All Figures

$\begin{figure} \par\includegraphics[width=7.8cm,clip]{8179fig1a.ps}\hspace*{0.7c... ....ps}\hspace*{0.7cm} \includegraphics[width=7.8cm,clip]{8179fig1d.ps}\end{figure}$ Figure 1: Final IRAC mosaics of M 33. The upper left image displays the 3.6 $\mu$ m, the upper right 4.5 $\mu$ m, the lower left 5.8 $\mu$ m and the lower right the 8.0 $\mu$ m channel. The images are about 1 $^\circ$ by 1.5 $^\circ$ , north is up and east is to the left. The region enclosed by the continuous line will be further investigated in the present paper.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{8179fig2a.ps}\hspace*{1cm} \... ...2c.ps}\hspace*{0.4cm} \includegraphics[width=8cm,clip]{8179fig2d.ps}\end{figure}$ Figure 2: MIPS images of M 33. The upper panels display the 24 $\mu$ m images: the full image (one single epoch) is on the left. The inner region enclosed by the continuous line will be further investigated in the present paper, and is shown in the upper right panel after overlapping the data from two different epochs. The bottom left panel shows the 70 $\mu$ m channel, and the lower right the 160 $\mu$ m channel. The images are about 30' $\times$ 50', north is up and east is left.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{8179fig3a.ps}\hspace*{0.4cm} \includegraphics[width=8cm,clip]{8179fig3b.ps}\end{figure}$ Figure 3: Colour images representing the ratio of the 3.6 over 4.5 $\mu$ m images ( left) and 4.5 over 8.0 $\mu$ m ( right). These images have a common resolution roughly equivalent to that in the 8.0 $\mu$ m image. White corresponds to low colour ratios, and black to high ones.
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In the text

$\begin{figure} \par\includegraphics[width=8.4cm,clip]{8179fig4.eps}\end{figure}$ Figure 4: M 33 three colour image, composite of the central region of the complete IRAC data. Blue is a weighted mean of 3.6 and 4.5 $\mu$ m, representing the evolved stellar population. Green depicts the 5.8 $\mu$ m light after removal of stellar emission, and red traces the 8.0 $\mu$ m channel also corrected for stellar emission. (See the electronic edition of the Journal for a colour version of this figure.)
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In the text

$\begin{figure} \par\includegraphics[angle=-90,width=8.2cm,clip]{8179fig5.ps}\end{figure}$ Figure 5: H ${\alpha }$ image of M 33 with the over-plot of 8.0 $\mu$ m contours.
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In the text

$\begin{figure} \par\includegraphics[angle=-90,width=8.2cm,clip]{8179fig6.ps}\end{figure}$ Figure 6: H ${\alpha }$ image of M 33 with the over-plot of 24 $\mu$ m contours.
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In the text

$\begin{figure} \par\includegraphics[width=7.4cm,clip]{8179fig7.eps}\end{figure}$ Figure 7: Detection rate in the different wavebands. Open symbols represent the H II regions selected in IR (squares), radio (triangles) and optical (circles). The (blue) crosses represent the optically selected SNRs and the (blue) plus signs the radio selected SNRs. The filled (red) diamonds represent the PNe. The (green) tripods represent our 24 $\mu$ m sources.
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In the text

$\begin{figure} \par\includegraphics[width=8.4cm,clip]{8179fig8a.eps}\par\includegraphics[width=8.4cm,clip]{8179fig8b.eps}\end{figure}$ Figure 8: The position of catalogued sources with photometry in all four IRAC bands superimposed on the 4.5 $\mu$ m IRAC image. The upper panel shows the IR selected (open squares), radio selected (pluses) and optically selected (open circles) H II regions. The lower panel shows the locations of the PNe (open circles), optically selected (crosses) and radio selected (pluses) SNRs. The images are about 1 $^\circ$ on a side, north is up and east to the left.
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In the text

$\begin{figure} \par\includegraphics[width=7.7cm,clip]{8179fig9.eps}\end{figure}$ Figure 9: Mean IR SEDs of the different kind of sources. Open symbols represent the H II regions selected in IR (squares), radio (triangles) and optical (circles). The (blue) crosses represent the SNRs optically selected and the (blue) pluses the SNRs radio selected. The filled (red) diamonds represent the PNe. The (green) tripods represent the 24 $\mu$ m unknown sources. The grey vertical stripes depict the widths of the IRAC and MIPS-24 filters.
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In the text

$\begin{figure} \par\includegraphics[width=8.2cm,clip]{8179fig10.ps}\end{figure}$ Figure 10: Spectra obtained with Cloudy code, for the four models. The parameter pairs in size-density space with density given first in cm^-3 followed by outer radius in pc are represented by the (blue) short-dashed line for (100,10); the (cyan) dotted line (20,30); the (green) long-dashed line (10,60); and the (red) solid line (1000,2).
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In the text

