All Figures

$\begin{figure} \par\includegraphics[width=8.1cm,clip]{2993_f1.ps}\end{figure}$ Figure 1: Position of the central slitlet on the B image taken at the 3.6 m ESO telescope. The spatial spectral analysis has been obtained by cutting the slit region in 17 slices, as numbered in the figure. The two circular apertures centerd on each nuclei and with a diameter of8 arcsecs correspond to those adopted by Mirabel et al. (1991). See text for more details.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{2993_f2.ps}\end{figure}$ Figure 2: The three-dimensional structure of the spectrum obtained with the pipeline described in Sect. 2. Different emission lines are already clearly visible, H $\alpha$ standing over all the others. The spatial axis corresponds to the one shown in Fig. 1.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{2993_f3.eps}\end{figure}$ Figure 3: The spectra of the two nuclei of IRAS 19254-7245 as extracted from an aperture of $1.2''\times 3.6''$ for the southern nucleus, and $1.2''\times 2.4''$ for the northern, respectively. A lot of emission lines typical of ULIRGs are recognizable.
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In the text

$\begin{figure} \par\includegraphics[angle=-90,width=17.5cm,clip]{2993_f4.eps}\end{figure}$ Figure 4: The seventeen different slices of Fig. 1 are plotted in sequence, to show the trend of the continuum and other spectral features, as a function of position along the slit.
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In the text

$\begin{figure} \par\includegraphics[angle=-90,width=16cm,clip]{2993_f5.eps}\end{figure}$ Figure 5: Spatial profile of the main emission lines. The solid histograms represent the profiles of the various lines, as indicated in the panels. To study the spatial profile of the lines, we plot the continuum normalized to the peak value of line emission in the sounthern nucleus (dotted histogram): going from the brightest nucleus to the outer regions, all the lines but [OII] fade more rapidly than the continuum.
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In the text

$\begin{figure} \par\includegraphics[width=7.6cm,clip]{2993_f6.eps}\end{figure}$ Figure 6: Upper panel: spatial dependence of the extinction, E(B-V), as computed from the Balmer decrement along the slit. Middle and lower panels: H $\alpha$ and H $\beta$ fluxes. The H $\beta$ flux scale is linear instead of logarithmic because the line is seen in absorption in the Northern nucleus.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{2993_f7.eps}\end{figure}$ Figure 7: [OII](3727Å)/H $\alpha$ and H $\beta$ /H $\alpha$ flux ratios vs. H $\alpha$ , before extinction correction. Each data point is labelled according to Fig. 1 (the southern nucleus refers to spectra #4, 5, 6; the northern to #13 and 14). There is a clear dependence of the ratios to the intensity of H $\alpha$ , suggesting a tight relationship between extinction and H $\alpha$ luminosity, indicative that the most active (luminous) regions are also the more extinguished. See Fritz et al. (2003) for a detailed discussion.
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In the text

$\begin{figure} \par\includegraphics[width=7.5cm,clip]{2993_f8.eps}\end{figure}$ Figure 8: Spatial profile along the slit, at the wavelength of H $\alpha$ , used to evaluate the amount of energy encircled within the 8'' aperture centered on each nucleus (delimited with shaded borders), which falls outside the slit (1.2'' shaded regions). After performing a Gaussian fit to the profile, slit losses were estimated assuming that the bidimensional light profile is an axial-simmetric Gaussian.
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In the text

$\begin{figure} \par\includegraphics[angle=-90,width=15.5cm,clip]{2993_f9.eps}\end{figure}$ Figure 9: Spectral energy distribution of the southern nucleus. Data are from Mirabel et al. (1991), Laurent et al. (2000) and Klaas et al. (2001) as described in the text. The thick solid line is the fit to the global SED, obtained combining an optical-NIR fit (dashed line short ward 5 $\mu$ m) to our spectra (corrected to the 8''aperture) with an AGN model obtained with the code DUSTY (dot-dashed line) and a semiempirical starburst Arp220-like template (dashed line long ward 5 $\mu$ m). The thin solid and dotted lines show the fit of the central and outer regions spectra. The N band width is slightly shifted upward with respect to the observed datapoint, in order not to be confused with the LW2's.
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In the text

$\begin{figure} \par\includegraphics[width=7.8cm,clip]{2993_f10.eps}\end{figure}$ Figure 10: The spectral energy distribution of the northern nucleus. The fit (thick solid line) is obtained by adding an optical-NIR model short ward 5 $\mu$ m, and a semiempirical starburst M 82-like template (long ward 5 $\mu$ m). The thin solid and dotted lines show the fit of the central and outer regions spectra. No AGN component is needed in this case.
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In the text

$\begin{figure} \par\includegraphics[width=7.7cm,clip]{2993_f11.ps}\end{figure}$ Figure 11: Best fit model for the southern nucleus. Top panel reports the observed (thick line) and the model (thin line) spectrum, the dotted line shows the residuals between model and observed spectra. Bottom panel depicts the contribution of each extinguished SSP (labeled from 1 to 10 with increasing age). The intermediate age population dominates the continuum luminosity, while the 2-6 Myr population provides the emission lines.
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In the text

$\begin{figure} \par\includegraphics[width=7.8cm,clip]{2993_f12.ps}\end{figure}$ Figure 12: SFR and extinction of the southern best fit model. Top panel: SFR (histogram and left scale) and colour excess (filled squares and right scale) for the different stellar populations. Middle panel: ratio between the bolometric FIR emission due to each SSP and the total V band flux. Notice that the sum over all populations is not 100, but turns out to be 57.6, which is the ratio between the total FIR and V fluxes. This number is the conversion factor to obtain the differential contribution of each SSPs to the integrated infrared light. Bottom panel: cumulative mass formed in the galaxy: about 35% of the total stellar mass has been processed by the intermediate age star formation episode, 60% belongs to the oldest. The youngest stars, significantly contributing to the FIR bolometric emission and dominating the lines fluxes, provide almost no contribution to the total luminous mass formed in the galaxy.
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In the text

