tokens: main age periods in the ring. Age, mass, and Av inferred from the fit are indicated. The data are circles, model is the line, squares are the model after convolution with the filter width used for the data, the horizontal bar indicates the width of the filter. The red point is the photometric emission in the HST /Ha + [N ii ] filter, which includes the line and continuum emission; the yellow point on the vertical grey line is the predicted H α emission from the ionizing budget of the cluster and added up to the continuum prediction, which includes the photosphere contribution, all convolved with the shape of the HST filter (hence the grey line in the younger clusters). The cluster number and its relative coordinates – left corner of each plot – are to be used to find its location in any of the images in this work. To illustrate the range of extinctions and its effect, for each age period two distinct clusters, one requiring minimum Av, other requiring larger Av, are shown. The discrepancy between the predicted H α emission and the data for the youngest 4 Myr burst (illustrated with the clusters 30 y 138), which is due to the diffuse nature of the nebular gas in the ring (discussed in Section 3 ), can be noticed. The cluster no 145 is an example of the few in which the measured emission is in line with the prediction. The simultaneous inclusion of UV, optical, and IR data in the fit was found to be critical for the age determination in the particular range covered in the ring, in the 1 Myr to a few 100 Myr (see below). Moreover, the moderate extinction at the clusters location (Section 5 ) also helped in better constraining ages as the observed SED is close to its intrinsic shape and minimum correction due to dust is required in the fit. On this basis, special features in the SED that were found determinant for the age estimates are: the steep UV–optical–IR spectral slope for assuring the ages of the younger, less than 10 Myr old population (in this case the cluster H α emission cannot be used (Section 3.1 ) but the wide wavelength coverage up to and minimum dust extinction makes the result very solid); the Balmer break for ages older than 20 Myr; the near-IR bands helped to constrain the oldest population beyond 60 Myr because of the progressive curvature of the SED, simultaneously, towards the UV and IR bands. Dating of the clusters detected only from onwards was found largely uncertain: the SSP fits are based on four spectral bins only and the rather featureless shape of the continuum in the workable range, , is alone insufficient to properly constrain the age. For this reason, these clusters are not further analysed in this work. The denser dusty environment of these clusters is evidenced by comparing the UV - with the K -band image of the ring (Fig. 1 ), to see that these clusters are often located behind the thicker dust filaments. Putting in context the cluster properties of NGC 1097’s ring as compared with those of massive young star
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