline3589.

However, Plate J4882 was found to be significantly desensitised over Fields C and D (by tex2html_wrap_inline3815 mag) and the magnitudes for those fields had therefore to be based on J9229 alone. Mean magnitudes (based on both the J4882 and J9229 values) were nevertheless adopted for Fields A and B. These considerations were found to lead to the VPC-sample completeness limits of:

(1a) tex2html_wrap_inline3817 for Fields A and B,
or
(1b) tex2html_wrap_inline3819 for Fields C and D,
and
(2) profile-slope parameter tex2html_wrap_inline3589,

which are derived at the end of this subsection.

  figure708
Figure 11: Frequency distribution of VPC galaxies unsaturated in tex2html_wrap_inline3309, as a function of peak surface brightness; the heavily shaded areas representing those galaxies brighter than the completeness limits (tex2html_wrap_inline3825 for Fields A and B; tex2html_wrap_inline3827 for Fields C and D)

Should populations of ultra-low-surface-brightness galaxies exist (of peak surface brightnesses fainter than 25.0 tex2html_wrap_inline3441 in tex2html_wrap_inline3309) they would therefore remain uncatalogued regardless of their apparent magnitude because isophotal magnitudes rather than total magnitudes were used to select the galaxy sample. Figure 11 (click here) would however suggest that galaxies with peak surface brightnesses fainter than tex2html_wrap_inline3833 tex2html_wrap_inline3441 do not normally have flat enough surface-brightness profiles to qualify as 18th-magnitude (or brighter) objects, unless they possess very extensive haloes of surface brightness slightly fainter than 25.0 tex2html_wrap_inline3441. Likewise, discrimination against ultra-low-surface-brightness galaxies on account of using isophotal magnitudes cannot be a problem unless such galaxies have very flat profiles and are quite large in angular extent (e.g. an object of mean surface brightness =26.00 tex2html_wrap_inline3441 would have to subtend 28 arcsec in diameter in order to be a 19.0 magnitude object or 35 arcsec in diameter in order to be an 18.5 magnitude object). Although giant galaxies of very low surface brightness are known to exist (e.g. Malin 1, which escaped detection in the VPC) it is unlikely that such objects could be prolific enough to be a major hazard.

  figure721
Figure 12: Frequency distribution of VPC galaxies as a function of the profile-slope parameter tex2html_wrap_inline3379 (see Sect. 3.6 (click here)); the heavily shaded areas representing those galaxies brighter than the completeness limits of (tex2html_wrap_inline3827 for Fields A and B; tex2html_wrap_inline3825 for Fields C and D)

It would appear from Fig. 12 (click here) that brighter than the completeness limit of tex2html_wrap_inline3827 galaxy loss due to the limit tex2html_wrap_inline3589 is probably not significant. However, from the same figure it can be seen that [due to this criterion alone] the number of galaxies lost from the sample within the range tex2html_wrap3809 is probably quite significant. In other words, for tex2html_wrap_inline3855, the galaxy profiles tend to be significantly more starlike than for tex2html_wrap_inline3857. Unfortunately, it is rather difficult to extrapolate the frequency distribution of galaxies as a function of tex2html_wrap_inline3379 beyond the tex2html_wrap_inline3581 cut off with great confidence, even though the indications are that the frequency is already in sharp decline as tex2html_wrap_inline3379 increases towards the cut off. An order of magnitude estimate based in Fig. 12 (click here) would suggest that somewhere in the region of ten or possibly twenty objects of tex2html_wrap_inline3865 might have been excluded by the limit in tex2html_wrap_inline3379 alone.

As for stellar contamination, this is unlikely to be a serious problem as objects for which tex2html_wrap_inline3379 exceeded -0.065 were excluded. Only in a couple of cases was there any uncertainty as to whether an object was a star or a galaxy, as all galaxy candidates were visually inspected. Overlapping stellar images were occasionally difficult to distinguish from elliptical galaxies, but it is extremely unlikely that stellar contamination could account for even 0.25tex2html_wrap_inline3873 of the objects catalogued (i.e. about three objects in total).

