5 Abell 496 photometric properties

In this section we analyse the general properties of the cluster. We examine closely the following points. First, by means of the Colour-Magnitude Relation, we select the main, early type, component of the cluster population. Second, we estimate the projected spatial distribution of the different types of galaxies and we measure the core radius of the cluster as tracked by bright galaxies. Third, we analyze the photometric properties of the cD central galaxy. Fourth, we study the distribution of galaxy colour as function of their position within the cluster core.

5.1 The colours of the galaxies

On the r/(g-r) plane (Fig. 12) we emphasize the narrow sequence of the linear Colour-Magnitude Relation (CMR): the sequence defines the locus of early type galaxies of the cluster within the plane (Visvanathan & Sandage 1977; Arimoto & Yoshii 1987). The continuous line is determined by fitting the locus of points as defined by elliptical galaxies brighter than magnitude 18, excluding the cD galaxy. The equation derived by the best fit

CMR(r) = -0.025 r + 0.914

has been extrapolated to the limiting magnitude of the frame. The slope of the CMR is consistent with that estimated by Visvanathan & Sandage (1977) for the Virgo cluster (see their Table 1 and Figs. 1 and 2) and very similar to the estimates given by Garilli et al. (1996). The cD galaxy fits quite nicely the locus of the elliptical galaxies and the CMR relation.

Figure 13: Colour-colour planes. Our data are superimposed on the expected colours of elliptical galaxies at different red-shifts: each cross on the continue line represents a 0.05 red-shift variation. The filled square represents the cD galaxy, perfectly placed on the theoretical path at red-shift 0.03. Redder galaxies show expected colours of elliptical galaxies at higher red-shift. Sequence galaxies are slightly bluer than cD galaxy with dispersion increasing with the magnitude (see Fig. 12) Finally, blue galaxies have colours unmatchable with the early type galaxy colours

Figure 14: Projected spatial distribution of all (upper panel), bright ( $r \le 20.0$) galaxies (central panel), and bright sequence galaxies (lower panel). We fit bright galaxies distributions with King functions and we show the two different core radius best values. Comparison between the upper and central panel suggests a luminosity segregation effect; comparison between central and lower panel suggest a colour segregation effect. The density profile is obtained as the average of 36 profiles, and the errors correspond to 1 standard deviation of the 36 values distribution. In the left panel the dotted lines show 1 e 2 core radius

Figure 15: Spatial distribution of red galaxies (upper panel) and blue galaxies (lower panel) as labelled on the magnitude-colour plane. Red galaxies do not show any particular behaviour linked to cluster structure. Blue galaxies remarkably crowd at 1 core radius distance from the centre of the cluster

Several authors have used the CMR to define cluster members (Metcalfe et al. 1994; Biviano et al. 1995; Secker 1996; Lopez-Cruz et al. 1997; De Propris & Pritchet 1998; Molinari & Smareglia 1998) since, by so doing, the contamination, due to the background galaxies, is largely reduced. Given the analytical formula of the linear relation CMR(r), determined above, we define the "sequence zone'' as the colour-magnitude plane region inside the curves

$$(g-r)(r) = CMR(r) \pm (\sqrt{\sigma_g(g)^2+\sigma_r(r)^2}+0.06)~,$$

where we take into account photometric uncertainty at $1\sigma$ level (see Sect. 4.4) and the inherent dispersion of the relation (estimated upon the most luminous galaxies). The plane redward of the sequence (red zone) is expected to be mainly populated by higher red-shift galaxies, while the blueward zone is likely the locus of cluster and foreground late-type galaxies.

To further clarify this concept of likely membership we plot our data in the colour-colour plane, g-r versus g-i (Fig. 13). The continuous line in the plane represents the locus of points defined by elliptical galaxies at different redshifts according to the models of Buzzoni et al. (1993). These plots are consistent with the previous discussion: a) the cD galaxy, filled square, is near the expected location of an E galaxy at the cluster redshift, b) galaxies located in the red zone of Fig. 13 are displayed along the sequence of higher redshift ellipticals, and c) blue galaxies do not match the redshift sequence for elliptical galaxies.

5.2 Spatial distribution

The strategy we adopt for the observations has the advantage of allowing measuring fields at a rather large distance, about 2700 pixels ($\sim 1275$ kpc) from the cluster centre in a reasonable amount of telescope time. On the other hand we are forced to select an ad hoc radial direction. That is we are more sensitive to cluster and background field density fluctuations. We proceed as follows. First, we build the density frame relative to the whole mosaic. Then we divide the density frame in 36 circular sectors centred on the cluster centre and average the contribution of each segment at fixed radius going from the centre to the external limit of the mosaic. The whole sample mean radial surface density profile (Fig. 14, upper panel) does not clearly make evident the excess of galaxies defining the cluster.

Due to the segregation effect of the most luminous galaxies, r < 20.0, a King profile well fits the density profile at these magnitudes (Fig. 14 central panel, and Table 10). The sequence galaxies as defined by the CMR, with r < 20.0, present a higher central concentration as indicated by the smaller core radius (Table 10). This is also to be expected in a relaxed cluster since the CMRsequence has been defined by using the bright elliptical cluster galaxies.

