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2. Sample definition, observations and datareduction

The sample was selected from the Virgo Cluster Catalogue (VCC) of Binggeli et al. (1985, 1993), which is complete to tex2html_wrap_inline2086, by applying the following criteria:

Limitations on telescope time dictated that not all the 207 VCC galaxies satisfying these criteria could be observed. Therefore, a further criterion based on projected position was introduced, as sketched in Fig. 1 (click here).

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 tex2html_wrap_inline2090 (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 (tex2html_wrap_inline2092) were also excluded.

The resulting sample of 117 galaxies is complete to tex2html_wrap_inline2094, and both the cluster-periphery and -core subsamples span the range tex2html_wrap_inline2096. 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 tex2html_wrap_inline2098, includes all the SO/a (6 objects), 62 of the 63 spirals and 25 of the 43 Im-BCDs.

2.1. Observations

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 tex2html_wrap_inline2102 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. tex2html_wrap_inline2104 arcmintex2html_wrap_inline2106, 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 tex2html_wrap_inline2108 arcmintex2html_wrap_inline2110.
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 tex2html_wrap_inline2112 arcmintex2html_wrap_inline2114. 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 tex2html_wrap_inline2116.

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) (tex2html_wrap_inline2120). These diameters are isophotal diameters at 25.5 mag tex2html_wrap_inline2122.
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 tex2html_wrap_inline2132m  and 2.1 tex2html_wrap_inline2134m  the sky brightness at Calar Alto was typically 13.8 and 13.1 mag arcsectex2html_wrap_inline2136   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 arcsectex2html_wrap_inline2150  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 tex2html_wrap_inline2152 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

2.2. Photometric calibration

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): tex2html_wrap_inline2168. The typical uncertainty of the measurements in photometric periods is 0.05 mag.

Table 4: Calibration stars

2.3. Image analysis

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 Astrophysicsgif.
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 tex2html_wrap_inline2180.
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 (tex2html_wrap_inline2182) were combined using a median filter to obtain tex2html_wrap_inline2184 for type ``A'' mosaics. For type ``B'' mosaics tex2html_wrap_inline2186 was obtained by combining the 9 frames containing target + SKY with a median filter.
The mean counts tex2html_wrap_inline2188 and tex2html_wrap_inline2190 were respectively determined for the tex2html_wrap_inline2192 target observation and the median sky. Individual ``normalized'' tex2html_wrap_inline2194 frames were then produced such that tex2html_wrap_inline2196. 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 (tex2html_wrap_inline2198) was processed to obtain a flat-field, sky subtracted, corrected frame: tex2html_wrap_inline2200.
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 (tex2html_wrap_inline2202) (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.

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