The sample was selected from the Virgo Cluster Catalogue (VCC) of Binggeli
et al. (1985, 1993), which is complete to 
, by applying the
following criteria:
.
Figure 1: Plot of all VCC galaxies classified as members by Binggeli et al.
(1985) taken from Fig. 1 of Sandage et al. (1985). The subsample of
late-type galaxies described in Sect. 2 are marked either
with 
(objects observed in this work) or with + (objects in the sample
still to be observed). The dots indicated all remaining VCC galaxies,
irrespective of type and membership.
The
2.0 degree radius circle centred on M 87 contains the cluster-core subsample.
The inner boundary of the cluster periphery subsample has a radius of 4
degrees about the position of maximum projected galaxy density
The sky areas from which galaxies were chosen define two
contrasting subsamples to optimise the statistical evaluation
of the cluster environment on observed properties. The cluster-core subsample
is comprised of 44 galaxies with projected positions within 2 degrees of
M 87, essentially from within the X-ray emitting ``atmosphere'' of M 87
(Boehringer et al. 1994).
The cluster-periphery subsample is comprised of 73 galaxies with angular
separations of greater than 4 degrees from the position of maximum projected
galaxy density given by Sandage et al. (1985), but excluding
galaxies within 1.5 degrees of the M49 sub-cluster. To limit the spread of
distances within the sample, galaxies towards the M and W clouds, and in the
Southern extension (
) were also excluded.
The resulting sample of 117 galaxies is complete to 
, and
both the cluster-periphery and -core subsamples span the range 
.
Both subsamples are approximately equally divided between giant
spirals on the one hand and dwarf and irregular galaxies on the other.
The distribution over Hubble type is summarised in Table 1 (click here).
Table 1: Distribution of sample over Hubble type for the cluster-periphery
and cluster-core subsamples
In this paper we report NIR observations of 94 of the 117 selected galaxies.
This subsample, which is complete to 
, includes all
the SO/a (6 objects), 62 of the 63 spirals and 25 of the 43 Im-BCDs.
The observations were carried out in February 1994 and
1995 with the Calar Alto 2.2-m telescope and in February 1996 with the Calar Alto 3.5-m
telescope. The Cassegrain focus of both
telescopes was equipped with the MAGIC 
pixel NICMOS3 infrared array
(Herbst et al. 1993).
In order to observe galaxies with large apparent sizes, the optical
configuration of the detector at the 2.2-m telescope
was chosen to give the largest possible field
of view, i.e. 
arcmin
, with a pixel size of 1.61 arcsec.
For one night in 1995 (February 15) we used an optical configuration with a
pixel size of 0.64 arcsec and a field of view of 
arcmin
.
Fifteen galaxies have been observed at the 3.5-m telescope: 4 of them were
objects observed in non-photometric conditions at the 2.2-m telescope for
which a calibration was necessary, 2 were low surface brightness galaxies
already observed at the 2.2-m telescope but with a poor signal to noise, and 9
were new measurements.
The optical configuration of the detector at the 3.5-m telescope gave a pixel size
of 0.81 arcsec and a field of view of 
arcmin
.
To improve the signal to noise,
the images of the galaxies observed with both telescopes were
scaled to the same resolution and combined.
Out of the 17 scheduled nights, 7 were useful, as specified in the log book
of the observations given in Table 2 (click here). All observations were obtained
with a seeing of typically 
.
Table 2: The logbook of the observations
Table 3 (only available in electronic form)
lists the relevant parameters of the galaxies observed in this work.
The Table is arranged in order of increasing VCC name, as follows:
Column 1: VCC denomination (Binggeli et al. 1985). Galaxies
marked * are not included in the sample as defined in Sect. 2.
Columns 2, 3: NGC and UGC denominations (Nilson 1973).
Columns 4, 5: adopted (1950) celestial coordinates.
Column 6: morphological type, from Binggeli et al. (1985).
Columns 7, 8: major and minor optical B band diameters, in arcminutes,
from Binggeli et al. (1985) (
).
These diameters are isophotal diameters at 25.5 mag 
.
Column 9: photographic magnitude, from Binggeli et al. (1985).
Columns 10 and 11: total H and K' magnitudes obtained by
interpolating the present photometric measurements to the optical diameter along
circular apertures as outlined in Gavazzi & Boselli (1996). The
H magnitudes of galaxies not observed in this work were determined using the
aperture photometry available in the literature, following Gavazzi &
Boselli (1996). These magnitudes are uncorrected for internal extinction,
extinction due to our Galaxy and K dimming.
Obtaining a satisfactory background subtraction is the main difficulty of IR
observations. At 1.65 
m and 2.1 
m the sky brightness at Calar Alto
was typically 13.8 and 13.1 mag arcsec
respectively.
Because of the predominance of faint
extended sources in the sample, the principal waveband
was chosen to be K' rather than H, as the sky was somewhat
more stable in the former band, allowing flatter fields to be obtained.
The K' band (Wainscoat & Cowie 1992) was preferred to K
to reduce the effects of thermal emission from the sky, and from the telescope.
