6 Analysis

The analysis presented in this Sect. is based on 818 galaxies listed in Table 1. Assuming an average B-H=3 mag, the optical completeness levels given in Sect. 2 translate into 100% completeness at ${\rm Log}~L_{\rm H_\odot}>10$ for the Coma supercluster, 100% at ${\rm Log}~L_{\rm H_\odot}>9.4$ and 50% at ${\rm Log}~L_{\rm H_\odot}>8.6$ for the Virgo cluster. The 50% unobserved Virgo galaxies with $16.0>m_{\rm p}>14.0$ mag includes objects whose surface brightness, as judged on the DSS plates, was fainter than what we could expect to detect with a 4 m telescope in one-hour integration.

6.1 The frequency of profile decompositions

We consider the distribution of profile decompositions along the Hubble sequence only for galaxies in the Virgo cluster, for which the morphological classification is most reliable. This is shown in Fig. 5. It is apparent that pure de Vaucouleurs profiles are present only in 40% of Es and in 30% of S0s. Their contribution drops to zero both for later types and for the early type dwarfs. The exponential profiles are nearly absent among early type giant systems up to Sab, while their frequency is high (44%) in dwarf E+S0s, and increases from 40% (Sc) to almost 100% for later types. Mixed (M) decompositions dominate among dwarf E and S0s (50%) and giant Es (50%), increasing up to 90% among Sb galaxies, then drop to zero for later types. Truncated profiles (T) are rare (their frequency is always <35%), and are absent from giant early-type galaxies up to Sc spirals.

Figure 6 shows the relative fraction of profile decompositions plotted as a function of the H band luminosity ( ${\rm Log}\, L_{\rm H}/\!\!~L_\odot=11.36-0.4H_{\rm T}+2{\rm Log}\,D$ (D in Mpc)) for the 818 objects in the Virgo+Coma sample (top-left panel), for the early type (dE-E-S0a) (bottom-left panel), late type (Sa-BCD) (bottom-right panel) and Sc-Sd galaxies alone (top-right panel). All panels show similar trends, indicating that the dependence of the frequency of profile decompositions on luminosity is independent of the morphological type. The fraction of pure de Vaucouleurs profiles strongly increases with the H luminosity, being absent for $L_{\rm H}<10^{9.5}$ $L_\odot$, a luminosity range where the pure exponential profiles dominate, since their frequency clearly anti-correlates with luminosity. At the faintest luminosities, however truncated profiles are abundant among late-type galaxies. The frequency of mixed profiles increases monotonically with luminosity among late-type galaxies, while it reaches a maximum at $L_{\rm H}\sim~10^{10}$ $L_\odot$ for the early-type ones, because for higher luminosities these galaxies have increasingly more frequently pure de Vaucuoleurs profiles.

6.2 The light concentration parameter $\mathsfsl{C_{31}}$

Figure 7 shows the remarkable dependence of C₃₁ on luminosity found by Scodeggio et al. (in preparation) and extended here to comprise dwarf galaxies. We confirm that high C₃₁ (cusps+extended haloes) are almost completely absent at $L_{\rm H}<10^{9.5}$ $L_\odot$. Faint galaxies cluster around C₃₁=2.80, which is the expected value for pure exponential profiles. This result is completely independent from galaxy morphology. On the other end high C₃₁ (bulge-dominated) objects are present only at high luminosity $L_{\rm H}>10^{10.5}$ $L_\odot$. These are a mixture of giant E and Early-type spirals. There exist however a significant class of high-luminosity (giant), low C₃₁ (bulge-less) galaxies which appears to be confined to Sc galaxies.

Figure 5: The fraction of pure de Vaucouleurs, mixed, pure exponential and truncated profiles along the Hubble sequence for galaxies in the Virgo cluster.

Figure 6: The fraction of pure de Vaucouleurs, mixed, exponential and truncated profiles as a function of the NIR luminosity among all galaxies (top-left), among the Sbc-Sd (top-right), among early types (dE-E-S0a) (bottom-left) and among spirals (Sa-BCD) (bottom-right). The completeness levels of 100% and 50% for the Virgo cluster, computed assuming an average B-H=3 mag, are at 9.4 and 8.6 ${\rm Log}~L_{\rm H_\odot}$ respectively.

