All Figures

$\begin{figure} \par\includegraphics[width=7.5cm,clip]{1811fig1.ps} \end{figure}$ Figure 1: R-band magnitude distributions of the galaxies of our spectroscopic sample (207 objects - dotted line), of all the galaxies with good velocity determination (122 - dashed line), and, among them, of those belonging to A3921 (104 - shaded area).
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In the text

$\begin{figure} \par\includegraphics[width=7.5cm,clip]{1811fig2.ps} \end{figure}$ Figure 2: Completeness of the spectroscopic sample as a function of the R-band magnitude in the central $18 \times 12.5~{\rm arcmin}^2$ field for all galaxies observed (dashed line) and for those that led to a Quality Flag = 1 (solid line).
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In the text

$\begin{figure} \par\includegraphics[width=7.6cm,clip]{1811fig3.ps} \end{figure}$ Figure 3: Apparent radial velocity (cz) histogram of the galaxies in the central field of the cluster A3921, with a binning of 500 km s^-1.
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In the text

$\begin{figure} \par\includegraphics[width=15.7cm,clip]{1811fig4.eps} \end{figure}$ Figure 4: Galaxies identified as members of A3921 (squares), superimposed on a fraction of the R-band image ( $27'\times 20'$ ).
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In the text

$\begin{figure} \par\includegraphics[width=15.7cm,clip]{1811fig5.eps} \end{figure}$ Figure 5: Projected galaxy density maps (with $R_{\rm AB}\le 19$ ) on a $34 \times 34 ~ {\rm arcmin}^2$ field centred on A3921. Left panel: all galaxies; central panel: the red sequence galaxies, and right panel: same as central panel but removing galaxies known not to be cluster members from spectroscopy. The white crosses show the positions of the four brightest galaxies indicated in Fig. 4.
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In the text

$\begin{figure} \par\includegraphics[width=15.8cm,clip]{1811fig6.eps} \end{figure}$ Figure 6: Velocity distributions of the main structures of the projected galaxy density map (same as the right panel of Fig. 5). The dotted lines in the histograms indicate the relative velocities of the neighbouring brightest galaxy. At the level of $R_{\rm AB}=19$ all the selected clumps have a 100% coverage except the eastern group with 70% completeness.
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In the text

$\begin{figure} \par\includegraphics[width=7.4cm,clip]{1811fig7.ps}\hspace*{3mm} \includegraphics[width=7.4cm,clip]{1811fig8.ps} \end{figure}$ Figure 7: $(V-R)_{\rm AB}$ vs. $R_{\rm AB}$ ( left) and $(R-I)_{\rm AB}$ vs. $I_{\rm AB}$ ( right) colour-magnitude diagrams. All galaxies within $34\times 34$ ${\rm arcmin}^2$ are shown. Big symbols correspond to confirmed cluster members whereas dots correspond to galaxies without spectroscopic information. Triangles correspond to emission line galaxies, squares to k+a type, circles to galaxies presenting an H-K inversion following the classification described in the text. The solid line is the best linear fit to the red sequence of the cluster, while the dotted line is at $\pm$ $1\sigma _{\rm RS}$ .
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In the text

$\begin{figure} \par\includegraphics[width=15cm,clip]{1811fig9.eps} \end{figure}$ Figure 8: Projected galaxy density maps (using red sequence galaxies and excluding galaxies known not to be cluster members from spectroscopy, as in the right panel of Fig. 5) on a $34\times 34$ ${\rm arcmin}^2$ field centred on A3921 and for three magnitude cuts. The white crosses indicate the positions of the four brightest galaxies indicated in Fig. 4.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{1811fig10.eps} \end{figure}$ Figure 9: Projected galaxy density map ( $34 \times 34 ~ {\rm arcmin}^2$ ) of the red-sequence galaxies (see Fig. 6). The symbols represent the galaxies with $R_{\rm AB}<20$ and bluer than the red sequence galaxies.
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In the text

