THE ANTENNAE GALAXIES (NGC 4038/4039) REVISITED: ADVANCED CAMERA FOR SURVEYS AND NICMOS OBSERVATIONS OF A PROTOTYPICAL MERGER

The Advanced Camera for Surveys and the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) have been used to obtain new Hubble Space Telescope images of NGC 4038/4039 (The Antennae). These new observations allow us to better differentiate compact star clusters from individual stars, based on both size and color. We use this ability to extend the cluster luminosity function (LF) by approximately 2 mag over our previous WFPC2 results, and find that it continues as a single power law, dN/dL proportional to L-alpha with alpha = -2.13 +/- 0.07, down to the observational limit of M-V approximate to -7. Similarly, the mass function (MF) is a single power law dN/dM proportional to M beta with beta = -2.10 +/- 0.20 for clusters with ages <3 x 10(8) yr, corresponding to lower mass limits that range from 10(4) to 10(5) M-circle dot, depending on the age range of the subsample. Hence, the power-law indices for the luminosity and MFs are essentially the same. The LF for intermediate-age clusters (i.e., similar to 100-300 Myr old objects found in the loops, tails, and outer areas) shows no bend or turnover down to M-V approximate to -6, consistent with relaxation-driven cluster disruption models which predict the turnover should not be observed until M-V approximate to -4. An analysis of individual similar to 0.5 kpc sized areas over diverse environments shows good agreement between values of alpha and beta, similar to the results for the total population of clusters in the system. There is tentative evidence that the values of both alpha and beta are flatter for the youngest clusters in some areas, but it is possible that this is caused by observational biases. Several of the areas studied show evidence for age gradients, with somewhat older clusters appearing to have triggered the formation of younger clusters. The area around Knot B is a particularly interesting example, with a similar to 10-50 Myr old cluster of estimated mass similar to 10(6) M-circle dot having apparently triggered the formation of several younger, more massive (up to 5 x 10(6) M-circle dot) clusters along a dust lane. A comparison with new NICMOS observations reveals that only 16% +/- 6% of the IR-bright clusters in the Antennae are still heavily obscured, with values of A(V) > 3 mag.