All Figures

$\begin{figure} \par\includegraphics[width=16.7cm,clip]{4996fig1.ps} \end{figure}$ Figure 1: Wavelet decomposition of the VLT-FORS2 spectrum of a distant LIRG (UDSR23) located at a redshift of z=0.7094) and forming stars at a rate of SFR $_{\rm IR}= 51.8$ $M_{\odot }$ yr^-1. The raw spectrum (thick grey line) is compared to its wavelet decomposition (thin dark line), starting from scale 7, corresponding to the spectrum smoothed with a B-spline (equivalent to a gaussian) at the lowest resolution of 89.6 Å, then adding up the wavelet scales by increasing the spectral resolution from 44.8 Å (scale 6) to 0.7 Å (scale 1), by dividing it by a factor 2 for each scale. The narrow vertical grey zones were masked in the analysis because of the presence of sky emission lines. The vertical dotted lines indicate the minimum and maximum wavelength for each absorption line. In this zoom, the absorption lines clearly appear as much shallower than the emission line in OII.
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In the text

$\begin{figure} \par\includegraphics[width=7.4cm,clip]{4996fig2.ps} \end{figure}$ Figure 2: Comparison of the VLT-FORS2 raw spectrum (grey line) of the distant LIRG UDSR23 (z=0.7094, 51.8 $M_{\odot }$ yr^-1) to its wavelet decomposition resulting from the combination of the four wavelet scales 4-7, i.e. from 11.2 to 89.6 Å (black line). While the absorption lines are clearly distinguishable, the emission lines such as [OIII], [OII], H $\beta$ , H $\gamma$ are partly diluted by this technique.
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In the text

$\begin{figure} \par\includegraphics[width=7.2cm,clip]{4996fig3.ps} \end{figure}$ Figure 3: Time evolution of the H8, H9, and H $\delta _{\rm A}$ indices and 4000 Å break (D4000) for a single stellar population synthesized with the GALAXEV code (Bruzual & Charlot 2003). Four different metallicities were used in each plot, demonstrating the marginal effect of metallicity on the three indices and on D4000 before 1 Gyear: Z= 0.004 (20% solar, dashed line), Z= 0.008 (40% solar, dash-dotted line), Z= 0.02 (solar, solid line), Z= 0.05 (2.5 times solar, dotted line). From top to bottom and left to right: a) H8 evolution; b) H9 evolution; c) H $\delta _{\rm A}$ evolution; d) D4000 evolution.
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In the text

$\begin{figure} \par\includegraphics[width=16cm,clip]{4996fig4.ps} \end{figure}$ Figure 4: H $\delta _{\rm A}$ versus D4000 diagram for the SDSS galaxies (black points) and distant LIRGs of the present sample (green filled circles with error bars). The sub-population of SDSS galaxies detected with IRAS are marked with triangles and separated into galaxies with optical spectra with (empty red triangles) and without (filled blue triangles) an AGN signature.
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In the text

$\begin{figure} \par\includegraphics[width=16.5cm,clip]{4996fig5.ps} \end{figure}$ Figure 5: Stellar mass as a function of D4000 for the SDSS galaxies (black points) and distant LIRGs of the present sample (large filled blue circle). The median stellar mass for the distant LIRGs was derived from Franceschini et al. (2003) as discussed in the text. The sub-population of SDSS galaxies detected with IRAS and without an AGN signature in their optical spectra (star-forming galaxies) are marked with orange filled circles. The median and 1- $\sigma$ error bars for the field SDSS galaxies are represented with thick light-blue crosses, while the thin dark-blue crosses are for the sub-sample of star-forming SDSS-IRAS galaxies.
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In the text