$\begin{figure} \par\includegraphics[width=7.5cm,clip]{1811fig11.ps} \end{figure}$ Figure 10: Top: stripe density plot of the radial velocity offsets of the 104 A3921 members from $\bar{z}$ (corrected for cosmological and relativistic effects). The position of the gap detected in the cluster velocity distribution is indicated by an arrow. Bottom: velocity histogram of the confirmed cluster galaxies in bins of 200 km s^-1. The Gaussian best-fit to the velocity distribution is superimposed.
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In the text

$\begin{figure} \par\includegraphics[width=7.7cm,clip]{1811fig12.ps}\hspace*{4mm}... ...4.ps}\hspace*{4mm} \includegraphics[width=7.7cm,clip]{1811fig15.ps} \end{figure}$ Figure 11: Profile of the velocity means ( top) and dispersions ( bottom) in the cluster: left: as a cumulative function of galaxies centred on the position of BG1 - right: differential values as a function of radius in rings with 13 galaxies; the last point is far from the others due to the lower degree of completeness reached by spectroscopic data in the outer regions of the cluster (beyond $\sim$ 12 arcmin from the center).
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In the text

$\begin{figure} \par\includegraphics[width=7.5cm,clip]{1811fig16.ps} \end{figure}$ Figure 12: Projected positions, centred on the coordinates of BG1, of the galaxies with v_r=25 400-30 400 km s^-1. Each galaxy is represented by a circle, whose dimension is weighted by the estimator of Dressler & Shectman (1988). Filled circles (red in the electronic version of the paper) are centred on the positions of the three BGs.
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In the text

$\begin{figure} \par\includegraphics[width=7.7cm,clip]{1811fig17.ps} \end{figure}$ Figure 13: Completeness of the spectroscopic sample for the two clumps A3921-A (solid line) and in A3921-B (dashed line).
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In the text

$\begin{figure} \par\includegraphics[width=6.7cm,clip]{1811fig18.eps} \end{figure}$ Figure 14: Iso-density contours of the projected distribution of the red-sequence galaxies (with $R_{\rm AB}\leq 18$ and after removing galaxies known not to be cluster members from spectroscopic data) superimposed on the central $22\times 22$ ${\rm arcmin}^2$ of the R-band image of A3921. The dynamical analysis of the two clumps A and B has been performed based on the galaxies selected inside the two dashed circles ( $R\simeq 0.34$ Mpc).
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In the text

$\begin{figure} \par\includegraphics[width=7.6cm,clip]{1811fig19.ps} \end{figure}$ Figure 15: Velocity histogram, with a binning of 200 km s^-1, of the galaxies in A3921-A (solid line) and in A3921-B (dotted line), according to the division showed in Fig. 14. The relative Gaussian best-fits to the velocity distributions are superimposed. Arrows show the radial velocities of the three brightest cluster galaxies.
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In the text

$\begin{figure} \par\includegraphics[width=6.2cm,clip]{1811fig20.ps}\hspace*{4mm}... ...2.ps}\hspace*{4mm} \includegraphics[width=6.2cm,clip]{1811fig23.ps} \end{figure}$ Figure 16: The total mass of the two clumps as a function of the angle $\alpha$ between the plane of the sky and the line connecting the centres of the two sub-clusters. The horizontal solid line represents the sum of the pairwise projected masses as estimated in Sect. 6.1; the associated 1 $\sigma$ confidence level domain is limited by the two dot-dashed lines. Top left (cases a1, a2, a3 in Table 6): the solid line shows the bound-incoming and -outgoing solutions, while the dashed line corresponds to the unbound solutions. These solutions have been estimated under the hypothesis that the two systems were at zero separation 12.6 Gyr ago (i.e. the age of the Universe in our cosmology). Top right (case b): the same but assuming that the two systems were at zero separation 0.3 Gyr ago. The only possible configuration shows that the two clumps are bound and the system is expanding. Bottom (cases c and d1, d2, d3): same as in the top right panel, but assuming that the collision took place 0.5 (left) and 1.0 (right) Gyr ago respectively.
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In the text