   

Table 10: Best fit values of King function for the distribution of bright galaxies (r<20)

Sample	$\sigma_0~(10^{3^{^{}}}/{\rm sq}^2)$	$R_{\rm c}~{\rm (arcsec)}$	$\sigma_{\infty}~(10^3/{\rm sq}^2)$
ALL	$2.077 \pm 0.2$		$0.22 \pm 0.02$
SEQUENCE	$1.807 \pm 0.2$		$0.12 \pm 0.01$

Galaxies belonging to the red region of colour-magnitude plane are identified as galaxies at higher red-shift (see Figs. 12 and 13). Their distribution is homogeneous over the observed field without any link to cluster structure (Fig. 15 upper panel). Galaxies belonging to the blue zone of the colour-magnitude plane are identified as cluster or foreground late type galaxies. Their projected distribution seems to be influenced by cluster potential: their density abruptly peaks at 1 core radius distance from the cluster centre. This effect has been noticed also in some of the other clusters that we are analysing.

5.3 The central cD galaxy

The cD central galaxy is the brightest member of the cluster: it is 2 magnitudes brighter than the second member. In Molinari et al. (1998) its luminosity is regarded as too bright to be consistent with other ellipticals and it is not included in the computation of LF. However, as seen in the previous subsection, the cD magnitude and colour are consistent with the CMR extrapolated from the population of the bright elliptical galaxies.

cD galaxies are generally characterised by a surface brightness (SB) profile that falls off more slowly with radius than most elliptical galaxies. In Fig. 16 the profile of the Abell 496 cD galaxy along the major axis is shown up to a distance of 100 arcsec ($\sim 92$ kpc) from the centre. In this profile the presence of the halo is particularly noticeable, it departs strongly from a de Vaucouleur law (the straight line in the figure). The comparison of the SB profile along the northern major semi-axis (N) with the one along the southern semi-axis (S) (Fig. 16) shows an evident asymmetry. The N region of the halo exhibits an excess of intensity with respect to the S in each of the 3 filters in the interval 25-50 arcsec of distance from the centre. This effect is clearly depicted by the isophotes in Fig. 17.

Figure 16: The intensity profiles of the cD galaxy of Abell 496 along the N and S major semi-axis are superimposed (the r and i profiles are shifted of 2 and 4 magnitude to make the figure clearer). The excess of intensity of the northern semi-axis is noticeable in the interval (25, 50) arcsec from the centre. The straight lines represents the de Vaucouleur profile

In spite of the large extension of the halo, this is somewhat fainter than the core. After fitting the core by a de Vaucouleur law, we could subtract it from the cD image and estimate the magnitude of the halo. The derived total magnitudes in the three filters are listed in Table 11. As already stated, the luminosity of the core is dominant.

Figure 17: The filter r halo isophotes are superimposed to the image of the cD galaxy (the North is toward the bottom of the image), the last isophote corresponding to the SB threshold. The asymmetry of the halo emission is clearly evident

The average colour index of the total profile presents a gradient toward the blue moving from the core to the outermost part of the galaxy. This is due to the colour of the halo that is bluer than that of the core. Within the halo itself a difference exists between the colour of the northern hemisphere of higher surface brightness, and the colour of the southern hemisphere. The northern zone is bluer (marked as colour excess in Fig. 18). In other cD galaxies (see for instance Molinari et al. 1994) the halo has been found redder than the core. Therefore, the characteristics of the halo population are undoubtedly related to the specific history of the cD under consideration.

Figure 18: The average colour index of the three components of the galaxy is shown. They are compared with the expected colours of the stars convoluted from the spectral catalogue of Vilnius et al. (1972) (stars are labelled with the name of spectral class) and also with the colours of the stars of our catalogue (small points)

   

Table 11: Photometric parameters of the cD galaxy

F	$r_{\rm e}$(arsc)	$\mu_{\rm e}$ (mag/arsc 2)	$m_{{\rm tot}}$	$m_{{\rm core}}$	$m_{{\rm halo}}$
g	$58.3 \pm 7.5$	$25.81 \pm 0.12$	$12.64 \pm 0.03$	$12.85 \pm 0.04$	$14.56 \pm 0.07$
r	$51.7 \pm 5.1$	$25.12 \pm 0.10$	$12.04 \pm 0.02$	$12.22 \pm 0.03$	$13.99 \pm 0.06$
i	$52.7 \pm 4.9$	$24.96 \pm 0.10$	$11.88 \pm 0.02$	$12.05 \pm 0.03$	$13.97 \pm 0.06$

5.4 Colour gradient of the galaxy population

Finally, the distribution of the g - r colours of the sequence galaxies is analysed as a function of their projected distance from the centre of the cluster. We find a significant correlation relative to the population of faint galaxies.

As partly expected, brighter galaxies tend to dominate in the central region of the cluster. Such galaxies (see also the discussion on the CMR relation) tend to be somewhat redder. Therefore we expect a mild correlation between the cluster integrated colour - defined as the mean colour derived from the galaxy population located at a given distance from the centre - and the distance from the centre. The total gradient expected to be < 0.2 in g-r. On the other hand if we limit ourselves to consider only the dwarf galaxies (bottom of Fig. 19), we do not measure any correlation between the mean galaxy magnitude and the distance from the cluster centre. In spite of this lack of correlation the faint cluster population shows a well-defined colour gradient moving outward from the centre (upper panel of Fig. 19). This effect is significant at a 4 sigma level and unrelated to the CMR relation. Indeed over the small range of magnitude we took into consideration (18 <r<21) such an effect would be at most of about 0.1 mag, while we observe a gradient of about 0.3 magnitudes.

Figure 19: The average colour index of dwarf sequence galaxies shows a gradient from red to blue going off the centre of the cluster (upper panel). This feature cannot be ascribed to the luminosity+colour segregation: the non correlation between radius cluster and medium magnitude of the dwarf sequence galaxies is shown (lower panel)

A very similar result is found by Secker (1996) in Coma cluster; conversely, Hilker et al. (1998) do not find any correlation between the projected distance from the centre and the colours of dwarf galaxies in the central region of Fornax cluster.