Shaw (1987) inspected visually the ESO/SERC J and
Palomar Observatory/National Geographical Survey (POSS) B- or R-band
scanned survey plates.
He derived a frequency
of b/p bulges of 20()% as a lower limit
detecting 23
b/p bulges in a sample of 117 disk galaxies. The sample was mainly selected
from the RC2 (Second Reference Catalogue of Bright Galaxies,
de Vaucouleurs et al. 1976) and included disk galaxies
with a diameter log
(
)
at the 25
(B) mag arcsec-2 isophotal level and a "sufficiently'' edge-on
aspect.
Until today the largest investigated sample of 555 galaxies was presented by
SA87. Their list was extracted from several catalogues and contained all disk
galaxies with restricted total B magnitude (
mag)
and axis ratio (b/a < 0.5).
The galaxies were inspected on film copies of the
ESO Quick Blue survey or the POSS prints by means of a microscope.
A "substantial
number'' of candidates
were scanned using the PDS microdensitometer at ESO and checked with an
image processing system (IHAP).
In this way SA87 found 74 b/p galaxies as a lower
limit. This is 13% of their total sample.
A much larger frequency of b/p bulges was proposed after the investigation
of 73 galaxies obtained with CCD surface photometry
(Dettmar & Barteldrees 1988; Dettmar 1989).
That sample gave
a frequency of )%
and Dettmar (1996) concluded from this sample a lower limit of
the total frequency of
b/p bulges of 35%.
However, only 15 objects
with b/p bulge from this sample were listed by name in a paper of Shaw et al.
(1990)
together with other known b/p bulges from the literature.
Since b/p bulges can only be observed at inclinations down to
75
(Shaw et al. 1990),
in a first step all face-on galaxies are excluded using the axis ratio
according to SA87. S0 galaxies (
)
with
and other disk galaxies with
are included (
).
These different limits are
used since the transformation of the axis ratio into
the inclination angle depends on the morphological type
(Bottinelli et al. 1983; Guthrie 1992). With different formulae
(the simplest is cos
;
see references in Guthrie 1992)
this restriction results
for disk galaxies in a limit for the inclination angle i between 60
and
70
degree. This limit ensures most likely
a detection of all observable b/p bulges.
The final sample meeting these selection criteria
contains 1343 galaxies.
The preferable method to classify a b/p bulge is the use of CCD images, but
for the whole sample the amount of observing time would be unreasonably large.
Therefore all galaxies are inspected using the Digitized Sky Survey (DSS).
This survey is complete over the
whole sky and an investigation of the images of the galaxies
with data analysis software is possible.
The DSS is based on photographic surveys
of the northern
POSS E plates (R-band,
20.0 mag),
the southern SERC J plates (equivalent to B-band,
23.0 mag),
and the southern Galactic plane
SERC V plates (V-band,
14.0 mag) (McLean 1999) and
has a scale of
pixel-1.
Galaxies which are saturated in their central regions are checked
(if possible) with the ESO Lauberts-Valentijn Archive
(Lauberts & Valentijn 1989) kindly made available by
ESO
.
While the images within the
ESO Lauberts-Valentijn Archive are not saturated and have a better scale
(
pixel-1), they
have a lower signal-to-noise ratio compared to the images of the DSS.
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