$\begin{figure} \par\includegraphics[width=7cm,clip]{1811fig24.ps} \end{figure}$ Figure 17: The histogram in absolute magnitude in the R_AB band of our high S/N spectroscopic sample (83 objects). The various fractions of galaxies of different spectral types in each bin are encoded as follows: passive (white), k+a (less shaded), k+a? (more shaded) and emission-line galaxies (black). The dashed and dotted lines show the magnitude limits corresponding to the MORPHS (Dressler et al. 1999) and "giants/dwarfs'' (Poggianti et al. 2004) cuts respectively. Note that the contribution of active (k+a, k+a? and e = emission line) galaxies creates a second peak at fainter magnitudes.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{1811fig25.eps} \end{figure}$ Figure 18: Projected galaxy density map (34 $\times$ 34 ${\rm arcmin}^2$ ) of the red-sequence galaxies with $R_{\rm AB}<19$ (see Fig. 6). The squares represent galaxies that are members of A3921 and classified as k+a and the circles as "k+a?'' (see text).
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In the text

$\begin{figure} \par\includegraphics[width=5.6cm,clip]{1811fig26.ps}\hspace*{2mm}... ...}\hspace*{2mm} \includegraphics[width=5.6cm,clip]{1811fig28.ps}\par \end{figure}$ Figure 19: From left to right, velocity distribution (with a binning of 400 km s^-1) of k, k+a and emission line galaxies (according to the definition of Dressler et al. 1999, see text for details). In the central figure, the white component corresponds to secure k+a's and the shaded part to k+a candidates. In the right panel, the shaded component of the histogram corresponds to e(a)'s, the black component to e(b)'s, the white part to e(c) type objects.
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In the text

$\begin{figure} \par\includegraphics[width=7.5cm,clip]{1811fig29.ps} \end{figure}$ Figure 20: Colour- ${\rm H}\delta$ diagram for galaxy members of A3921. Crosses indicate k type galaxies, squares k+a type galaxies, circles galaxies with H-K inversion and triangles emission line galaxies.
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In the text

$\begin{figure} \par\includegraphics[width=7.5cm,clip]{1811fig30.ps} \end{figure}$ Figure 21: Colour-[OII] diagram for galaxy members of A3921. Crosses indicate k type galaxies, squares k+a type galaxies, circles galaxies with H-K inversion and triangles emission line galaxies.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{1811fig31.eps} \end{figure}$ Figure 22: Projected galaxy density map ( $34 \times 34 ~ {\rm arcmin}^2$ ) of the red-sequence galaxies with $R_{\rm AB}<19$ (see Fig. 6). The triangles show the location of emission line cluster member galaxies (see text).
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{1811fig32.eps} \end{figure}$ Figure 23: X-ray residuals after subtraction of a 2D- $\beta$ model (Belsole et al. 2005) overlaid on the red sequence galaxy density map of the central part of A3921 ( $22\times 18~{\rm arcmin}^2$ ).
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In the text

