The Antennae Galaxies (Ngc 4038/4039) Revisited: Advanced Camera for Surveys and Nicmos Observations of a Prototypical Merger

Abstract

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 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 by approximately two magnitudes over our previous WFPC2 results, and find that it continues as a single power law, dN/dL ∝ L α with α = −2.13 ± 0.07, down to the observational limit of M V ≈ −7. Similarly, the mass function is a single power law dN/dM ∝ M β with β = −2.10 ± 0.20 for -2clusters with ages < 3 × 10 8 yr, corresponding to lower mass limits that range from 10 4 to 10 5 M ⊙ , 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., ∼100-300 Myr old objects found in the loops, tails, and outer areas) shows no bend or turnover down to M V ≈ −6, consistent with relaxation-driven cluster disruption models which predict the turnover should not be observed until M V ≈ −4. An analysis of individual ∼0.5-kpc sized areas over diverse environments shows good agreement between values of α and β, similar to the results for the total population of clusters in the system. There is tentative evidence that the values of both α and β 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 ∼10-50 Myr old cluster of estimated mass ∼ 10 6 M ⊙ having apparently triggered the formation of several younger, more massive (up to 5 × 10 6 M ⊙ ) 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.

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A catalog of extended clusters and ultra-compact dwarf galaxies

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