A new red giant-based distance modulus of 13.3 Mpc to the antennae galaxies and its consequences

The Antennae galaxies are the closest example of an ongoing major galaxy merger and, as such, represent a unique laboratory for furthering the understanding of the formation of exotic objects ( e. g., tidal dwarf galaxies, ultraluminous X- ray sources, super stellar clusters). In a previous paper HST WFPC2 observations were used to demonstrate that the Antennae system might be at a distance considerably less than that conventionally assumed in the literature. Here we report new, much deeper HST ACS imaging that resolves the composite stellar populations and, most importantly, reveals a well- defined red giant branch. The tip of this red giant branch ( TRGB) is unambiguously detected at I(0)(TRGB) = 26: 65 +/- 0: 09 mag. Adopting the most recent calibration of the luminosity of the TRGB then yields a distance modulus for the Antennae of ( m - M)(0) 30. 62 +/- 0.17 corresponding to a distance of 13. 3 +/- 1. 0Mpc. This is consistent with our earlier result, once the different calibrations for the standard candle are considered. We briefly discuss the implications of this now well- determined shorter distance.