From Fig. 13 (click here) it is evident that the VPC galaxy sample is complete to tex2html_wrap_inline3825 for Fields A and B (assuming tex2html_wrap_inline3877). It should be noted that the slight asymmetry in the distribution of data points about tex2html_wrap_inline3879 is due to the sensitivity of J4882 being less consistent than that of J9229. As a consequence, the most wayward data-points tend to correspond to images that yielded higher magnitudes on J4882 than on J9229. For Fields C and D, there is almost certainly completeness at tex2html_wrap_inline3881, but galaxy loss probably only becomes significant beyond tex2html_wrap_inline3827, as can be seen from Fig. 14 (click here).

  figure748
Figure 13: The disparity between magnitudes generated from different tex2html_wrap_inline3309 plates as a function of mean magnitude [as quoted in the VPC] for Fields A and B combined. The completeness limit (dashed line) and the observational selection criterion, tex2html_wrap_inline3811, (solid diagonal line) are shown for reference

  figure755
Figure 14: The completeness limit (dashed line) to the galaxy sample in the cases of Fields C and D. Note that due to pronounced de-sensitisation of Plate J4882's emulsion over Fields C and D, a mean zero-point shift of 0.31 mag was observed between those magnitudes from Fields C and D of Plate J9229 [which were included in the VPC] and those due to J4882 for the same fields [which were discarded]

6.3. Galaxy colours

Equal-area and total (tex2html_wrap_inline3607) and (tex2html_wrap_inline3609) colours have been computed for most VPC galaxies; the main exceptions generally being those whose images were saturated in one or both relevant pass bands, and/or those that appeared heavily merged with adjacent images (whether galaxies or stars). For the equal-area colours, the areas were generally defined by the tex2html_wrap_inline3901 tex2html_wrap_inline3441 or the tex2html_wrap_inline3905 tex2html_wrap_inline3441 isophote. The total colours were based on the differences between extrapolated tex2html_wrap_inline3909, tex2html_wrap_inline3741 and tex2html_wrap_inline3913 values, and as a whole are probably less susceptible to systematic effects even though they exhibit a considerably larger scatter. However, for any individual galaxy, the equal-area colours are generally more reliable than the total ones (see Sects. 7.2 (click here) and 7.3 (click here)). It was also possible to estimate very approximate total (tex2html_wrap_inline3607) colours for many of the saturated objects by means of Eq. (5 (click here)) (which is an approximation based on Eq. (1)) from values of tex2html_wrap_inline3617 and tex2html_wrap_inline3919 whenever listed in the RC3. Unfortunately though, published (tex2html_wrap_inline3609) colours could not be found for those objects whose images were saturated.


 equation777

6.4. Radial velocities

  figure785
Figure 15: Frequency distribution of VPC galaxies still lacking published velocities (unshaded areas), those measured for the first time by Drinkwater et al. (1996) (vertically shaded areas) and those measured previously (unshaded); all as a function of tex2html_wrap_inline3571

Only a minority of galaxies fainter than tex2html_wrap_inline3925 have published radial velocities, but the situation has improved since Drinkwater et al.'s (1996) study. The present situation is depicted in Fig. 15 (click here).

6.5. Morphological types

Morphological types are essentially complete to tex2html_wrap_inline3929, but sporadic beyond this limit as shown by Fig. 16 (click here). As the Du Pont plates of Binggeli et al. (1985) had considerably larger plate scales than did our UKST plates, VCC types have been adopted for almost all of the galaxies in common between the VPC and the VCC galaxy samples. Those galaxies which were brighter than tex2html_wrap_inline3929 yet not included in the VCC (generally because they were deemed to be background objects) were classified by visual inspection of a copy plate of UKST plate J9229. Due to the small scale of this copy plate and the problem of image saturation in many cases, no attempt has been made to provide detailed morphological classifications (e.g. spiral subclasses or luminosity classes) in the VPC.

  figure800
Figure 16: Frequency distribution of VPC galaxies that have been typed (heavily shaded areas) and untyped (unshaded areas) as a function of tex2html_wrap_inline3571. Note that any unshaded area brightward of tex2html_wrap_inline3929 represents galaxies which defied typing

6.6. Astrometry

Unambiguous identification of the vast majority of galaxies within the central Virgo field (background objects included) brighter than tex2html_wrap_inline3939 should now be possible, as the use of a measuring machine has enabled improved positions to be computed. Only for a subset of objects saturated in tex2html_wrap_inline3309, was it necessary to extract the coordinates from the RC3 or occasionally from the VCC, though these objects are generally so extended that the positional accuracy required to identify them unambiguously is not that high. The VPC also contains orientation and ellipticity information.


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