$\begin{figure} \par\includegraphics[width=7.5cm,clip]{1811fig1.ps} \end{figure}$	Figure 1: R-band magnitude distributions of the galaxies of our spectroscopic sample (207 objects - dotted line), of all the galaxies with good velocity determination (122 - dashed line), and, among them, of those belonging to A3921 (104 - shaded area).
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$\begin{figure} \par\includegraphics[width=7.5cm,clip]{1811fig2.ps} \end{figure}$	Figure 2: Completeness of the spectroscopic sample as a function of the R-band magnitude in the central $18 \times 12.5~{\rm arcmin}^2$ field for all galaxies observed (dashed line) and for those that led to a Quality Flag = 1 (solid line).
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$\begin{figure} \par\includegraphics[width=7.6cm,clip]{1811fig3.ps} \end{figure}$	Figure 3: Apparent radial velocity (cz) histogram of the galaxies in the central field of the cluster A3921, with a binning of 500 km s^-1.
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$\begin{figure} \par\includegraphics[width=15.7cm,clip]{1811fig4.eps} \end{figure}$	Figure 4: Galaxies identified as members of A3921 (squares), superimposed on a fraction of the R-band image ( $27'\times 20'$ ).
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$\begin{figure} \par\includegraphics[width=15.8cm,clip]{1811fig6.eps} \end{figure}$	Figure 6: Velocity distributions of the main structures of the projected galaxy density map (same as the right panel of Fig. 5). The dotted lines in the histograms indicate the relative velocities of the neighbouring brightest galaxy. At the level of $R_{\rm AB}=19$ all the selected clumps have a 100% coverage except the eastern group with 70% completeness.
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$\begin{figure} \par\includegraphics[width=15cm,clip]{1811fig9.eps} \end{figure}$	Figure 8: Projected galaxy density maps (using red sequence galaxies and excluding galaxies known not to be cluster members from spectroscopy, as in the right panel of Fig. 5) on a $34\times 34$ ${\rm arcmin}^2$ field centred on A3921 and for three magnitude cuts. The white crosses indicate the positions of the four brightest galaxies indicated in Fig. 4.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{1811fig10.eps} \end{figure}$	Figure 9: Projected galaxy density map ( $34 \times 34 ~ {\rm arcmin}^2$ ) of the red-sequence galaxies (see Fig. 6). The symbols represent the galaxies with $R_{\rm AB}<20$ and bluer than the red sequence galaxies.
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$\begin{figure} \par\includegraphics[width=7.5cm,clip]{1811fig16.ps} \end{figure}$	Figure 12: Projected positions, centred on the coordinates of BG1, of the galaxies with v_r=25 400-30 400 km s^-1. Each galaxy is represented by a circle, whose dimension is weighted by the estimator of Dressler & Shectman (1988). Filled circles (red in the electronic version of the paper) are centred on the positions of the three BGs.
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$\begin{figure} \par\includegraphics[width=7.7cm,clip]{1811fig17.ps} \end{figure}$	Figure 13: Completeness of the spectroscopic sample for the two clumps A3921-A (solid line) and in A3921-B (dashed line).
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$\begin{figure} \par\includegraphics[width=7.6cm,clip]{1811fig19.ps} \end{figure}$	Figure 15: Velocity histogram, with a binning of 200 km s^-1, of the galaxies in A3921-A (solid line) and in A3921-B (dotted line), according to the division showed in Fig. 14. The relative Gaussian best-fits to the velocity distributions are superimposed. Arrows show the radial velocities of the three brightest cluster galaxies.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{1811fig25.eps} \end{figure}$	Figure 18: Projected galaxy density map (34 $\times$ 34 ${\rm arcmin}^2$ ) of the red-sequence galaxies with $R_{\rm AB}<19$ (see Fig. 6). The squares represent galaxies that are members of A3921 and classified as k+a and the circles as "k+a?'' (see text).
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$\begin{figure} \par\includegraphics[width=7.5cm,clip]{1811fig29.ps} \end{figure}$	Figure 20: Colour- ${\rm H}\delta$ diagram for galaxy members of A3921. Crosses indicate k type galaxies, squares k+a type galaxies, circles galaxies with H-K inversion and triangles emission line galaxies.
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$\begin{figure} \par\includegraphics[width=7.5cm,clip]{1811fig30.ps} \end{figure}$	Figure 21: Colour-[OII] diagram for galaxy members of A3921. Crosses indicate k type galaxies, squares k+a type galaxies, circles galaxies with H-K inversion and triangles emission line galaxies.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{1811fig31.eps} \end{figure}$	Figure 22: Projected galaxy density map ( $34 \times 34 ~ {\rm arcmin}^2$ ) of the red-sequence galaxies with $R_{\rm AB}<19$ (see Fig. 6). The triangles show the location of emission line cluster member galaxies (see text).
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$\begin{figure} \par\includegraphics[width=8cm,clip]{1811fig32.eps} \end{figure}$	Figure 23: X-ray residuals after subtraction of a 2D- $\beta$ model (Belsole et al. 2005) overlaid on the red sequence galaxy density map of the central part of A3921 ( $22\times 18~{\rm arcmin}^2$ ).
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