$\begin{figure} \par\includegraphics[width=7.8cm,clip]{2993_f13.ps}\end{figure}$ Figure 13: A good fit to the optical spectrum of the southern nucleus (not shown here) is found even by imposing a constant long burst of star formation. However the fitting procedure "obscures'' unnecessary populations, which only contribute to the FIR. Due to this behaviour of the color excess we exclude such a star formation scenario.
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In the text

$\begin{figure} \par\includegraphics[width=7.7cm,clip]{2993_f14.eps}\end{figure}$ Figure 14: Best fit to the northern nucleus spectrum. The mismatch at the edges of the spectral range analysed is found in all other models we have tried.
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In the text

$\begin{figure} \par\includegraphics[width=7.8cm,clip]{2993_f15.eps}\end{figure}$ Figure 15: SFR and colour excess of the northern nucleus best fit model. The star formation history of this nucleus closely resembles that of the southern nucleus, with two main star formation episodes, forming 75% and 25% of the total stellar mass respectively. The intermediate age population provides the bulk of the FIR flux. Extinction is significately lower than in the southern nucleus.
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In the text

$\begin{figure} \par\includegraphics[width=7.5cm,clip]{2993_f16.eps}\end{figure}$ Figure 16: The spatial dependence of the extinction, as computed on the basis of the Balmer decrement and through the spectrophotometric modelling of the spectra (see Table 3). The Balmer decrement has not been applied in regions 9 to 17 where H $\beta$ is not detected or appears in absorption.
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In the text

$\begin{figure} \par\includegraphics[width=8.1cm,clip]{2993_f1.ps}\end{figure}$	Figure 1: Position of the central slitlet on the B image taken at the 3.6 m ESO telescope. The spatial spectral analysis has been obtained by cutting the slit region in 17 slices, as numbered in the figure. The two circular apertures centerd on each nuclei and with a diameter of8 arcsecs correspond to those adopted by Mirabel et al. (1991). See text for more details.
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$\begin{figure} \par\includegraphics[width=8.8cm,clip]{2993_f2.ps}\end{figure}$	Figure 2: The three-dimensional structure of the spectrum obtained with the pipeline described in Sect. 2. Different emission lines are already clearly visible, H $\alpha$ standing over all the others. The spatial axis corresponds to the one shown in Fig. 1.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{2993_f3.eps}\end{figure}$	Figure 3: The spectra of the two nuclei of IRAS 19254-7245 as extracted from an aperture of $1.2''\times 3.6''$ for the southern nucleus, and $1.2''\times 2.4''$ for the northern, respectively. A lot of emission lines typical of ULIRGs are recognizable.
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$\begin{figure} \par\includegraphics[angle=-90,width=17.5cm,clip]{2993_f4.eps}\end{figure}$	Figure 4: The seventeen different slices of Fig. 1 are plotted in sequence, to show the trend of the continuum and other spectral features, as a function of position along the slit.
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$\begin{figure} \par\includegraphics[width=7.6cm,clip]{2993_f6.eps}\end{figure}$	Figure 6: Upper panel: spatial dependence of the extinction, E(B-V), as computed from the Balmer decrement along the slit. Middle and lower panels: H $\alpha$ and H $\beta$ fluxes. The H $\beta$ flux scale is linear instead of logarithmic because the line is seen in absorption in the Northern nucleus.
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$\begin{figure} \par\includegraphics[width=7.8cm,clip]{2993_f10.eps}\end{figure}$	Figure 10: The spectral energy distribution of the northern nucleus. The fit (thick solid line) is obtained by adding an optical-NIR model short ward 5 $\mu$ m, and a semiempirical starburst M 82-like template (long ward 5 $\mu$ m). The thin solid and dotted lines show the fit of the central and outer regions spectra. No AGN component is needed in this case.
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$\begin{figure} \par\includegraphics[width=7.8cm,clip]{2993_f13.ps}\end{figure}$	Figure 13: A good fit to the optical spectrum of the southern nucleus (not shown here) is found even by imposing a constant long burst of star formation. However the fitting procedure "obscures'' unnecessary populations, which only contribute to the FIR. Due to this behaviour of the color excess we exclude such a star formation scenario.
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$\begin{figure} \par\includegraphics[width=7.7cm,clip]{2993_f14.eps}\end{figure}$	Figure 14: Best fit to the northern nucleus spectrum. The mismatch at the edges of the spectral range analysed is found in all other models we have tried.
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$\begin{figure} \par\includegraphics[width=7.8cm,clip]{2993_f15.eps}\end{figure}$	Figure 15: SFR and colour excess of the northern nucleus best fit model. The star formation history of this nucleus closely resembles that of the southern nucleus, with two main star formation episodes, forming 75% and 25% of the total stellar mass respectively. The intermediate age population provides the bulk of the FIR flux. Extinction is significately lower than in the southern nucleus.
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$\begin{figure} \par\includegraphics[width=7.5cm,clip]{2993_f16.eps}\end{figure}$	Figure 16: The spatial dependence of the extinction, as computed on the basis of the Balmer decrement and through the spectrophotometric modelling of the spectra (see Table 3). The Balmer decrement has not been applied in regions 9 to 17 where H $\beta$ is not detected or appears in absorption.
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