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$\begin{figure} \par\includegraphics[width=7.2cm,clip]{9129fig1.eps}\end{figure}$ Figure 1: a) Percentage of luminosity that is required to be subtracted from the measured H ${\alpha }$ luminosity to correct for contamination from the [N II] emission lines (at 5648 Å and 6584 Å) in the on-line filter, and from the inclusion of emission lines in the continuum R-band filter, as a function of the [N II]5684/H ${\alpha }$ ratio. The dotted line indicates the correction applied to the H ${\alpha }$ luminosity of the bar H II regions of NGC 1530. The dashed lines show the corresponding correction for the two H II regions observed by Márquez et al. (2002). b) Same as a) but as a function of the metallicity for two extreme values of the ionization parameter (q). We have employed the [N II] $\lambda$ 5684/H ${\alpha }$ parametrisation as a function of q and metallicity (from Kewley & Dopita 2002), and the transmission curve of the on and off-line filters for creating the plot. The contribution of [S II] $\lambda \lambda$ 6717, 6731 Å to the continuum flux is negligible. Therefore, a) and b) can be used to obtain the overestimation in the measured $EW_{\rm {H\alpha }}$ due to the inclusion of the emission lines mentioned above (see Sect. 4.1).
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In the text

$\begin{figure} \par\includegraphics[width=16.9cm,clip]{9129fig2.eps}\end{figure}$ Figure 2: a) H ${\alpha }$ continuum subtracted image of the bar region of NGC 1530. b) R band image. c) Observed projected velocity gradient map in the direction perpendicular to the bar (from Zurita et al. 2004). d) B-R colour map. e) $B-K{\rm s}$ colour map. f) B-R colour map of the NW side of the bar with overlaid contours of the velocity gradients in the direction of the bar (velocity gradients from Zurita et al. 2004). We note the spatial correlation between the loci of maximum velocity gradient in the direction parallel to the bar major axis, and the dust spurs in the trailing side of the bar (the information on the velocity gradients is spatially-limited to zones with detectable H ${\alpha }$ emission). The colour maps were not corrected for Galactic extinction. This correction amounts to -0.25 mag/sq $^{\prime \prime }$ and -0.6 mag/sq $^{\prime \prime }$ , for the B-R and $B-K{\rm s}$ maps, respectively (Schlegel et al. 1998). Images a) to e) show the same area of the galaxy. The circles indicate the position and size of the H II regions of the bar. Their identification number, as provided in the catalogue (Relaño et al. 2005) is indicated in Fig. 1a. The arrows in panels d) and e) indicate blue knots with no counterpart in H ${\alpha }$ (see Sect. 3.1 for details.).
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In the text

$\begin{figure} \par\includegraphics[width=7.3cm,clip]{9129fig3.ps}\end{figure}$ Figure 3: Surface brightness profile along the major axis of the bar (PA = 121 $^{\circ }$ ) of NGC 1530 for the B, V, R and $K{\rm s}$ bands. The data were corrected for Galactic extinction (Schlegel et al. 1998). The dotted line indicates the approximate location of the bar end.
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In the text

$\begin{figure} \par\includegraphics[width=17.15cm,clip]{9129fig4.ps}\end{figure}$ Figure 4: Bar minor-axis profiles for the B, V, R and $K{\rm s}$ broad bands for the SE a) and NW b) half sides of the bar of NGC 1530. The dashed vertical line indicates the location of the bar major axis. Negative and positive distances correspond to the trailing and leading sides of the bar respectively. Each profile was normalized to its maximum value and afterwards the $K{\rm s}$ , R, V and B band profiles were displaced by 0, -0.15, -0.30 and -0.40 arbitrary units respectively, to facilitate inspection.
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In the text

$\begin{figure} \includegraphics[width=16.7cm,clip]{9129fig5.ps}\end{figure}$ Figure 5: B-R ( top) and $B-K{\rm s}$ ( bottom) colour profiles across the bar of NGC 1530 for both semi-bar halves (SE on the left panels, a) and c), and NW on the right panels, b) and d)). As in Fig. 4, the dashed vertical line indicates the bar major-axis position, and negative distances correspond to the bar trailing side. We note the steeper colour gradient of the bar leading side. The dust-lanes appear in the plots as local (redder) peaks at $\sim$ 2-4 $^{\prime \prime }$ from the bar major axis on the leading side. The profiles were corrected for Galactic extinction (Schlegel et al. 1998).
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In the text

$\begin{figure} \par\includegraphics[width=15.2cm,clip]{9129fig6.eps}\end{figure}$ Figure 6: a) and b): colour-colour plots for single stellar-population models (Bruzual & Charlot 2003) for different metallicities: $Z/Z{_\odot }$ = 0.02 (solid line), 1.0 (dashed), 2.5 (dotted). c) and d): colour-colour plots for stellar population synthesis models assuming solar metallicity and a different star formation history: a star formation rate (SFR) decreasing with time through a dependence on the galaxy gas mass fraction (solid line) ( ${\it SFR}=0.3$ $\times$ 10^-3 $M_\odot$ Myr^-1 $\times$ ( $M_{\rm gas}$ / $M_{\rm total}$ ); Fioc & Rocca-Volmerange 1997); a constant rate of star formation (dashed line) ( ${\it SFR}=0.5$ $\times$ 10^-4 $M_\odot$ Myr^-1; Fioc & Rocca-Volmerange 1997); and an exponentially decreasing with time SFR (dotted line) ( ${\it SFR} = 0.25$ $\times$ 10^-3 $M_\odot$ Myr^-1 $\times$ $\exp{(- t / 4~{\rm Gyr})}$ ; Bruzual & Charlot 2003). The numbers along the tracks indicate the stellar population age (in Gyr) at each position. Filled squares and triangles represent our measurements of local background values after and before extinction correction, respectively. Filled magenta symbols indicate local background determinations for the H II regions located furthest away from the bar dust lane on the leading side, while green filled symbols correspond to the rest of the measurements. The extinction correction includes Galactic extinction (Schlegel et al. 1998), and extinction implied by an empirically determined colour excess E(B-V) = 0.3 and Calzetti et al. (2000) attenuation curve, which gives the following extinction in magnitudes for each band: A_B = 1.6 mag, $A_{\rm V}$ = 1.2 mag, A_R = 1.0 mag, $A_{K{\rm s}}$ = 0.1 mag. The arrow in each plot indicates the extinction correction corresponding to $A_{\rm V}$ = 1 mag.
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In the text