$\begin{figure} \par\includegraphics[width=4.2cm,clip]{8179fig11a.ps}\includegrap... ...lip]{8179fig11c.ps}\includegraphics[width=4.2cm,clip]{8179fig11d.ps}\end{figure}$ Figure 11: The resulting DUSTY spectra. We plot only 5 values of A_V, namely 0.1, 2.15, 10.0, 21.5, and 100 (shown as long-dashed, short-dashed, dotted, dot-dashed, and solid lines, respectively). Also shown (as a heavy solid line which ends at $\lambda \sim 100~\mu$ m) is the input stellar spectrum of a 3 Myr starburst from SB99. The upper panels show $T_{\rm in}$ = 300 K, and the lower ones $T_{\rm in}$ = 600 K. The left panels show thick shells ( $R_{\rm out}$ = 10 000), and in the right panels thin ones ( $R_{\rm out}$ = 100).
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In the text

$\begin{figure} \par\includegraphics[width=7.7cm,clip]{8179fig12a.eps}\vspace*{4.... ...s}\vspace*{4.8mm} \includegraphics[width=7.7cm,clip]{8179fig12c.eps}\end{figure}$ Figure 12: Bottom panel: blackbodies are depicted by filled (black) circles. NGC 7023 is represented by a (yellow) triangle, the ISM by Draine by an inverted (cyan) triangle, the SB99 simulations by (grey) diamonds. The Cloudy H II region is represented by filled (green) squares. Finally, the DUSTY models are represented by filled ( $T_{\rm in}$ = 300 K) and empty ( $T_{\rm in}$ = 600 K) circles, for a variety of shell thickness $Y_{\rm out}$ = 10 (solid), 100 (dashed), 1000 (dash-dot) and 10 000 (dotted line). Different values of A_V (logarithmically from 0.1 to 100 mag) for a given $T_{\rm in}$ and $Y_{\rm out}$ are connected by the different lines; the highest value of A_V is marked by $\times$ . Middle panel: the open symbols represent the H II regions selected in IR (squares), radio (triangles) and optical (circles). The (blue) crosses represent the SNRs optically selected and the (blue) plus the SNRs radio selected. The filled (red) diamonds represent the PNe. The open rectangles delineate the zones occupied by the majority of sources of a given type. The grey rectangle delimits the zone of molecular shocked SNRs (Reach et al. 2006). Top panel: the 24 $\mu$ m sources are depicted by (black) pluses.
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In the text

$\begin{figure} \par\includegraphics[width=7.7cm,clip]{8179fig13a.eps}\vspace*{5m... ...eps}\vspace*{5mm} \includegraphics[width=7.7cm,clip]{8179fig13c.eps}\end{figure}$ Figure 13: Same as Fig. 12, but for different IRAC flux ratios.
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In the text

$\begin{figure} \par\includegraphics[width=7.7cm,clip]{8179fig14a.eps}\vspace*{5m... ...eps}\vspace*{5mm} \includegraphics[width=7.7cm,clip]{8179fig14c.eps}\end{figure}$ Figure 14: Same as Fig. 12, but now including the MIPS 24 $\mu$ m flux ratio.
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In the text

$\begin{figure} \par\includegraphics[width=7.7cm,clip]{8179fig15.eps}\end{figure}$ Figure 15: Diagnostic diagrams in magnitude, for comparison with Cohen et al. (2006). The open symbols represent the H II regions selected in IR (squares), radio (triangles) and optical (circles). The (blue) crosses represent the SNRs optically selected and the (blue) plus the SNRs radio selected. The filled (red) diamonds represent the PNe. Two radio H II regions are lying outside the limits of the displayed plot.
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In the text

$\begin{figure} \par\includegraphics[width=7.7cm,clip]{8179fig16a.eps}\par\includegraphics[width=7.7cm,clip]{8179fig16b.eps}\end{figure}$ Figure 16: Bottom panel: cumulative distribution of the 24 $\mu$ m sources. The 24 $\mu$ m fluxes are reported $F_\nu$ (24 $\mu$ m) (bottom axis), as well as the TIR luminosity $L_\nu$ (TIR) (top axis). The linear best fits are depicted by a (red) solid line for the $F_\nu$ (24 $\mu$ m) fluxes between 1 and 70 mJy, and by a (green) dashed line for the $F_\nu$ (24 $\mu$ m) fluxes greater than 70 mJy. Top panel: cumulative distribution of the 24 $\mu$ m sources detected also at 8.0 $\mu$ m, the TIR luminosity is calculated using for each source the effective 8 $\mu$ m/24 $\mu$ m flux ratio.
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In the text

