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$\begin{figure} \par\includegraphics[width=7.9cm,clip]{0276fg1a.eps}\hspace*{3mm} \includegraphics[width=7.9cm,clip]{0276fg1b.eps} \end{figure}$ Figure 1: HDFS/LW3 source S40 at z=1.27: best fit mass is M=1.63e11 $M_\odot$ . Left Panel: comparison of the observed SEDs to the best fits. The solid line is the sum of all the used SSPs; the dashed line is the sum of all the young SSPs (age <10⁹ yrs); the dotted line represents the old populations (age $\ge$ 10⁹ yrs); the dashed thick line longward 5 $\mu$ m (restframe) is the k-corrected rescaled M 82 template; the square black dots are the observed photometric datapoints; the triangular open dot at $\lambda\sim 15 ~\mu$ m is the synthetic LW3 flux (overlapping to the observed LW3 when the FIR constraint is satisfied). Right Panel top: values of SFR (histogram, left y-scale) and E(B-V) (dots, right y-scale) for each simple stellar population. Middle: ratio between the FIR and V-band fluxes for the different SSPs. This quantity is used to asses both the contribution of various stellar populations to the FIR luminosity and the effect of extinction on each population (depressing the optical light and boosting the IR). The global ratio between the FIR and V fluxes of the composite final synthetic spectrum is reported on the right. Bottom: cumulative stellar mass formed in the galaxy. The youngest stars, significantly contributing to the FIR bolometric emission, provide almost no contribution to the total luminous mass assembled in the galaxy.
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In the text

$\begin{figure} \par\includegraphics[width=7.7cm,clip]{0276fg2a.eps}\includegraphics[width=8cm,clip]{0276fg2b.eps} \end{figure}$ Figure 2: Left: $\chi ^2$ contours on the M vs. $L_{\rm FIR}$ diagram, for source S40. Contours are drawn at 1, 2, 3 $\sigma$ levels, corresponding to 68.3%, 95.5% and 99.7% confidence (decreasing as shade darkness increases). The solid and dashed horizontal lines represent the far-IR luminosity and uncertainty as derived from the observed LW3 flux, based on fitting with the M 82 template. Right: $\chi ^2$ values as a function of the stellar mass of the models, for those solutions reproducing the LW3 flux within 1%. The mass uncertanty range is $[0.70,3.80]\times 10^{11}~M_\odot$ .
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In the text

$\begin{figure} \par\includegraphics[width=5.7cm,clip]{0276fg3a.eps}\hspace*{2mm}... ....eps}\hspace*{2mm} \includegraphics[width=5.7cm,clip]{0276fg3f.eps} \end{figure}$ Figure 3: Comparison of three different fits of the SED of source E5 in the ellipticals HDFS sample by Rodighiero et al. (2001), obtained with the three different adopted methods. Left: analytic star formation history, given by a Schmidt law; center: two simple stellar populations (10⁹ and $1.2 \times 10^{10}$ yrs old) with solar metallicity; right: two old populations ( $1.2 \times 10^{10}$ yrs) with different metallicities (Z=0.02, 0.008). Top: the best fit models SEDs (solid line) have been split into two components: on the left into the contributions of SSPs younger than 10⁹ yrs (dashed line) and older (dotted line); at center into the contribution of the 10⁹yrs old SSP (dashed) and of the $1.2 \times 10^{10}$ yrs old one (dotted): on the right into the contribution of the Z=0.008 SSP (dashed) and of the Z=0.02 one (dotted). Bottom: star formation history and contribution of each population to the total stellar mass assembled in the galaxy. In the bottom-right panel the two old SSPs with different metallicities have been distinguished both in the SFH plot (shaded histogram is the Z=0.02 SSP) and cumulative-mass diagram. The main contributions to the $\chi ^2$ for the central and right solutions come from the J and V data respectively. No extinction is plotted, since it was assumed constant for all populations (see Table 2).
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In the text

$\begin{figure} \par\mbox{\subfigure[E1: $z=0.512$ , best fit $M=1.8\times 10^{10... ...e{-0.3cm} \includegraphics[width=0.245\textwidth]{0276fg4h.eps} }}\end{figure}$ Figure 4: Best-fits spectral solutions for representative elliptical galaxies, based on the Schimdt star formation history model (see Sect. 3.3). See also Figs. 1 and 3.
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In the text

$\begin{figure} \par\includegraphics[width=7.2cm,clip]{0276fg5a.eps}\hspace*{2mm} \includegraphics[width=7.2cm,clip]{0276fg5b.eps} \end{figure}$ Figure 5: SEDs fits of some Lyman-break galaxies. Left: best fit solutions for sources Ly2, Ly3 and Ly5, as obtained involving only young SSPs. Right: consequences of introducing the $\ge$ 10⁹ yrs old populations on the fit of source Ly1. The relative contribution of old stars to the total assembly of stellar mass increases from top to bottom: the fit accuracy does not change, but in the restframe $1{-}2~ \mu$ m domain the model becomes significatively steeper (see also right panel of Fig. 7 and text for details).
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In the text

$\begin{figure} \par\includegraphics[angle=-90,width=14cm,clip]{0276fg6.eps} \end{figure}$ Figure 6: Comparison of stellar masses between mid-IR selected starburst (filled squares), K-band selected E/S0 (open circles) and Ly-break galaxies (triangles), as a function of redshift. With the exception of very massive MIR sources belonging to the HDFS well-known group at $z\simeq 0.6$ , the stellar masses we find for starbursts and E/S0 are comparable; the small sample of Ly-break galaxies seems to consist of less massive objects than those observed at lower redshift.
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In the text

$\begin{figure} \par\includegraphics[width=7.9cm,clip]{0276fg7a.eps}\hspace*{3mm} \includegraphics[width=7.9cm,clip]{0276fg7b.eps} \end{figure}$ Figure 7: Simulations of SIRTF/IRAC observations to constrain the estimate of stellar masses in starbursts and Lyman-break galaxies. Left panel: three different solutions for ISO/LW3 source S40: all three fits reproduce comparatively well both the optical data and the LW3 observed flux (even if not displayed for plotting reasons). SIRTF synthetic data (open squares) have been obtained simulating an IRAC observation of the best fit model. Right panel: the three different fits to optical-NIR data of source Ly1 already discussed in Fig. 5. IRAC synthetic fluxes have been computed for the best fit solution, which is the youngest and least massive among the three considered. In both panels the three models differ in their star formation history: the reddest are dominated by old populations more than the others. Small boxes show the transmissivity curves of the four IRAC filters; the numbers of the models refer to those appearing in Figs. 1 and 5.
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In the text

$\begin{figure} \par\mbox{\subfigure[E1: $z=0.512$ , best fit $M=1.8\times 10^{10... ...e{-0.3cm} \includegraphics[width=0.245\textwidth]{0276fg4h.eps} }}\end{figure}$	Figure 4: Best-fits spectral solutions for representative elliptical galaxies, based on the Schimdt star formation history model (see Sect. 3.3). See also Figs. 1 and 3.
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