$\begin{figure} \par\includegraphics[width=16.5cm,clip]{4996fig6.ps} \end{figure}$ Figure 6: Influence of the attenuation, velocity dispersion, and metallicity for a typical star formation history synthesized with the GALAXEV code (Bruzual $\&$ Charlot 2003). In all three plots, the loops correspond to a burst of star formation that started 3 Gyear (big black circles) after the galaxy's birth. a) Influence of the attenuation $\tau _{\rm v}$ : for a continuous star formation ( $\gamma =1.0$ , $V_{\rm disp}~=~200$ km s^-1, $\mu =0.3$ , $Z=Z_{\odot }$ ) and a single burst ( $\tau _{\rm B}=0.1$ Gyear and ${\it fb}=10\%$ ); the solid line: $\tau _{ \rm v}=1.0$ . The dotted line: $\tau _{\rm v}=2.0$ . The dashed line: $\tau _{ \rm v}=3.0$ . The long dashed line: $\tau _{\rm v}=4.0$ . b) Influence of the velocity dispersion $V_{\rm disp}$ : for a continuous star formation ( $\gamma =1.0$ , $\tau _{ \rm v}=3.0$ , $\mu =0.3$ , $Z=Z_{\odot }$ ) and a single burst ( $\tau _{\rm B}=0.1$ Gyear and ${\it fb}=10\%$ ). The solid line: $V_{\rm disp}=100$ km s^-1. The dotted line: $V_{\rm disp}=200$ km s^-1. The dashed line: $V_{\rm disp}=300$ km s^-1. c) Influence of the metallicity Z: for a continuous star formation ( $\gamma =1.0$ , $V_{\rm disp}=200$ km s^-1, $\tau _{ \rm v}=3.0$ and $\mu =0.3$ ) and a single burst ( $\tau _{\rm B}=0.1$ Gyear and ${\it fb}=10\%$ ). The solid line: Z=0.5 $Z_{\odot }$ . The dotted line: $Z=Z_{\odot }$ . The dashed line: Z=2.5 $Z_{\odot }$ .
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In the text

$\begin{figure} \par\includegraphics[width=15cm,clip]{4996fig7CMJN.eps} \end{figure}$ Figure 7: Location of the 200 000 Monte Carlo realizations using the parameters of SIM1 (see Table 5) in the H8 versus 4000 Å (D4000) break diagram. Light blue points ( lower-left): starbursting galaxies. Dark blue points ( upper part): post-starburst galaxies (galaxies having experienced a recent starburst which ended less than 2 Gyear ago). Black points: galaxies with continuous star formation in the two past Gyears. Bold red line: track followed by an individual galaxy with continuous star formation (numbers in squares = age in Gyear) generated with GALAXEV ( $\gamma =2$ ., $\mu =0.3$ , $\tau _{\rm V}=3.0$ , $V_{\rm disp}=200$ km s^-1). Orange loops: effect of starbursts of $5\times 10$ ⁷ (dashed line) and 10⁸ (plain line) years starting after 2 Gyear (light orange) and 3 Gyear (dark orange) of continuous star formation. The line is hatched during the starburst phase. The size of the orange dots is proportional to the time counted in 10⁸ years units after the beginning of the burst. The dark rectangle with dashed contours: area where the distant LIRGS are located. The dark rectangle with a solid line: area where 75 $\%$ of the sample lie (see Fig. 8 for the position of the individual galaxies with error bars). Triangles: galaxies corrected for the underlying nebular emission line in H8 as derived from the observed lower-order Balmer lines corrected for attenuation from the Balmer lines ratio. Stars: galaxies with negligible emission Balmer line in emission, hence uncorrected for H8 emission. Squares: corrected for the underlying H8 emission line using only one Balmer emission line available with dust attenuation computed from the IR (from the ratio of SFR(IR) over SFR(H $\beta$ )). Circles: no correction for the underlying potential emission line because no information is available on the Balmer emission lines.
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In the text

$\begin{figure} \par\includegraphics[width=13cm,clip]{4996fig8CMJN.eps} \end{figure}$ Figure 8: Same as in Fig. 7 for the SIM2 Monte Carlo simulation of 200 000 model galaxies. The distant LIRGs are represented with green points with error bars.
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In the text

