We have obtained NIR
surface photometry of a complete volume-limited sample
of late-type galaxies in the
Virgo cluster.
Some obvious features of this sample can be briefly discussed here.
Total B and K' magnitudes, taken from Table 3, will be
used in the following analysis.
The relation between the optical diameters 
determined at the 25.5
mag arcsec
isophote by Binggeli et al. (1985) and the K' diameters
determined in this
work is shown in Fig. 4 (click here). Near infrared diameters at the 22.0 mag arcsec
isophote are systematically smaller than the optical ones at the 25.5 mag arcsec
isophote.
The linear regression analysis gives the following relations:
which imply that the present NIR observations cover a substantial fraction
of the optical size of the galaxies.
Figure 3: a): Left hand panels: Grey-level/contour representations of the
K' band brightness distributions of galaxies measured with the 2.2 m telescope in
both H and K' bands. N is up, E to the left. The faintest contour level
shown is 20.5 mag arcsec
with a step of 0.5 mag arcsec
; the grey scale is
arbitrary. The scale given in the image is in pixels. The angular scale of the
images can be determined using the pixel size given in Table 5.
Right hand panels: Differential H and K' band radial surface brightness
and colour profiles obtained by azimuthal integration along elliptical annuli.
The horizontal scale gives the radius in arcsec as measured from the galaxy
centre along the major axis of the ellipse.
b): as for a) but for galaxies with only one band image available.
c): as for b) but for galaxies observed with the 3.5 m telescope;
(if both the observations are available the combined image is given).
The faintest contour level shown
is 21.0 mag arcsec
with a step of 0.5 mag arcsec
Figure 5 (click here) shows that the ratio between the optical (
) to
NIR (
) diameters correlates with the B-K'
colour index, (bluer galaxies have larger optical-to-NIR ratios)
(given the uncertainty on the B photographic magnitudes, the estimated
error on the B-K' colour index is 
).
Since the ratio of the optical to NIR surface brightness is not constant
in galaxies of different luminosity and colour (Gavazzi et al.
1996c), the trend observed in Fig. 4 (click here) is not entirely due to
the non-linearity between the optical and NIR diameters.
Figure 4: Relation between K' band diameters at the 22
mag arcsec
isophote determined in
this work and the optical B diameters from Binggeli et al. (1985)
Figure 5: The relation between the optical (
) to NIR
(
) diameter ratio versus the B-K'
colour index
This result indicates that in the red galaxies, which are generally bright spirals (see Sect. 4.2), the old and young stellar components are distributed on discs of comparable dimensions. In the bluest objects, which are generally dwarfs, young stars are characterized by an higher surface density, thus by a larger isophotal diameter, than the old stellar component.
Figure 6: Relation between the ellipticity (1-b/a) in the K' band
determined in this work and the corresponding ellipticity derived in the visible.
Galaxies with 
are marked with filled dots,
open dots indicate objects with 
The relation between the ellipticity determined in the
K' band and
the one determined in the B band by Binggeli et al. (1985) is given
in Fig. 6 (click here). Two different symbols are used
for bright (
) and weak (
)
objects. As expected, there is a better agreement for bright galaxies,
while a large scatter is observed for dwarf irregulars and blue compact objects, for which
ellipses are just a crude representation of their shape.
Part of the scatter is
probably due to the different techniques used to determine the ellipticity by
Binggeli et al. (1985) and by us:
while in the B band the ellipticity is calculated from the ratio of the minor
to the major diameter at 25.5 mag arcsec
isophote, in the NIR it is given by the ellipse which
best fits the 
mag arcsec
isophote of the galaxy.
Figure 7: Relation between the uncorrected colour B-K' and the morphological
type
Figure 8: Relation between the concentration index
(at the 22.0 mag arcsec
isophote) and the morphological
type
The relation between the uncorrected colour index B-K' and the
morphological type is given in Fig. 7 (click here), while Fig. 8 (click here) shows the
relation between the concentration index (at the 22.0 mag arcsec
) and the
morphological type.
Galaxies of earliest morphological classes have in general redder B-K' colours
and higher concentration indices than dwarfs and late type systems, indicating
that they are generally dominated by old stars mainly distributed in the bulge
and/or in steep discs. The
scatter in both relations is however large. Pure exponential discs
(
; Gavazzi et al. 1996c) are found
generally
in late-type systems, with some exceptions in early spirals.
This result is in agreement with that found by Gavazzi et al. (1996a,b). It is interesting to note that, although BCD galaxies have B-K' colours similar to Sm and Im galaxies, they have higher K' concentration indices. This indicates that in BCD galaxies the stars emitting in the NIR bands have a centrally concentrated distribution, as for the blue component.
Figure 9: Relation between the B-K' colour and the K' magnitude
The relation between the B-K' colour and the K'
magnitude is given in Fig. 9 (click here).
Since the galaxies are at the same distance, this relation
corresponds to a colour - absolute magnitude relation.
Furthermore, as shown by Gavazzi (1993) and Gavazzi et al.
(1996c), the NIR absolute magnitude is a good indicator of the total mass
of galaxies. Figure 9 (click here) thus indicates that brighter and
more massive galaxies have redder colours than low mass objects. The scatter in
this relation is significantly lower than in the colour - morphological type
relation, indicating that the history of star formation, as traced by the
B-K' colour, is probably more closely related to the mass of the
galaxies than to the morphological class. No systematic differences are
observed between cluster core and periphery objects.
