The Stellar Content of the Southern Tail of NGC 4038/4039 and a Revised Distance

Abstract

We have used the Hubble Space Telescope and Wide Field Planetary Camera 2 to image the putative tidal dwarf galaxy located at the tip of the Southern tidal tail of NGC 4038/4039, the Antennae. We resolve individual stars and identify two stellar populations. Hundreds of massive stars are present, concentrated into tight OB associations on scales of 200 pc, with ages ranging from 2 to 100 Myr. An older stellar population is distributed roughly following the outer contours of the neutral hydrogen in the tidal tail; we associate these stars with material ejected from the outer disks of the two spirals. The older stellar population has a red giant branch tip at I = 26.5 ± 0.2 from which we derive a distance modulus (m - M)0 = 30.7 ± 0.25. The implied distance of 13.8 ± 1.7 Mpc is significantly smaller than commonly quoted distances for NGC 4038/4039. In contrast to the previously studied core of the merger, we find no super–star clusters (SSCs). One might conclude that SSCs require the higher pressures found in the central regions in order to form, while spontaneous star formation in the tail produces the kind of OB star associations seen in dwarf irregular galaxies. The youngest population in the putative tidal dwarf has a total stellar mass of ≈2 × 105 M⊙, while the old population has a stellar mass of ≈7 × 107 M⊙. If our smaller distance modulus is correct, it has far-reaching consequences for this prototypical merger. Specifically, the luminous to dynamical mass limits for the tidal dwarf candidates are significantly less than 1, the central SSCs have sizes typical of Galactic globular clusters, rather than being 1.5 times as large, and the unusually luminous X-ray population becomes both less luminous and less populous.