Figure 7: The dependence of C31 on luminosity. Points are coded in three classes of Hubble type (dE-S0a = triangles; Sa-Scd = squares; Sd-BCD = circles). Measurements reported in this paper are given with filled symbols. Open symbols are from Paper V. Two completeness levels for the Virgo cluster, computed assuming an average B-H=3 mag, are indicated with dashed lines.

6.3 Color gradients

Only 22 dE/dS0 in Virgo analyzed in this paper have either B-V or B-Hcolor profile (see Fig. 4). Nine among these 22 (41%) have no radial color gradients. Another 9 have a red central excess consistent with an age or metallicity gradient toward the center. The remaining 4 (VCC 781, 951, 1499, 1684, representing a non-negligeable 18% of our sample) have instead a blue central excess consistent with a nuclear post star-burst phase (see below the discussion on VCC 1499). In spite of the paucity of the available data, we find that the color of the central excess correlates with the global color ( $<B-V>_{\rm B excess}$ = 0.61, $<B-V>_{\rm R excess}$ = 0.82) and that the 4 $B_{\rm excess}$objects have on average slightly lower $<L_{\rm H}>$ = 8.9 than those with $R_{\rm excess}$ ( $<L_{\rm H}>$ = 9.1).
This evidence is consistent with the analysis by Kormendy & Djorgovski (1989) (see their Fig. 4).

We also find that the color distribution of dEs overlaps with that of dIs (see Fig. 8), though the mean colors of the two groups differ significantly.

Excluding galaxies with $m_{\rm p}>16.0$, because we don't have measurements of dEs fainter than this limit, we explored the possible continuity in the structural parameters of dEs and dIs, as proposed by Sung et al. (1998). dEs (9 objects) and nucleated dE-Ns (27 objects) are indistinguishable from each other both in colors ( $<B-H>~\sim3.1$) and in C₃₁ ( $<C_{31}>\ \sim3.4$). The only subclass of dEs with significantly bluer colors and lower C₃₁ are dE-pecs (6 objects) which have $<B-H>\ \sim 2.7$ and $< C_{31} >\ \sim 2.7$, consistent with $<B-H>\ \sim 2.5$ and $< C_{31} >\ \sim 2.5$ of dIs (16 objects).

Figure 8: The distribution of dE/dS0 and dI in bins of B-H.

Figure 9: B-H color map of VCC1491-1499 (white = red,black = blue). Grey levels span from B-H=2.35 (darkest) to B-H=3.75. North is up, East to the left.

Figure 10: Long-slit spectra of VCC1491-1499. The flux is normalized at $5500~\rm\AA$.

An illustrative and meaningful example of the wide range spanned in color and color gradient by dEs is offered by the two galaxies VCC1491 and 1499 which happen to lie 1.5 arcmin apart in the same frame (see Fig. 1). The two have V mag differing by 0.01 mag, thus they are indistinguishable galaxies in all respects. However VCC1491 is almost as red (B-V=0.82, B-H=3.42) as a giant elliptical, while on the opposite VCC1499 (dE-pec) is almost as blue (B-V=0.49, B-H=2.67) as a typical dI. Moreover VCC1499 shows a strong blue central excess, while VCC1491 presents a shallower red gradient toward its centre (see Fig. 9 and profiles in Fig. 4). Using the Carelec spectrograph (Lemaitre et al. 1990) attached to the OHP 1.93 m telescope we obtained in February 2000 long slit spectra for the two objects, shown in Fig. 10. The spectral signatures of the two galaxies are significantly different: 1491 resembles a typical dE galaxy, while 1499 has a much bluer continuum and strong Balmer absorption lines (EW $H_\delta \sim 8~\rm\AA$) typical of E+A galaxies which have experienced an intense burst of star formation ended about 1-2 Gyrs ago (Poggianti & Barbaro 1996).