Given the low scatter, many authors
(Wyse 1982; Tully et al. 1982; Bothun
1984) suggested to use the NIR colour-magnitude relation as a distance
indicator. Figure 9 (click here) shows however that the dispersion in the B-K'
versus K' relation is 
, thus significantly
higher than the one obtained with other distance indicators such as the
Tully-Fisher relation (Bothun 1984; Jacoby et al.
1992).
Figure 10: Relation between the concentration index (at the
22.0 mag arcsec
isophote)
and the K' magnitude
All galaxies with 
have a concentration index 
, which
is consistent with an exponential disc (Gavazzi et al. 1996c),
independent of their morphological type (see Fig. 10 (click here)). Stars are more
concentrated in bulges and/or in
discs only in luminous spirals (
).
The presence of bulges and of steep discs
in galaxies is thus better discriminated by the NIR luminosity (or by the mass) than by
the morphological type.
An accurate analysis of the colour-magnitude relation and of
the concentration index-magnitude relation extended to a complete sample including dwarf
and giant galaxies is addressed in Gavazzi et al. (1996c).
Inspection of Fig. 3 (click here) clearly shows that the majority of the surveyed galaxies can be represented with exponential light profiles, typical of disks. Only a minority - generally bright early type spirals - show the characteristic signature of the bulge. Other features which characterize the morphology in the visible bands are also present in the NIR. Spiral arms, bars and rings are evident in spiral galaxies (see for example VCC 1615) albeit more diffuse and less clumpy. Many of the morphological differences observed between B and K' images are due to the more drastic effect of dust obscuration at optical wavelengths. A typical case is the edge-on VCC 92 (NGC 4192), which is one of the largest galaxies in the Virgo cluster. In the B band (Boselli et al., in preparation) a prominent dust lane is observable across the nucleus, and two spiral arms are clearly visible in the outer parts. The K' image shows a bright boxy-shaped central region and the two spiral arms close to the centre of the galaxy, not observable in the B band because of dust extinction.
Figure 11: a) The B luminosity function of the complete volume-limited
sample of 88 late-type galaxies (
Sa) with 
observed
in this work. The dashed line indicates the 62 spirals (Sa 
type 
Sm), while the full line shows the 26 dwarfs (Im, BCD). b) as for a)
but in the K' band
In some spiral galaxies, however, giant HII regions can occasionally be seen
in the NIR images. An example is VCC 66 (NGC 4178),
where two giant HII regions are observable in the
S-W part of the disc, about 
from the nucleus. At these
wavelengths, the emission of the HII regions is dominated by relatively young
(
) red supergiant stars (Mas-Hesse & Kunth 1991),
while in the B band and in particular in 
the emission is
dominated by young and massive OB stars (
) (Kennicutt
1990). Low surface brightness galaxies are faint and diffuse in the NIR.
Compact structures in the NIR are more common in BCD galaxies and contribute
significantly to the integrated emission. As for giant spirals, these clumpy
structures are HII regions, and in the NIR their emission is probably dominated
by relatively young red supergiants. On the other hand, HII regions visible in
and in B band are sometimes not seen in the NIR; in this case
the HII regions are probably too young (
)
to have produced red supergiants. The comparison of NIR with optical and
images could be a useful tool to evaluate the age of HII regions
within galaxies; some preliminary comparisons between 
and
K' images are given by Boselli et al.
(1995b). We plan to use the available NIR
images in conjunction with new 
(Hippelein et al. 1996) images to study
the recent star formation history of these objects and
to investigate dependencies on morphological type and
environment.
The subsample of spiral galaxies with available K' data constitutes
a complete, volume-limited sample suitable for statistical studies. At present
all galaxies in the sample with 
and of morphological type
S0a or later have been imaged (94 objects). In Fig. 11 (click here)a we compare
the B luminosity function of the 62 spiral galaxies (Sa to Sm;
dashed line) with the one of the 26 dwarfs (Im, BCD, Im/BCD, Sm/BCD; continuum line)
observed in this work (using B magnitudes from Binggeli et al.
1985). Given the limiting magnitude of the VCC catalogue of 
, the bell-shaped distribution centered at 
represents the
intrinsic luminosity function of spiral galaxies in Virgo and, as emphasised
by Sandage et al. (1985) is not due to an observational cut-off. On
the other hand, the distribution of the dwarfs represents
the bright tail (B > 13.0) of their luminosity function (see Fig. 19
of Sandage et al. 1985);
the cut-off at 
corresponds to
the magnitude limit of the present observations.
Figure 11 (click here)b shows the
K' luminosity function of spirals and dwarfs
(notice however that
this is an ``hybrid" K' luminosity function since galaxies were
selected in the B band, thus the sample is not magnitude limited
in the K' band).
Two features are worth mentioning: 1) the average B-K' of spirals is 
4.0 mag, while that of dwarfs is 
2.0 mag; 2) the K' band spiral
distribution is significantely broader than the corresponding B band
distribution. The difference between the two distributions is in accordance with
the B-K' versus K' relation shown in Fig. 9 (click here) (see also
Gavazzi et al. 1995b).
Galaxies at the faint end (11< K' <14)
populate the 14<B<16 bins, thus they have <B-K'> 
2.0, bluer
than average.