Context :

Aims : Approximately 20 % of all spiral galaxies display starburst activity in nuclear rings of a few hundred parsecs in diameter .
It is our main aim to investigate how the starburst ignites and propagates within the ring , leading to the formation of massive stellar clusters .

Methods : We observed the ring galaxy NGC 7552 with the mid-infrared ( MIR ) instrument VISIR at an angular resolution of 0 \aas@@fstack { \prime \prime } 3 - 0 \aas@@fstack { \prime \prime } 4 and with the near-infrared ( NIR ) integral-field spectrograph SINFONI on the VLT , and complement these observations with data from ISO and Spitzer .

Results : The starburst ring is clearly detected at MIR wavelengths at the location of the dust-extincted , dark ring seen in HST observations .
This “ ring ” , however , is a rather complex annular region of more than 100 parsec width .
We find a large fraction of diffuse [ Ne ii ] and PAH emission in the central region that is not associated with the MIR peaks on spatial scales of \sim 30 pc .
We do not detect MIR emission from the nucleus of NGC 7552 , which is very prominent at optical and NIR continuum wavelengths .
However , we have identified nine unresolved MIR peaks within the ring .
The average extinction of these peaks is A _ { V } = 7.4 and their total infrared luminosity is L _ { IR } = 2.1 \times 10 ^ { 10 } L _ { \odot } .
The properties of these peaks are typical for MIR-selected massive clusters found in other galaxies .
The ages of the MIR-selected clusters are in the range of 5.9 \pm 0.3 Myr .
The age spread among the clusters of 0.8 Myr is small compared to the travel time of \sim 5.6 Myr for half an orbit within the starburst ring .
We find no strong evidence for a scenario where the continuous inflow of gas leads to the ongoing formation of massive clusters at the contact points between galactic bar and starburst ring .
Instead , it appears more likely that the gas density build up more gradually over larger ring segments , and that the local physical conditions govern cluster formation .
We note that the fundamental limitation on the accurate derivation of cluster age , mass and IMF slope is the lack of higher angular resolution .
Resolving the highly embedded , massive clusters requires milli-arcsecond resolution at infrared wavelengths , which will be provided by the next generation of instruments on extremely large telescopes ( ELTs ) .

Conclusions :