The Advanced Camera for Surveys ( ACS ) and the Near Infrared Camera and Multi-Object Spectrometer ( NICMOS ) have been used to obtain new Hubble Space Telescope images of NGCÂ 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 by approximately two magnitudes over our previous WFPC2 results , and find that it continues as a single power law , dN / dL \propto L ^ { \alpha } with \alpha = -2.13 \pm 0.07 , down to the observational limit of \mbox { $M _ { V } $ } \approx - 7 .
Similarly , the mass function is a single power law dN / dM \propto M ^ { \beta } with \beta = -2.10 \pm 0.20 for clusters with ages < 3 \times 10 ^ { 8 } Â yr , corresponding to lower mass limits that range from 10 ^ { 4 } to 10 ^ { 5 } \mbox { $M _ { \odot } $ } , depending on the age range of the subsample .
Hence the power law indices for the luminosity and mass functions are essentially the same .
The luminosity function for intermediate-age clusters ( i.e. , \sim 100–300 Myr old objects found in the loops , tails , and outer areas ) shows no bend or turnover down to \mbox { $M _ { V } $ } \approx - 6 , consistent with relaxation-driven cluster disruption models which predict the turnover should not be observed until \mbox { $M _ { V } $ } \approx - 4 .
An analysis of individual \sim 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 an \sim 10–50 Myr old cluster of estimated mass \sim 10 ^ { 6 } \mbox { $M _ { \odot } $ } having apparently triggered the formation of several younger , more massive ( up to 5 \times 10 ^ { 6 } \mbox { $M _ { \odot } $ } ) clusters along a dust lane .
A comparison with new NICMOS observations reveals that only 16 \pm 6 % of the IR-bright clusters in the Antennae are still heavily obscured , with values of \mbox { $A _ { V } $ } > 3 mag .