$\begin{figure} \par\includegraphics[width=16.8cm,clip]{9129fig7.ps}\end{figure}$ Figure 7: Representation of ( from top to bottom and from left to right) the H ${\alpha }$ equivalent width ( $EW_{\rm {H\alpha }}$ ), H ${\alpha }$ + [N II] luminosity, and B-V and B-R broad-band colours of the bar H II regions of NGC 1530 as a function of their deprojected distance from the bar dust-lane in arc seconds. Zero means that the H II region is located on the bar dust-lane, negative and positive values refer to H II regions located on the trailing and leading side of the bar dust-lane respectively. The $EW_{\rm {H\alpha }}$ have been corrected for [N II] contamination as described in Sect. 2.3. The broad-band colours have only been corrected for Galactic extinction (Schlegel et al. 1998).
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In the text

$\begin{figure} \par\includegraphics[width=16.8cm,clip]{9129fig8.ps}\end{figure}$ Figure 8: Correlations between observed properties of the H II regions of the bar of NGC 1530. Green dots correspond to the H II regions with lowest observed H ${\alpha }$ equivalent widths (located in the leading side of the bar dust-lane), pink dots correspond to H II regions located in the trailing side of the bar dust-lane. The broad-band colours have only been corrected for Galactic extinction (Schlegel et al. 1998). The arrow in the B-V vs. V-R plot indicates the direction of the extinction correction corresponding to $A_{\rm V}$ = 1 and assuming Calzetti et al. (2000) attenuation curve. The $EW_{\rm {H\alpha }}$ have been corrected for [N II] contamination as described in Sect. 2.3. Note the lack of any systematic colour difference between H II regions of the leading and trailing sides.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{9129fig9.ps}\end{figure}$ Figure 9: Required percentage of Lyc photons absorbed by dust and/or escaping from the H II regions of the leading side of bar dust-lane of NGC 1530, as a function of the same percentage for the H II regions of the trailing side, under the hypothesis that ionizing-photon extinction and photon leakage are the only causes of the observed difference in $EW_{\rm {H\alpha }}$ (Fig. 7a).
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In the text

$\begin{figure} \par\includegraphics[width=8.2cm,clip]{9129fig10.eps}\end{figure}$ Figure 10: Evolution of the H ${\alpha }$ equivalent width as a function of the age predicted by theoretical models (Leitherer et al. 1999), for an IMF with exponents 1.3 and 2.35, for masses below and above 0.5 $M_\odot$ , respectively. Different IMF upper mass limits were assumed: 30 $M_\odot$ (solid lines), 60 $M_\odot$ (dotted lines), 100 $M_\odot$ (dashed lines), 150 $M_\odot$ (dot-dash-dot lines). Red and black lines indicate metallicities of Z=0.008 and Z=0.02 (solar), respectively.
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In the text

$\begin{figure} \par\includegraphics[width=7.6cm,clip]{9129fig11.eps}\end{figure}$ Figure 11: Estimated age of the H II regions of the bar of NGC 1530 as a function of their deprojected distance across the bar of the galaxy. Zero means that the H II region is located on the bar dust-lane. Negative and positive values refer to H II regions located on the trailing and leading sides of the bar dust-lanes respectively. Asterisks, diamonds and triangles indicate ages estimated from IMFs with upper mass limits of 60 $M_\odot$ , 100 $M_\odot$ and 150 $M_\odot$ , respectively. Top and bottom figures show ages when a metallicities of Z=0.008 and Z=0.02 (solar) are assumed, respectively. The error bars come from the conversion into age of the upper and lower bounds to the measured $\log$ $EW_{\rm {H\alpha }}$ .
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In the text

$\begin{figure} \par\includegraphics[width=7.3cm,clip]{9129fig3.ps}\end{figure}$	Figure 3: Surface brightness profile along the major axis of the bar (PA = 121 $^{\circ }$ ) of NGC 1530 for the B, V, R and $K{\rm s}$ bands. The data were corrected for Galactic extinction (Schlegel et al. 1998). The dotted line indicates the approximate location of the bar end.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{9129fig9.ps}\end{figure}$	Figure 9: Required percentage of Lyc photons absorbed by dust and/or escaping from the H II regions of the leading side of bar dust-lane of NGC 1530, as a function of the same percentage for the H II regions of the trailing side, under the hypothesis that ionizing-photon extinction and photon leakage are the only causes of the observed difference in $EW_{\rm {H\alpha }}$ (Fig. 7a).
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