$\begin{figure} \par\includegraphics[width=7.8cm,clip]{8179fig17.eps}\end{figure}$ Figure 17: Comparison of the SFR calculated from the H ${\alpha }$ luminosity and from the IR emission based on the 8.0 and 24 $\mu$ m emissions. The dashed line shows the SFR(H ${\alpha }$ ) = SFR(IR) relation.
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In the text

$\begin{figure} \par\includegraphics[width=7.7cm,clip]{8179fig18.eps}\end{figure}$ Figure 18: Diagnostic diagram involving IRAC colours for the 24 $\mu$ m sources having a H ${\alpha }$ counterpart (open circles) and for the 24 $\mu$ m sources with no H ${\alpha }$ counterpart (crosses).
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In the text

$\begin{figure} \par\includegraphics[width=7.8cm,clip]{8179fig1a.ps}\hspace{0.7c... ....ps}\hspace{0.7cm} \includegraphics[width=7.8cm,clip]{8179fig1d.ps}\end{figure}$	Figure 1: Final IRAC mosaics of M 33. The upper left image displays the 3.6 $\mu$ m, the upper right 4.5 $\mu$ m, the lower left 5.8 $\mu$ m and the lower right the 8.0 $\mu$ m channel. The images are about 1 $^\circ$ by 1.5 $^\circ$ , north is up and east is to the left. The region enclosed by the continuous line will be further investigated in the present paper.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{8179fig3a.ps}\hspace*{0.4cm} \includegraphics[width=8cm,clip]{8179fig3b.ps}\end{figure}$	Figure 3: Colour images representing the ratio of the 3.6 over 4.5 $\mu$ m images ( left) and 4.5 over 8.0 $\mu$ m ( right). These images have a common resolution roughly equivalent to that in the 8.0 $\mu$ m image. White corresponds to low colour ratios, and black to high ones.
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$\begin{figure} \par\includegraphics[angle=-90,width=8.2cm,clip]{8179fig5.ps}\end{figure}$	Figure 5: H ${\alpha }$ image of M 33 with the over-plot of 8.0 $\mu$ m contours.
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$\begin{figure} \par\includegraphics[angle=-90,width=8.2cm,clip]{8179fig6.ps}\end{figure}$	Figure 6: H ${\alpha }$ image of M 33 with the over-plot of 24 $\mu$ m contours.
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$\begin{figure} \par\includegraphics[width=7.4cm,clip]{8179fig7.eps}\end{figure}$	Figure 7: Detection rate in the different wavebands. Open symbols represent the H II regions selected in IR (squares), radio (triangles) and optical (circles). The (blue) crosses represent the optically selected SNRs and the (blue) plus signs the radio selected SNRs. The filled (red) diamonds represent the PNe. The (green) tripods represent our 24 $\mu$ m sources.
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$\begin{figure} \par\includegraphics[width=8.2cm,clip]{8179fig10.ps}\end{figure}$	Figure 10: Spectra obtained with Cloudy code, for the four models. The parameter pairs in size-density space with density given first in cm^-3 followed by outer radius in pc are represented by the (blue) short-dashed line for (100,10); the (cyan) dotted line (20,30); the (green) long-dashed line (10,60); and the (red) solid line (1000,2).
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$\begin{figure} \par\includegraphics[width=7.7cm,clip]{8179fig13a.eps}\vspace{5m... ...eps}\vspace{5mm} \includegraphics[width=7.7cm,clip]{8179fig13c.eps}\end{figure}$	Figure 13: Same as Fig. 12, but for different IRAC flux ratios.
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$\begin{figure} \par\includegraphics[width=7.7cm,clip]{8179fig14a.eps}\vspace{5m... ...eps}\vspace{5mm} \includegraphics[width=7.7cm,clip]{8179fig14c.eps}\end{figure}$	Figure 14: Same as Fig. 12, but now including the MIPS 24 $\mu$ m flux ratio.
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$\begin{figure} \par\includegraphics[width=7.8cm,clip]{8179fig17.eps}\end{figure}$	Figure 17: Comparison of the SFR calculated from the H ${\alpha }$ luminosity and from the IR emission based on the 8.0 and 24 $\mu$ m emissions. The dashed line shows the SFR(H ${\alpha }$ ) = SFR(IR) relation.
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$\begin{figure} \par\includegraphics[width=7.7cm,clip]{8179fig18.eps}\end{figure}$	Figure 18: Diagnostic diagram involving IRAC colours for the 24 $\mu$ m sources having a H ${\alpha }$ counterpart (open circles) and for the 24 $\mu$ m sources with no H ${\alpha }$ counterpart (crosses).
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