The Luminosity Function of Young Star Clusters in “the Antennae” Galaxies (NGC 4038/4039)

Abstract

The Wide Field Planetary Camera 2 of the Hubble Space Telescope has been used to obtain high-resolution images of NGC 4038/4039 that go roughly 3 mag deeper in V than previous observations made during cycle 2. These new images allow us to measure the luminosity functions (LFs) of clusters and stars over a range of 8 mag (-14 < M V < -6). To first order, the LF is a power law, with exponent α = -2.12 ± 0.04. However, using a variety of different techniques to decouple the cluster and stellar LFs, which overlap in the range -9 ≲ M V ≲ -6, we find an apparent bend in the young cluster LF at approximately M V = -10.4. Brightward of this magnitude the LF has a power-law exponent α = -2.6 ± 0.2, while faintward the slope is α = -1.7 ± 0.2. The bend corresponds to a mass ≈ 1 × 10 5 M ⊙ , only slightly lower than the characteristic mass of globular clusters in the Milky Way (≈2 × 10 5 M ⊙ ). It is currently not feasible to determine the cluster LF fainter than M V ≈ -8, where individual stars are likely to dominate. The stellar LF in the range -9 < M V < -6 is much steeper, with α = -2.9 ± 0.1, and is dominated by young red and blue supergiants. The star clusters of the Antennae appear slightly resolved, with median effective radii of 4 ± 1 pc, similar to or perhaps slightly larger than those of globular clusters in our Galaxy. However, the radial extents of some of the very young clusters (ages less than 10 Myr) are much larger than those of old globular clusters (e.g., the outer radius of knot S exceeds 450 pc). This may indicate that the tidal forces from the galaxies have not had time to remove some of the outer stars from the young clusters. A combination of the UBVI colors, Hα morphology, and Goddard High Resolution Spectrograph (GHRS) spectra enables us to age date the clusters in different regions of the Antennae. Star clusters around the edge of the dust overlap region appear to be the youngest, with ages ≲5 Myr, while clusters in the western loop appear to be 5–10 Myr old. Many star clusters in the northeastern star formation region appear to be ∼100 Myr old, with an LF in V that has shifted faintward by ∼1.0 mag relative to the younger (0–20 Myr) clusters that dominate over most of the rest of the galaxy. A third cluster population consists of intermediate-age clusters (∼500 Myr) that probably formed during the initial encounter responsible for ejecting the tails. A handful of old globular clusters from the progenitor galaxies are also identified. Most of these lie around NGC 4039, where the lower background facilitates their detection. Age estimates derived from GHRS spectroscopy yield 3 ± 1 Myr for knot K (just south of the nucleus of NGC 4038) and 7 ± 1 Myr for knot S in the western loop, in good agreement with ages derived from the UBVI colors. Effective gas outflow velocities from knots S and K are estimated to be about 25–30 km s -1 , based on the above cluster ages and the sizes of the surrounding Hα bubbles. However, the measured widths of the interstellar absorption lines suggest dispersion velocities of ∼400 km s -1 along the lines of sight to knots S and K.