$\begin{figure} \par\includegraphics[width=13cm,clip]{4996fig9CMJN.eps} \end{figure}$ Figure 9: Location of the 200 000 Monte Carlo realizations using the parameters of SIM2 (see Table 5) in the H $\delta _{\rm A}$ versus 4000 Å (D4000) break diagram. Light blue points ( lower-left): starbursting galaxies. Dark blue points ( upper part): post-starburst galaxies (galaxies having experienced a recent starburst that ended less than 2 Gyear ago). Black points: galaxies with continuous star formation in the two past Gyears. Bold red line: track followed by an individual galaxy with continuous star formation (numbers in squares = age in Gyear) generated with GALAXEV ( $\gamma =4$ ., $\mu =0.3$ , $\tau _{\rm V}=3.0$ , $V_{\rm disp}=200$ km s^-1). Orange loops: effect of starbursts of $5\times 10$ ⁷ (dashed line) and 10⁸ (plain line) years starting after 2 Gyear (light orange) and 3 Gyear (dark orange) of continuous star formation. The line is hatched during the starburst phase. The size of the orange dots is proportional to the time counted in 10⁸ years units after the beginning of the burst. Triangles: galaxies corrected for the underlying nebular emission line in H $\delta _{\rm A}$ as derived from the observed lower order Balmer lines corrected for attenuation from the Balmer lines ratio. Stars: galaxies with a negligible emission Balmer line in emission, hence uncorrected for H $\delta _{\rm A}$ emission. Squares: corrected for the underlying H $\delta _{\rm A}$ emission line using only one Balmer emission line available with dust attenuation computed from the IR (from the ratio of SFR(IR) over SFR(H $\beta$ )). Circles: no correction for the underlying potential emission line because no information is available on the Balmer emission lines.
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In the text

$\begin{figure} \par\includegraphics[width=8.4cm,clip]{4996fig10.ps} \end{figure}$ Figure 10: Example of PDF obtained for 1 galaxy: a) the Scalo parameter (Sect. 6.2); b) burst duration (Sect. 6.2); c) burst fraction (Sect. 6.2); d) effective burst fraction (Sect. 6.2).
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In the text

$\begin{figure} \par\includegraphics[width=7.3cm,clip]{4996fig11.ps} \end{figure}$ Figure 11: Distribution of the Scalo parameter and the burst duration; a) the Scalo parameter in the H8-D4000 diagram; b) the burst duration in the H8-D4000 diagram; c) the Scalo parameter in the H $\delta _{\rm A}$ -D4000 diagram; d) the burst duration in the H $\delta _{\rm A}$ -D4000 diagram.
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In the text

$\begin{figure} \par\includegraphics[width=7.4cm,clip]{4996fig12.ps} \end{figure}$ Figure 12: The lines are the same as in Figs. 7 and 8. The black circles are proportional to t₂₀ defined before, while the grey ones to t₅₀.
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In the text

$\begin{figure} \par\includegraphics[width=7.4cm,clip]{4996fig13.ps} \end{figure}$ Figure 13: The lines are the same as in Figs 7 and in 8. The black circles are proportional to t₂₀ defined before, while the grey ones to t₅₀.
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In the text

$\begin{figure} \par\includegraphics[width=16cm,clip]{4996fig4.ps} \end{figure}$	Figure 4: H $\delta _{\rm A}$ versus D4000 diagram for the SDSS galaxies (black points) and distant LIRGs of the present sample (green filled circles with error bars). The sub-population of SDSS galaxies detected with IRAS are marked with triangles and separated into galaxies with optical spectra with (empty red triangles) and without (filled blue triangles) an AGN signature.
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$\begin{figure} \par\includegraphics[width=13cm,clip]{4996fig8CMJN.eps} \end{figure}$	Figure 8: Same as in Fig. 7 for the SIM2 Monte Carlo simulation of 200 000 model galaxies. The distant LIRGs are represented with green points with error bars.
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$\begin{figure} \par\includegraphics[width=8.4cm,clip]{4996fig10.ps} \end{figure}$	Figure 10: Example of PDF obtained for 1 galaxy: a) the Scalo parameter (Sect. 6.2); b) burst duration (Sect. 6.2); c) burst fraction (Sect. 6.2); d) effective burst fraction (Sect. 6.2).
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$\begin{figure} \par\includegraphics[width=7.3cm,clip]{4996fig11.ps} \end{figure}$	Figure 11: Distribution of the Scalo parameter and the burst duration; a) the Scalo parameter in the H8-D4000 diagram; b) the burst duration in the H8-D4000 diagram; c) the Scalo parameter in the H $\delta _{\rm A}$ -D4000 diagram; d) the burst duration in the H $\delta _{\rm A}$ -D4000 diagram.
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$\begin{figure} \par\includegraphics[width=7.4cm,clip]{4996fig12.ps} \end{figure}$	Figure 12: The lines are the same as in Figs. 7 and 8. The black circles are proportional to t₂₀ defined before, while the grey ones to t₅₀.
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$\begin{figure} \par\includegraphics[width=7.4cm,clip]{4996fig13.ps} \end{figure}$	Figure 13: The lines are the same as in Figs 7 and in 8. The black circles are proportional to t₂₀ defined before, while the grey ones to t₅₀.
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