High Resolution IR Observations of the Starburst Ring in NGC 7552 -- One
  Ring to Rule Them All?
  We observed the ring galaxy NGC 7552 with the mid-infrared (MIR) instrument
VISIR at an angular resolution of 0.3"- 0.4" and with the near-infrared (NIR)
integral-field spectrograph SINFONI on the VLT, and complement these
observations with data from ISO and Spitzer. 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 \sim30 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*10^10 Lo. 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\pm0.3 Myr. The age spread
among the clusters of 0.8 Myr is small compared to the travel time of \sim5.6
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.