At K' the sky brightness varied over the time scale of an observation
by typically 1% in photometric conditions. The sky variability ranged up to
5% in the worst conditions encountered.
Reaching a brightness limit 8-9 mag arcsec
fainter than the sky requires a careful
subtraction of the sky, necessitating mosaicing techniques (which mimic the
function of the chopping secondary used with aperture photometry IR
measurements).
Three types of mosaic maps, obtained by programming the telescope pointing
along different patterns, were used.
Galaxies with optical diameter larger than half of the size of the field of view
of the array were observed using a
mosaic in which 50% of the time is devoted to the target of interest and
50% to the surrounding sky (``A'' mosaic, Fig. 2 (click here)a). This pattern was
obtained alternating 8 fields centred on the target with 8 observations of the
sky chosen along a circular path around the galaxy (off-set by a field of view
from the centre). The 8 on-target fields were dithered by 10 arcsec in
order to help the elimination of bad pixels.
Galaxies with optical diameter smaller than half of the size of the field of view
of the array
were observed with a mosaic consisting of 9 pointings along a circular path
and displaced from one-another by 2 arcmin such that the target galaxy is
always
in the field (``B'' mosaic; Fig. 2 (click here)b). To avoid saturation
each pointing was split into several short elementary integrations of 
which were added by the on-line MAGIC software.
There were three galaxies with angular sizes larger than the dimension of the
detector; these were
mapped using mosaics expressly prepared according to the shape and orientation
of the galaxy in the sky in order to cover the entire surface
of the target.
In order to get a higher signal-to-noise the mosaics were repeated for
low surface brightness galaxies.
Calibration stars were observed with a third mosaic (``C'', Fig. 2 (click here)c).
This is composed of 5 pointings, starting with the target star near to the
centre of the array followed by pointings in each of the 4 quadrants of the
array.
Figure 2: Typical mosaic patterns used during the observations with the 2.2-m
telescope and the 1.61 arcsec/pixel scale: a) ``A'' mosaic,
b) ``B'' mosaic
and c) ``C'' mosaic. The figures show the outline of the detector array
at the positions observed. The dashed lines in mosaic ``A'' delineate the
sky reference positions
The observations were calibrated and the fluxes transformed into the H and
K' photometric system using the standard stars listed in Table 4 (click here)
(Elias et al. 1982), observed hourly throughout the
night. The
observations of the standard stars
were obtained with a defocused telescope to avoid saturation.
Only HD 40335 has a reference K' mag (Wainscoat & Cowie 1992). For
the others stars, having almost null spectral slopes, K' was determined using
the relation given by Wainscoat & Cowie (1992):
.
The typical uncertainty of the measurements in photometric periods is 0.05 mag.
The reduction of two-dimensional IR frames followed a procedure based on the
IRAF data reduction package developed by NOAO and on the SAOIMAGE and PROS
packages developed at the Center for Astrophysics
.
To remove the detector response, two sets
of flat-field exposures were obtained on the telescope dome with (lamp-on) and
without (lamp-off) illumination with a quartz lamp. The response of the
detector is then contained in the normalized frame
.
Specific reduction strategies were used for the various mosaics, according to
the stability of the sky during the observations. When the sky was stable to
within a few percent during the observation of a galaxy (the large majority of
the observations), the 8 SKY exposures
(
) were combined using a median filter
to obtain 
for type ``A''
mosaics. For type ``B'' mosaics 
was obtained by combining the 9 frames containing
target + SKY with a median filter.
The mean counts 
and 
were respectively determined for the 
target observation and the
median sky. Individual ``normalized'' 
frames were then produced
such
that 
.
This removed the time variations of the sky level, but, due to the source
emission, introduced an (additive)
offset; this was subsequently removed (see below).
Occasionally, when the average response of the detector to the sky changed by
about 5% during an observation, significant temporal variations in the spatial
response of the detector to the sky became discernable. Under these
circumstances, only
the three sky frames closest in time to each target frame were used to
determine the sky.
After sky removal, each target frame
(
) was processed to obtain a flat-field, sky subtracted, corrected
frame: 
.
Sky-subtracted and flat-fielded frames were then registered using
field
stars and combined together with a median filter. This provided a satisfactory
removal of the bad pixels in the final combined image. Tests on the data
showed that the photometry obtained from this use of a median filter was
identical to that obtained with averaging techniques.
Star-subtracted frames were produced by a manual ``editing'' of
the contribution from pointlike sources which are clearly not
associated with the target galaxies.
The residual sky background and its rms noise (
) (in the
individual pixels) were determined in each star-subtracted frame in
concentric object-free annuli around the objects of interest.
We checked the quality of the final images on large and small scales. On small scales the measured noise was always consistent with the expected statistical fluctuations in the photon count from the sky background accumulated over the total integration time. Over scales comparable with the sizes of the galaxies, the images are limited by imperfections in the sky subtraction. Typically, the deviation from flatness from this effect is 0.05% or better for photometric conditions, and 0.08% for non-photometric conditions. Although it has an amplitude of only about 10% of the statistical fluctuations on individual pixels, this deviation is the limiting factor in the virtual aperture photometry.
