An ultraviolet through infrared look at star formation and super star clusters in two circumnuclear starburst rings

We present broadband (U, V, I, and H) and narrowband (H alpha+[N II] and Pa alpha) images of the circumnuclear starburst rings in two nearby spiral ga laxies, NGC 1512 and NGC 5248, obtained with the WFPC2 and NICMOS cameras o n HST. Combined with previously published ultraviolet (UV) HST images at 23 00 Angstrom, these data provide a particularly wide wavelength range with w hich to study the properties of the stellar populations, the gas, and the d ust in the rings. The young star clusters and the line-emitting gas have di fferent spatial distributions, with some large (50 pc scale) line-emitting regions that have little associated continuum emission, but a Pa alpha equi valent width indicating an embedded stellar population a few megayears old. The observed H alpha /Pa alpha intensity ratios suggest the gas is mixed w ith dust, making it effective at completely obscuring some of the young clu sters. We identify the major (about 500 in each galaxy) compact continuum s ources (super star clusters and individual stars) and analyze their spectra l energy distributions (SEDs) from 0.2 to 1.6 mum by fitting them with a gr id of spectral synthesis models with a range of ages and dust extinction. M ost of the visible clusters are only mildly reddened, with A(V) = 0 to 1 ma g, suggesting that the processes that clear out the gas and dust of the ste llar birth clouds are efficient and fast. The patchiness of the dust distri bution makes it difficult to reliably estimate the star formation rate, bas ed on UV continuum slope or hydrogen emission-line ratios, in starbursts su ch as these. The cluster SEDs are consistent with a range in ages, from 1 t o 300 Myr, but with only a minority older than a few tens of megayears. We point out an age bias, the result of the steep luminosity function of the c lusters combined with the fading of clusters as they age, which causes youn g clusters to be overrepresented at any luminosity. Accounting for this bia s, the fraction of old clusters is consistent with continuous star formatio n in the rings over the past similar to 300 Myr. Because of the uncertainti es in dating the clusters, we cannot rule out episodic, similar to 20 Myr l ong bursts of star formation, but the presence of UV-bright rings in about 10% of spiral galaxies argues against this possibility. Although most of th e observed SEDs are well fitted by a range of models, some of the brightest young clusters have excess emission in the IR that is not predicted by the models and may be thermal reradiation by circumstellar dust. The cluster m ass functions follow a power-law distribution with index -2, similar to tha t recently derived for the starburst in the merging Antennae galaxies, and extending to similar to 10(5) The lack of a mass scale means that subsequen t evolution of the mass function is required, if some of the SSCs are to ev olve into globular clusters. The clusters are spatially unresolved or margi nally resolved, corresponding to V-band Gaussian radii of less than a few p arsecs, at an assumed distance of 10 Mpc. In NGC 5248, we report a previous ly unknown, 60 pc radius, inner emission-line ring, and in NGC 1512, a pecu liar compact (0."1 diameter) source with an H alpha+[N II] equivalent width of similar to 7000 Angstrom, which may be a so-called Balmer-dominated sup ernova remnant.